Geologic units in Montana (state in United States)

Additional scientific data in this geographic area

Tongue River Member of Fort Union Formation (Tertiary) at surface, covers 11 % of this area

Yellowish orange sandstone, sandy and silty carbonaceous shale, and coal. Alluvial plain. Thickness as much as 300 m (984 ft).

Bearpaw Formation (Cretaceous) at surface, covers 8 % of this area

Dark gray shale with several zones of calcareous concretions, a basal zone of ferruginous concretions, and numerous thin bentonite beds. Marine. Thickness as much as 300 m (984 ft).

Judith River Formation (Cretaceous) at surface, covers 5 % of this area

Light brown to light gray, fine- to coarse-grained sandstone with interbeds of gray to black carbonaceous shale, silty shale, and thin coal. Local Parkman Sandstone Member (lower part of formation):yellowish gray to brownish gray and olive green, fine- to medium-grained, cross-bedded sandstone interbedded with yellowish gray, silty shale. Estuarine, brackish, and nearshore marine. Thickness as much as 305 m (1000 ft).

Alluvium (Quaternary) at surface, covers 4 % of this area

Gravel, sand, silt, and clay deposits of stream and river channels, and floodplains.

Lebo Member of Fort Union Formation (Tertiary) at surface, covers 4 % of this area

Dark gray carbonaceous shale, bentonitic claystone, sandstone, and coal. Alluvial plain. Thickness as much as 185 m (607 ft).

Hell Creek Formation (Cretaceous) at surface, covers 3 % of this area

Light gray, bentonitic claystone that alternates with gray to brown sandstone interbedded with carbonaceous shale. Laterally equivalent to Lance Formation. Fluvial and flood plain. Thickness as much as 335 m (1,100 ft).

Tullock Member of Fort Union Formation (Tertiary) at surface, covers 3 % of this area

Yellow sandstone interbedded with subordinate grayish brown and black shale and thin beds of coal. Alluvial plain. Thickness as much as 180 m (590 ft).

Gravel (Quaternary) at surface, covers 3 % of this area

Variable deposits that range from pebble to boulder size and include sand, silt, and clay. Dominantly alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.

Piegan Group: Helena and Wallace Formations (Middle Proterozoic) at surface, covers 3 % of this area

Helena Formation (Yh): Northwestern Montana: (formerly “lower Wallace”): cycles of basal white quartzite or intraclast beds overlain by couplets of green siltite and argillite, capped by dolomite beds. Calcite pods and ribbons (molar tooth structure) common. West-central Montana: gray to dark gray limestone and dolomitic limestone with siltite partings. Thickness as much as 2,000 m (6,562). Wallace Formation: Tan-weathering, dolomitic quartzite and siltite, and black argillite with calcite ribbons (molar tooth structure) in graded pinch-and-swell couples and couplets. Thickness as much as 2,500 m (8,202 ft).

Ravalli Group: St. Regis, Revett, and Burke Formations; or Empire and Grinnell Formations; or Empire and Spokane Formations (Middle Proterozoic) at surface, covers 3 % of this area

St. Regis Formation: Green and purplish gray quartzite, siltite, and argillite in mud-cracked couples and couplets. Thickness as much as 900 m (2,953 ft). Revett Formation: Light gray, cross-bedded, felspathic, fine-grained quartzite. Thickness as much as 900 m (2,953 ft). Burke Formation: Gray or purple quartzite, siltite, and argillite in mud-cracked couples and couplets. Thickness as much as 1,000 m (3,281 ft). Empire Formation (Ye): Grayish green and pale olive gray argillite and siltite with subordinate thin beds of quartzite and sandy limestone. Thickness as much as 610 m. Grinnell Formation: White, cross-bedded sandstone or quartzite with red to purple siltite and argillite beds. Thickness as much as 1,160 m (3,806 ft). Spokane Formation (Ysp): Red siltite and argillite in mudcracked couplets. Thickness as much as 1,500 m (492 ft).

Upper Missoula Group: McNamara, Bonner, and Mount Shields Formations; locally includes lower Libby Formation in northwestern Montana (Middle Proterozoic) at surface, covers 3 % of this area

McNamara Formation: Dense green and red siltite and argillite in mudcracked couplets containing diagnostic chert beds and rip-up clasts. Thickness as much as 1,650 m (5,413 ft). Bonner Formation: Pink, cross-bedded, feldspathic, medium- to coarse grained quartzite. Thickness as much as 580 m (1,903 ft). Mount Shields Formation: Upper part: red quartzite, siltite, and argillite in mud-cracked couples and couplets with abundant salt casts. Lower part: light gray, flat-laminated, feldspathic, fine-grained quartzite. Thickness as much as 2,000 m (6,562 ft). In northwestern Montana - Libby Formation:Light to dark gray and greenish gray siltite and argillite with subordinate quartzite in mud-cracked couplets. Thickness as much as 2,300 m (7,546 ft).

Kevin Member of Marias River Formation (Cretaceous) at surface, covers 3 % of this area

Dark gray shale with calcareous concretions and numerous bentonite beds. Upper part calcareous. MacGowan Concretionary Bed (middle part of member): grayish brown concretionary dolostone and limestone with phosphatic pellets, and gray to black chert pebbles. Marine. Thickness as much as 350 m (1,148 ft).

Gravel (Quaternary and Tertiary) at surface, covers 2 % of this area

Variable deposits that range from pebble to boulder size and include sand, silt, and clay. Dominantly alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.

Claggett Formation (Cretaceous) at surface, covers 2 % of this area

Dark gray to gray shale that weathers brown, with thin, gray sandstone laminae and beds in upper or middle part and calcareous concretions in lower part. Marine. Thickness as much as 170 m (558 ft).

Glacial deposit (Quaternary) at surface, covers 2 % of this area

Dominantly till, outwash, and local glacial lake deposits. Shown only in western and south-central Montana. Laurentide glacial deposits are indicated with a map pattern.

Granitic rock (Cretaceous) at surface, covers 2 % of this area

Granitic rock

Lower Missoula Group: Shepard and Snowslip Formations; or Shepard Formation and unresolved Snowslip Formation equivalent (Middle Proterozoic) at surface, covers 2 % of this area

Shepard Formation: Tan-weathering, dolomitic, green siltite and argillite in couplets and microlaminae. In the west formerly considered part of the “upperWallace.” Thickness as much as 1,100 m (3,609 ft). Snowslip Formation: Green and red siltite and argillite in couplets. Western Snowslip equivalent is black siltite and argillite in couplets and microlaminae, formerly considered part of the “upper Wallace.” A new name is under consideration for the western Snowslip equivalent. Thickness as much as 1,200 m (3,937 ft).

Two Medicine Formation (Cretaceous) at surface, covers 2 % of this area

Gray and greenish gray volcanic sandstone, siltstone, and mudstone, andesite–pebble conglomerate, and thin beds of very light gray and pink ash-fall tuff. Fluvial and alluvial plain. Thickness as much as 660 m (2,165 ft).

Quartzofeldspathic gneiss (Archean) at surface, covers 1 % of this area

Quartzofeldspathic gneiss

Gravel (Tertiary) at surface, covers 1 % of this area

Variable deposits that range from pebble to boulder size and include sand, silt, and clay. Dominantly alluvial terrace, abandoned channel and floodplain, remnant alluvial fan, and local glacial outwash.

Eagle Formation (Cretaceous) at surface, covers 1 % of this area

Gray or grayish brown, thick-bedded sandstone, sandy shale, and thin interbedded coal. Chert-pebble lag deposit common at top of formation. Virgelle Member (lower part of formation in parts of central Montana): very light gray, gray, or brown sandstone that coarsens upward. Root zone and thin carbonaceous beds at top of member. Delta, coastal plain, tidal flat, and marine shoreface and foreshore. Thickness as much as 150 m (492 ft).

Sediment or sedimentary rock (Tertiary) at surface, covers 1 % of this area

Sediment or sedimentary rock (no unique unit description on map).

Lance Formation (Cretaceous) at surface, covers 1 % of this area

Orangish brown, coarse- to fine-grained sandstone with subordinate interbeds of shale and mudstone. Laterally equivalent to Hell Creek Formation. Fluvial. Thickness as much as 150 m (492 ft).

Ludlow Member of Fort Union Formation (Tertiary) at surface, covers 0.9 % of this area

Gray and brown shale, siltstone, silty or bentonitic claystone, sandstone, and coal. Alluvial plain with marine-influenced tongues. Thickness as much as 230 m (755 ft).

Pierre Formation (Cretaceous) at surface, covers 0.9 % of this area

Dark gray, partly silty shale with abundant bentonite beds and zones of gray, calcareous concretions. Marine. Thickness as much as650 m (2,133 ft). Only upper 50 m (164 ft) exposed.

Upper Prichard or Appekunny Formation (Middle Proterozoic) at surface, covers 0.8 % of this area

Upper Prichard Formation: Black and gray, pyritic siltite and argillite in planar couplets. Appekunny Formation: Gray and greenish gray argillite interbedded with light gray quartzite. Thickness as much as 1,700 m (5,577 ft).

Fox Hills Formation (Cretaceous) at surface, covers 0.8 % of this area

Yellowish orange to gray, fine- to medium-grained, non-calcareous sandstone in upper part, and interbedded sandstone, siltstone, and black shale with calcareous concretion zone in lower part. Marginal marine. Thickness 30–45 m (98–148 ft).

Madison Group: Mission Canyon and Lodgepole Formations; or Castle Reef and Allan Mountain Formations (Mississippian) at surface, covers 0.8 % of this area

Mission Canyon Formation (Mmc): Gray, massive limestone with chert beds and nodules, and solution breccia zones. Shallow marine. Thickness as much as 520 m (1,706 ft). Lodgepole Formation (Ml): Woodhurst Member (upper part of formation): light gray, well-bedded limestone, typically with much dark chert, interbedded with thinner calcareous mudstone beds. Paine Member (middle part of formation):dark gray, thin-bedded, silty or fossiliferous limestone. Cottonwood Canyon Member (lower part of formation): black shale with basal conglomeratic lag deposit. Shallow marine. Thickness as much as 305 m (1000 ft). In Northwest Montana - Castle Reef Formation: Medium to light gray, thick-bedded limestone or dolomite. Sun River Member (upper part of formation): light gray dolomite with thick fossiliferous lenses. Shallow marine. Thickness as much as 300 m (984 ft). Allan Mountain Formation: Dark gray, thinly bedded limestone with thin mudstone and shale partings, and nodular chert. Shallow marine. Thickness as much as 200 m (656 ft).

Granitic rock (Tertiary) at surface, covers 0.7 % of this area

Granitic rock

Eagle and Telegraph Creek Formations (Cretaceous) at surface, covers 0.7 % of this area

Eagle Formation (Ke): Gray or grayish brown, thick-bedded sandstone, sandy shale, and thin interbedded coal. Chert-pebble lag deposit common at top of formation. Virgelle Member (lower part of formation in parts of central Montana): very light gray, gray, or brown sandstone that coarsens upward. Root zone and thin carbonaceous beds at top of member. Delta, coastal plain, tidal flat, and marine shoreface and foreshore. Thickness as much as 150 m (492 ft). Telegraph Creek Formation (Ktc): Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft).

Upper Tertiary sediment or sedimentary rock (Tertiary) at surface, covers 0.6 % of this area

Conglomerate, tuffaceous sandstone and siltstone, marlstone, and equivalent sediment and ash beds. Local volcanic flows in southwestern Montana. Includes Sixmile Creek Formation. Dominantly fluvial, alluvial fan, mudflow, and palustrine. Composite thickness for Tsu and Tsm exceeds 4,880 m (16,010 ft).

Fort Union Formation (Tertiary and Cretaceous) at surface, covers 0.6 % of this area

Undivided Fort Union Formation - Sentinel Butte (Tfsb), Tongue River (Tftr), Linley (Tflc), Ludlow (Tfld), Lebo (Tfle), Ekalaka (Tfe), and Tullock (Tft) members.

Absaroka Volcanics Supergroup (Tertiary) at surface, covers 0.5 % of this area

Calc-alkalic andesite and dacite extrusive rock with lesser amounts of potassic, alkalic, and mafic lava; minor amounts of rhyodacitic ash-flow tuff associated with mafic lava; and dark gray, very fine-grained basalt or andesite intrusive breccia.

Middle Tertiary sediment or sedimentary rock (Tertiary) at surface, covers 0.5 % of this area

Tuffaceous siltstone, sandstone, bentonitic mudstone, conglomerate, limestone, and equivalent sediment and ash beds. Local volcanic flows in southwestern Montana. Includes Renova Formation. Dominantly fluvial, floodplain, palustrine, lacustrine, debris flow, and eolian. Composite thickness for Tsu and Tsm exceeds 4,880 m (16,010 ft). Includes Anaconda Formation: Light gray, reddish tan, or brownish red, coarse breccia that laterally grades to conglomerate, sandstone, and siltstone. Locally intertongues with Lowland Creek Volcanics. Derived from overlying unmetamorphosed plate of metamorphic core complex. Thickness as much as much as 200 m (656 ft).

Kootenai Formation (Cretaceous) at surface, covers 0.5 % of this area

Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft).

Niobrara Formation (Cretaceous) at surface, covers 0.5 % of this area

Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft).

Marias River Formation (Cretaceous) at surface, covers 0.5 % of this area

Marias River Formation (Kmr) - Kevin Member (Kmk): Dark gray shale with calcareous concretions and numerous bentonite beds. Upper part calcareous. MacGowan Concretionary Bed (middle part of member): grayish brown concretionary dolostone and limestone with phosphatic pellets, and gray to black chert pebbles. Marine. Thickness as much as 350 m (1,148 ft). Ferdig Member (Kmf): Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft). Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft). Floweree Member (Kmfl): Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft).

Mission Canyon Formation: locally includes Big Snowy Formation in south-central Montana (Mississippian) at surface, covers 0.4 % of this area

Gray, massive limestone with chert beds and nodules, and solution breccia zones. Shallow marine. Thickness as much as 520 m (1,706 ft). In south-central Montana - Big Snowy Formation: Olive gray, thick-bedded limestone, yellowish gray siltstone, dark gray shale, and yellowish brown, thin bedded siltstone and mudstone. Nearshore marine. Thickness 120 m (394 ft).

Lower Prichard Formation (Middle Proterozoic) at surface, covers 0.4 % of this area

Dark and light gray, pyrite- and pyrrhotite-rich siltite and argillite in planar couplets with subordinate gray, flat-laminated, fine-grained quartzite.

Three Forks through Maywood Formation: Three Forks, Jefferson, and Maywood Formations or Palliser and Alexo Formations, and Fairholme Group in northwestern Montana near Canadian border (Mississippian and Devonian) at surface, covers 0.4 % of this area

Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Maywood Formation: Thin-bedded dolomitic limestone and dolomite interbedded with calcareous shale. Shallow marine. Thickness as much as 120 m (394 ft). In northwestern Montana near Canadian border - Palliser Formation: Gray and greenish gray limestone and dolomitic limestone with locally abundant black chert. Pinkish gray, very thinly laminated, calcareous mudstone at base. Marine. Thickness as much as 300 m (984 ft). Alexo Formation: Pale yellowish orange, medium grained, calcareous siltstone with calcareous mudstone at the base. Shallow marine. Thickness as much as 595 m (1,952 ft). Fairholme Group: Brownish black, bioturbated, calcareous mudstone, gray algal limestone and dolomite, and pale yellowish orange, medium-grained, calcareous sandstone at the base. Thickness as much as 550 m(1,805 ft).

Telegraph Creek Formation (Cretaceous) at surface, covers 0.4 % of this area

Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft).

Carlile Formation (Cretaceous) at surface, covers 0.4 % of this area

Dark to light gray, noncalcareous, partly silty or sandy shale with upper zone of white-weathering, calcareous concretions, middle zone of sandy, orange-weathering, calcareous concretions, and basal zone of ferruginous concretions. Marine. Thickness as much as 195 m (640 ft).

Mafic volcanic rock (Tertiary) at surface, covers 0.4 % of this area

Mafic volcanic rock

Alluvial fan deposit (Quaternary) at surface, covers 0.3 % of this area

Variable deposits with fan-shaped morphology developed where slope gradient changes abruptly. Shown only where relatively extensive.

Virgelle and Telegraph Creek Formations (Cretaceous) at surface, covers 0.3 % of this area

Virgelle Formation (Kvi): Light gray, fine- to coarse-grained, locally titaniferous sandstone. Marine shoreface and foreshore. Thickness as much as 90 m (295ft). Telegraph Creek Formation (Ktc): Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft).

Mowry Formation (Cretaceous) at surface, covers 0.3 % of this area

Light gray to silvery gray, platy to blocky, siliceous shale and subordinate thin-bedded, gray siltstone or very fine-grained sandstone laminae or beds. Fish scales common in central Montana. Ledge-forming, chert-bearing, fine- to medium-grained sandstone at top throughout west-central Montana. Marine. Thickness 240 m (787 ft).

Belle Fourche Formation (Cretaceous) at surface, covers 0.3 % of this area

Gray to black shale with ironstone concretions and numerous bentonite beds. Mosby Sandstone Member (upper part of formation in north- and east-central Montana): brown sandstone locally with chert pebbles, interbedded with gray shale. Big Elk Sandstone Member (lower part of formation in southwest-central Montana): light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black clayey shale. Marine shelf. Thickness as much as 260 m (853 ft).

Gammon Formation (Cretaceous) at surface, covers 0.3 % of this area

Dark gray mudstone and shale with numerous red-weathering ferruginous concretions, and thin, fine-grained sandstone or siltstone beds interbedded with shale. Offshore marine. Thickness as much as 260 m (853).

Elkhorn Mountains Volcanics (Cretaceous) at surface, covers 0.3 % of this area

Light gray to dark gray, grayish red, greenish gray, and brown mostly andesitic to latitic welded tuff and tuff breccia, tuff, lapilli tuff, lava flows, flow breccia, and related hypabyssal intrusive rock. Subordinate quartz latitic welded tuff and basalt flows.

Greyson Formation (Middle Proterozoic) at surface, covers 0.3 % of this area

Greenish gray siltite and dark gray argillite with interbedded bands of light brown, sandy argillite and feldspathic quartzite. Thickness as much as 915 m (3,000 ft).

St. Mary River Formation (Cretaceous) at surface, covers 0.3 % of this area

Gray calcareous shale and siltstone with lenticular argillaceous sandstone and thin interbeds of black, nodular limestone. Lagoonal and fluvial. Thickness as much as 300 m (948 ft).

Lodgepole Formation (Mississippian) at surface, covers 0.3 % of this area

Woodhurst Member (upper part of formation): light gray, well-bedded limestone, typically with much dark chert, interbedded with thinner calcareous mudstone beds. Paine Member (middle part of formation):dark gray, thin-bedded, silty or fossiliferous limestone. Cottonwood Canyon Member (lower part of formation): black shale with basal conglomeratic lag deposit. Shallow marine. Thickness as much as 305 m (1000 ft).

Devils Glen through Flathead Formation: Devils Glen, Switchback, Steamboat, Pentagon, Pagoda, Dearborn, Damnation or Elko, Gordon, and Flathead Formations (Cambrian) at surface, covers 0.3 % of this area

Devils Glen Formation: Light gray dolomite with subordinate algal limestone and shale. Shallow marine. Thickness as much as 200 m (656ft). Switchback Formation: Green and gray, locally calcareous, fissile shale or mudstone, interbedded with thin beds of gray, flaggy limestone or dolomite. Shallow marine. Thickness as much as 95 m (312 ft). Steamboat Formation: Tan, thick-bedded, hard limestone with thin layers of green fissile shale and nodular shaly limestone. Shallow marine. Thickness as much as much as 90 m (295 ft). Pentagon Formation: Black fissile shale and brownish gray, platy limestone beds. Laterally replaced by Pagoda Formation. Shallow marine. Thickness as much as 88 m (289 ft). Pagoda Formation: Green shale interbedded with maroon calcareous mudstone and gray limestone, and lower fissile black shale and thin limestone-pebble conglomerate. Shallow marine. Thickness as much as 120 m (394 ft). Dearborn Formation: Upper brownish gray to gray limestone and lower interbedded green shale, shale-pebble conglomerate, and calcareous sandstone. Shallow marine shelf. Thickness as much as 115 m (377 ft). Damnation Formation: Brownish gray or bluish gray fossiliferous, locally oolitic limestone and subordinate dark gray shale. Shallow marine. Thickness as much as 70 m (230 ft). Elko Formation: Light brownish gray dolomite and subordinate mudstone. Thickness as much as 220 m (722 ft). Gordon Formation: Upper part of formation: dark green to dark brown, fissile shale with thin beds of sandstone and gray, glauconitic limestone. Lower part of formation: ferruginous sandstone, sandy shale, and grayish green shale. Shallow marine. Thickness as much as 90 m (295 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft).

Ferdig Member of Marias River Formation (Cretaceous) at surface, covers 0.3 % of this area

Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft).

Newland Formation (Middle Proterozoic) at surface, covers 0.3 % of this area

Dark bluish gray limestone. Thickness as much as2,000 m (6,562 ft).

Thermopolis and Fall River Formations (Cretaceous) at surface, covers 0.3 % of this area

Thermopolis Formation (Kt): Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft). Fall River Formation (Kfr): Gray and brown, fine- to medium-grained, quartzose sandstone with thin interbeds of dark gray shale. Nearshore marine. Thickness as much as 70 m (1,214 ft).

Pilgrim through Flathead Formation: Pilgrim, Park, Meagher, Wolsey, and Flathead Formations (Cambrian) at surface, covers 0.2 % of this area

Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft).

Andesite and basalt (Tertiary) at surface, covers 0.2 % of this area

Andesite and basalt

Frontier Formation (Cretaceous) at surface, covers 0.2 % of this area

Western Montana: gray, fine- to medium-grained, locally conglomeratic sandstone with interbeds of dark gray carbonaceous shale. Local conglomerate, porcellanite, and coal. Central Montana: dark gray, carbonaceous shale interbedded with siltstone and sandstone, chert-pebble conglomerate, and coal. Big Elk Sandstone Member: light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black, clayey shale. Boulder River Sandstone Member: gray to greenish gray, very fine-grained to conglomeratic, glauconitic sandstone. Brackish to nonmarine. Thickness exceeds 2,135 m (7,005 ft) in Lima Peaks region, and exceeds 915m in the Greenhorn, Snowcrest, Gravelly, and Pioneer Mountains of southwestern Montana. Elsewhere thickness as much as 215 m (705 ft).

Beaverhead Group (Tertiary and Cretaceous) at surface, covers 0.2 % of this area

Reddish gray conglomerate with limestone and quartzite clasts, gray limestone, and grayish brown sandstone. Syntectonic alluvial fan and braided stream. Thickness as much as 3,250 m (10,663 ft).

Missoula Group: Pilcher, Garnet Range, McNamara, Bonner, and Mount Shields Formations; and either Shepard and Snowslip Formations or Shepard Formation and unresolved Snowslip Formation equivalent (Middle Proterozoic) at surface, covers 0.2 % of this area

Upper Missoula Group (Ymu) - Pilcher Formation: Light gray and red cross-bedded, feldspathic, fine grained quartzite. Garnet Range Formation: Dark green, micaceous, fine-grained quartzite with subordinate argillite interbeds. Thickness as much as 1,200 m (3,937 ft). McNamara Formation: Dense green and red siltite and argillite in mudcracked couplets containing diagnostic chert beds and rip-up clasts. Thickness as much as 1,650 m (5,413 ft). Bonner Formation: Pink, cross-bedded, feldspathic, medium- to coarse grained quartzite. Thickness as much as 580 m (1,903 ft). Mount Shields Formation: Upper part: red quartzite, siltite, and argillite in mud-cracked couples and couplets with abundant salt casts. Lower part: light gray, flat-laminated, feldspathic, fine-grained quartzite. Thickness as much as 2,000 m (6,562 ft). Lower Missoula Group (Yml) - Shepard Formation: Tan-weathering, dolomitic, green siltite and argillite in couplets and microlaminae. In the west formerly considered part of the “upperWallace.” Thickness as much as 1,100 m (3,609 ft). Snowslip Formation: Green and red siltite and argillite in couplets. Western Snowslip equivalent is black siltite and argillite in couplets and microlaminae, formerly considered part of the “upper Wallace.” A new name is under consideration for the western Snowslip equivalent. Thickness as much as 1,200 m (3,937 ft).

Glacial lake deposit (Quaternary) at surface, covers 0.2 % of this area

Light brown laminated silt, fine-grained sand, and clay. Shown only in northwestern and central-western Montana.

Wasatch Formation (Tertiary) at surface, covers 0.2 % of this area

Southeastern Montana: orangish brown, arkosic sandstone, lenticular conglomerate and siltstone, dark gray carbonaceous shale, coal, and varicolored claystone. Bears Paw Mountains: variegated red, pink, lavender, light green, yellow, gray, and very light gray shale, bentonitic claystone, and siltstone; light gray, brown, and green cross-bedded sandstone; and lenses of boulder conglomerate. Syntectonic alluvial fan and alluvial plain. Thickness as much as 120 m (394 ft) in Montana; upper part removed by erosion.

Adel Mountain Volcanics (Cretaceous) at surface, covers 0.2 % of this area

Gray, light brownish gray, and dark gray trachyandesite and trachybasalt flows; dark red, purplish red, and dark grayish green volcanic breccia containing angular clasts of trachybasalt and trachyandesite in matrix of coarse tuff; and dark gray, dark grayish red, brownish gray, and dark grayish green porphyritic shonkinite intrusive rock with augite phenocrysts.

Two Medicine and Virgelle Formations (Cretaceous) at surface, covers 0.2 % of this area

Volcaniclastic facies: gray, greenish gray, maroon, and reddish gray volcaniclastic rocks, volcanic flows, and tuff. Fluvial and alluvial plain. Thickness as much as 660 m (2,165 ft).

Middle Kootenai Formation (Cretaceous) at surface, covers 0.2 % of this area

Sunburst Member (middle to lower part of the formation in northwest-central Montana) dominantly quartzose sandstone.

Sliderock Mountain volcanics (Cretaceous) at surface, covers 0.2 % of this area

Gray, pale purplish gray, and pale greenish gray andesite lahar breccia with interbeds of andesitic conglomerate and coarse-grained sandstone. Subordinate flows of porphyritic andesite and basaltic andesite containing plagioclase, hornblende, and pyroxene phenocrysts.

Kootenai Formation through Ellis Group: Kootenai, Morrison, and Ellis Group (Cretaceous and Jurassic) at surface, covers 0.2 % of this area

Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft). Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Ellis Group (Je) - Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft).

Lowland Creek Volcanics: locally includes interfingering Anaconda Conglomerate (Tertiary) at surface, covers 0.2 % of this area

Quartz latite flows, quartz latite welded tuff and tuff breccia, and basal conglomerate with tuffaceous debris that increases upward.

Belt Supergroup (Middle Proterozoic) at surface, covers 0.2 % of this area

Extremely large lumping of over 25+ formations. See individual unit descriptions. Belt Supergroup consists of: Missoula Group (Ym, Ymu, Yml), Piegan Group (Ypg, Yh), Ravalli Group (Yr, Ye, Ysp), and all the Lower Belt Formations (Ypu, Ypl, Yaw, Yla, Yg, Yn, Ynla, Ych, Yne).

Mount Pablo Formation through Ellis Group: Mount Pablo, Morrison, Swift, Rierdon, and Sawtooth Formations (Cretaceous and Jurassic) at surface, covers 0.2 % of this area

Mount Pablo Formation: Reddish gray and grayish green mudstone and dark gray shale interbedded with gray limestone and sandstone. Gray basal conglomerate or conglomeratic sandstone. Alluvial plain. Thickness as much as 30 m (98 ft). Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Ellis Group (Je) - Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft).

Volcanic rock: locally includes Challis Volcanics in southwestern Montana (Tertiary) at surface, covers 0.2 % of this area

Volcanic rock; In southwestern Montana - Challis Volcanics: Basaltic andesite, andesite, quartz latite, latite, rhyodacite, and rhyolite flows, and tuff, most of which is nonporphyritic and commonly spherulitic; glassy to devitrified. Subordinate water-laid tuff.

Amsden Formation (Pennsylvanian and Mississippian) at surface, covers 0.2 % of this area

Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft).

Thermopolis Formation (Cretaceous) at surface, covers 0.2 % of this area

Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft).

Metasedimentary rock: Variously assigned to the Swauger Formation, Yellowjacket Formation, Lemhi Group, and/or Belt Supergroup along southest Montana-Idaho border (Middle Proterozoic) at surface, covers 0.2 % of this area

Beaverhead Mountains, southwestern Montana - Light gray quartzite with subordinate argillite, siltite, and calc-silicate rocks; unresolved unit that has been variously assigned to Yellowjacket Formation, Lemhi Group, Swauger Formation, or Belt Supergroup. Thickness as much as 2,700 m (8,858 ft).

Blackleaf Formation (Cretaceous) at surface, covers 0.2 % of this area

Bootlegger Member (Kbb): Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft). Vaughn Member (Kbv): Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft). Taft Hill Member (Kbt): Black shale, gray bentonitic siltstone, and glauconitic sandstone. Marine. Thickness as much as 180 m (590 ft). Flood Member (Kblf): Upper part: medium gray, cliff-forming fine- to medium-grained chert-rich, quartzose sandstone, middle part medium to dark gray shale or gray siltstone; lower part: yellowish brown, gray, or olive gray fine-grained sandstone and siltstone with carbonaceous laminae and thin films of coal and dark gray silty, bentonitic shale. Sandstone beds thin eastward. Quartzose basal sandstone in southwestern Montana. Nearshore marine, tidal flat, and lagoon. Thickness as much as 230 m (755 ft).

Garnet Range Formation: locally includes upper Libby Formation and Pilcher Formation in northwestern Montana (Middle Proterozoic) at surface, covers 0.2 % of this area

Dark green, micaceous, fine-grained quartzite with subordinate argillite interbeds. Thickness as much as 1,200 m (3,937 ft). In northwestern Montana - Libby Formation: Light to dark gray and greenish gray siltite and argillite with subordinate quartzite in mud-cracked couplets. Thickness as much as 2,300 m (7,546 ft). Pilcher Formation: Light gray and red cross-bedded, feldspathic, fine grained quartzite. Garnet Range Formation: Dark green, micaceous, fine-grained quartzite with subordinate argillite interbeds. Thickness as much as 1,200 m (3,937 ft).

Morrison Formation and Ellis Group (Cretaceous and Jurassic) at surface, covers 0.2 % of this area

Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Ellis Group (Je) - Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft).

Huckleberry Ridge Tuff (Tertiary) at surface, covers 0.1 % of this area

Upper part: pinkish gray, gray, or brown welded or unwelded tuff that contains abundant phenocrysts of sanidine and quartz, and uncompacted pumice fragments at the top.

Flood Member of Blackleaf Formation (Cretaceous) at surface, covers 0.1 % of this area

Upper part: medium gray, cliff-forming fine- to medium-grained chert-rich, quartzose sandstone, middle part medium to dark gray shale or gray siltstone; lower part: yellowish brown, gray, or olive gray fine-grained sandstone and siltstone with carbonaceous laminae and thin films of coal and dark gray silty, bentonitic shale. Sandstone beds thin eastward. Quartzose basal sandstone in southwestern Montana. Nearshore marine, tidal flat, and lagoon. Thickness as much as 230 m (755 ft).

Bootlegger Member of Blackleaf Formation (Cretaceous) at surface, covers 0.1 % of this area

Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft).

Lower Kootenai Formation (Cretaceous) at surface, covers 0.1 % of this area

Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north.

Hell Creek and Fox Hills Formations (Cretaceous) at surface, covers 0.1 % of this area

Hell Creek Formation (Khc): Light gray, bentonitic claystone that alternates with gray to brown sandstone interbedded with carbonaceous shale. Laterally equivalent to Lance Formation. Fluvial and flood plain. Thickness as much as 335 m (1,100 ft). Fox Hills Formation (Kfh): Yellowish orange to gray, fine- to medium-grained, non-calcareous sandstone in upper part, and interbedded sandstone, siltstone, and black shale with calcareous concretion zone in lower part. Marginal marine. Thickness 30–45 m (98–148 ft).

Taft Hill Member of Blackleaf Formation (Cretaceous) at surface, covers 0.1 % of this area

Black shale, gray bentonitic siltstone, and glauconitic sandstone. Marine. Thickness as much as 180 m (590 ft).

Kishenehn Formation (Tertiary) at surface, covers 0.1 % of this area

Light to dark bluish gray, locally sandy clay with interbeds of light gray, nodular sandstone, conglomerate, and coal. Dominantly fluvial, palustrine, and lacustrine. Exposed thickness 230 m (755 ft).

Red Lion through Flathead Formation: Red Lion, Pilgrim, Park, Meagher, Wolsey, and Flathead Formations; or Red Lion, Hasmark, Silver Hill, and Flathead Formations (Cambrian) at surface, covers 0.1 % of this area

Red Lion Formation: Gray limestone with siliceous lamina and black calcareous shale interbedded with thin limestone. Shallow marine. Thickness as much as 115 m (377 ft). Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft). Hasmark Formation: Light gray or bluish gray limestone and dark-gray calcareous shale. Shallow marine. Thickness as much as 560 m (1,823 ft). Silver Hill Formation: Brown, green, and very light gray banded calcareous shale interbedded with laminated limestone. Shallow marine. Thickness as much as 120 m (394 ft).

Gneiss and amphibolite (Archean) at surface, covers 0.1 % of this area

Gneiss and amphibolite

Tensleep Formation (Pennsylvanian) at surface, covers 0.1 % of this area

Very light gray sandstone interbedded with subordinate carbonate, shale, and anhydrite. Cyclical marine, eolian, and sabkha. Thickness about 60 m (197 ft).

Willow Creek Formation (Tertiary and Cretaceous) at surface, covers 0.1 % of this area

Beaverhead Group. Reddish gray, olive gray, and purple mudstone, and gray, greenish gray, and yellow sandstone. Alluvial plain. Thickness as much as 330 m (1,083 ft).

Upper Kootenai Formation (Cretaceous) at surface, covers 0.1 % of this area

Greybull Member (top of the formation in south-central Montana) dominantly quartzose sandstone.

Horsethief Formation (Cretaceous) at surface, covers 0.1 % of this area

Gray to light brown, medium- to coarse-grained, locally titaniferous sandstone with argillaceous sandstone and mudstone near base. Brackish to littoral marine. Thickness as much as 60 m (197 ft).

Flathead Formation (Cambrian) at surface, covers 0.1 % of this area

Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft).

Spokane Formation (Middle Proterozoic) at surface, covers 0.1 % of this area

Red siltite and argillite in mudcracked couplets. Thickness as much as 1,500 m (492 ft).

Mowry through Fall River Formation: Mowry, Thermopolis, and Fall River Formations (Cretaceous) at surface, covers 0.1 % of this area

Mowry Formation (Km): Light gray to silvery gray, platy to blocky, siliceous shale and subordinate thin-bedded, gray siltstone or very fine-grained sandstone laminae or beds. Fish scales common in central Montana. Ledge-forming, chert-bearing, fine- to medium-grained sandstone at top. throughout west-central Montana. Marine. Thickness 240 m (787 ft). Thermopolis Formation (Kt): Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft). Fall River Formation (Kfr): Gray and brown, fine- to medium-grained, quartzose sandstone with thin interbeds of dark gray shale. Nearshore marine. Thickness as much as 70 m (1,214 ft).

Sentinel Butte Member of Fort Union Formation (Tertiary) at surface, covers 0.1 % of this area

Dark gray shale with interbedded lignite and gray sandstone. Nonmarine. Thickness 200 m (656 ft).

Bootlegger and Vaughn Members of Blackleaf Formation (Cretaceous) at surface, covers 0.1 % of this area

Bootlegger Member (Kbb): Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft). Vaughn Member (Kbv): Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft).

Marias River through Kootenai Formation: Marias River, Blackleaf, and Kootenai Formations (Cretaceous) at surface, covers 0.1 % of this area

Marias River Formation (Kmr) - Kevin Member (Kmk): Dark gray shale with calcareous concretions and numerous bentonite beds. Upper part calcareous. MacGowan Concretionary Bed (middle part of member): grayish brown concretionary dolostone and limestone with phosphatic pellets, and gray to black chert pebbles. Marine. Thickness as much as 350 m (1,148 ft). Ferdig Member (Kmf): Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft). Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft). Floweree Member (Kmfl): Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft). Blackleaf Formation (Kbl) - Bootlegger Member (Kbb): Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft). Vaughn Member (Kbv): Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft). Taft Hill Member (Kbt): Black shale, gray bentonitic siltstone, and glauconitic sandstone. Marine. Thickness as much as 180 m (590 ft). Flood Member (Kblf): Upper part: medium gray, cliff-forming fine- to medium-grained chert-rich, quartzose sandstone, middle part medium to dark gray shale or gray siltstone; lower part: yellowish brown, gray, or olive gray fine-grained sandstone and siltstone with carbonaceous laminae and thin films of coal and dark gray silty, bentonitic shale. Sandstone beds thin eastward. Quartzose basal sandstone in southwestern Montana. Nearshore marine, tidal flat, and lagoon. Thickness as much as 230 m (755 ft). Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft).

Schist and gneiss (Archean) at surface, covers 0.1 % of this area

Schist and gneiss

Rhyolite (Tertiary) at surface, covers 0.1 % of this area

Rhyolite

Otter Formation (Mississippian) at surface, covers 0.1 % of this area

Green, grayish green, gray, and subordinate reddish brown shale interbedded with light brown or gray limestone. Open and semirestricted marine. Thickness as much as 150 m (492 ft).

Trail City Member of Fox Hills Formation (Cretaceous) at surface, covers 0.1 % of this area

Yellowish orange, wavy-bedded siltstone and black shale with calcareous concretion zone. Thickness 10 m (33 ft).

Billman Creek Formation (Cretaceous) at surface, covers 0.1 % of this area

Livingston Group. Olive gray, grayish brown, and dusky red tuffaceous claystone interbedded with subordinate grayish yellow sandstone, conglomeratic sandstone, and siltstone beds. Fluvial and lacustrine. May be brackish in part. Thickness as much as 915 m (3,002 ft).

Ekalaka Member of Fort Union Formation (Tertiary) at surface, covers 0.1 % of this area

Yellowish orange or tan, fine- to medium-grained sandstone interbedded with mudstone and thin shale and coal beds. Estuarine. Thickness as much as 55 m (180 ft).

Heath Formation (Mississippian) at surface, covers 0.1 % of this area

Black, fissile, commonly petroliferous shale with subordinate sandstone, gypsum, and coal. Restricted marine to coastal plain. Thickness as much as 150 m (492 ft).

Alaska Bench Formation: locally includes Devils Pocket Formation in central Montana (Pennsylvanian) at surface, covers 0.1 % of this area

Gray and bluish gray, hard, fossiliferous limestone with subordinate interbeds of red mudstone. Marine. Thickness as much as 105 m (345 ft). In central Montana: Devils Pocket Formation: Red to gray siliceous dolomite, gray, finely crystalline, thin-bedded limestone, and red, gray, or pink, fine- to medium-grained quartz sandstone. May include massive breccia of coarse, cherty dolomite, limestone, red sandstone, shale, and chert. Intertidal to open marine. Thickness 43 m (140ft) at type section.

Pilgrim through Wolsey Formation: Pilgrim, Park, Meagher, and Wolsey Formations (Cambrian) at surface, covers < 0.1 % of this area

Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft).

Sedan Formation (Cretaceous) at surface, covers < 0.1 % of this area

Varicolored welded tuff and dark brown conglomerate interbedded with sandstone, conglomeratic sandstone, and dark gray or olive gray mudstone. Nonmarine including mudflow, and marginal marine. Thickness915–1470 m (3,002–4,823 ft).

Floweree Member of Marias River Formation (Cretaceous) at surface, covers < 0.1 % of this area

Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft).

Felsic volcanic rock (Tertiary) at surface, covers < 0.1 % of this area

Felsic volcanic rock

Latite (Tertiary) at surface, covers < 0.1 % of this area

Latite

Frontier and Blackleaf Formations (Cretaceous) at surface, covers < 0.1 % of this area

Frontier Formation (Kf): Western Montana: gray, fine- to medium-grained, locally conglomeratic sandstone with interbeds of dark gray carbonaceous shale. Local conglomerate, porcellanite, and coal. Central Montana: dark gray, carbonaceous shale interbedded with siltstone and sandstone, chert-pebble conglomerate, and coal. Big Elk Sandstone Member: light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black, clayey shale. Boulder River Sandstone Member: gray to greenish gray, very fine-grained to conglomeratic, glauconitic sandstone. Brackish to nonmarine. Thickness exceeds 2,135 m (7,005 ft) in Lima Peaks region, and exceeds 915m in the Greenhorn, Snowcrest, Gravelly, and Pioneer Mountains of southwestern Montana. Elsewhere thickness as much as 215 m (705 ft). Blackleaf Formation (Kbl) - Bootlegger Member (Kbb): Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft). Vaughn Member (Kbv): Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft). Taft Hill Member (Kbt): Black shale, gray bentonitic siltstone, and glauconitic sandstone. Marine. Thickness as much as 180 m (590 ft). Flood Member (Kblf): Upper part: medium gray, cliff-forming fine- to medium-grained chert-rich, quartzose sandstone, middle part medium to dark gray shale or gray siltstone; lower part: yellowish brown, gray, or olive gray fine-grained sandstone and siltstone with carbonaceous laminae and thin films of coal and dark gray silty, bentonitic shale. Sandstone beds thin eastward. Quartzose basal sandstone in southwestern Montana. Nearshore marine, tidal flat, and lagoon. Thickness as much as 230 m (755 ft).

Chugwater Formation (Triassic) at surface, covers < 0.1 % of this area

Red siltstone, sandstone, and shale with subordinate gypsum and local limestone bed at top. Shallow restricted marine and nonmarine. Thickness as much as 180 m (590 ft).

Dacite (Tertiary) at surface, covers < 0.1 % of this area

Dacite

Quartzofeldspathic gneiss (Early Proterozoic) at surface, covers < 0.1 % of this area

Quartzofeldspathic gneiss

Snowy Range through Flathead Formation: Snowy Range, Pilgrim, Park, Meagher, Wolsey, and Flathead Formations; or Snowy Range, Hasmark, Silver Hill, and Flathead Formations (Cambrian) at surface, covers < 0.1 % of this area

Snowy Range Formation (CAsr): Sage Member (upper part of formation): gray limestone and grayish green shale interbedded with grayish green flat-pebble conglomerate. Dry Creek Member (lower part of formation): greenish gray shale and grayish orange calcareous sandstone. Shallow marine. Thickness as much as 85 m (280 ft). Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft). Hasmark Formation: Light gray or bluish gray limestone and dark-gray calcareous shale. Shallow marine. Thickness as much as 560 m (1,823 ft). Silver Hill Formation: Brown, green, and very light gray banded calcareous shale interbedded with laminated limestone. Shallow marine. Thickness as much as 120 m (394 ft).

Phosphoria through Amsden Formation: Phosphoria, Quadrant, and Amsden Formations (Permian through Mississippian) at surface, covers < 0.1 % of this area

Phosphoria Formation (Pp): Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft).

Snowcrest Range and Madison Groups; or Snowcrest Range and Tendoy Groups; or Surret Canyon through McGowan Creek Formation: Surrett Canyon, South Creek, Scott Peak, Middle Canyon, and McGowan Creek Formations (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Surrett Canyon Formation: Dark gray, massive limestone with much scattered white bioclastic debris. Marine. Thickness at type section 67 m (220 ft). South Creek Formation: Dark gray, thin-bedded, chert bearing limestone that alternates with dark gray, argillaceous limestone. Marine. Thickness at type section 91 m (300 ft). Scott Peak Formation (Msp): Dark gray, chert-bearing, fine grained limestone interbedded with subordinate light brown, calcareous, quartzose siltstone to fine sandstone. Marine. Thickness at type section 685 m (2,247 ft). Middle Canyon Formation (Mmd): Dark gray, silty limestone with chert beds and nodules. Shallow marine. Thickness at type section 335 m (1,100 ft). McGowan Creek Formation: Upper part: dark gray and pale yellowish brown, thin-bedded, calcareous siltstone interbedded with dark gray, silty limestone. Lower part: dark gray, carbonaceous, thin-bedded argillite with interbeds of dark gray siltite, medium gray, fine-grained, conglomeratic sandstone and quartzite, and dark gray, silty limestone. Marine. Thickness as much as 61 m (200 ft).

Phosphoria and Quadrant Formations (Permian and Pennsylvanian) at surface, covers < 0.1 % of this area

Phosphoria Formation (Pp): Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana.

Three Forks and Jefferson Formations (Mississippian and Devonian) at surface, covers < 0.1 % of this area

Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft).

Cone Member of Marias River Formation (Cretaceous) at surface, covers < 0.1 % of this area

Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft).

Timber Lake Member of Fox Hills Formation (Cretaceous) at surface, covers < 0.1 % of this area

Yellowish orange to gray, fine- to medium grained, noncalcareous, hummocky-bedded sandstone. Thickness 15–22 m (50–72 ft).

Lacustrine deposit (Quaternary) at surface, covers < 0.1 % of this area

Light brown to brown, well-sorted, unconsolidated, laminated sand, silt, and clay. Lima Reservoir and Red Rocks Lake area, southwestern Montana.

Morrison Formation (Jurassic) at surface, covers < 0.1 % of this area

Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft).

Schist (Archean) at surface, covers < 0.1 % of this area

Schist

Cody and Frontier Formations (Cretaceous) at surface, covers < 0.1 % of this area

Cody Formation (Kco): Gray to dark gray shale with some sandstone beds. Eldridge Creek Member: greenish gray, glauconitic, fine-grained sandstone. Marine. Thickness as much as 465 m (1,526 ft). Frontier Formation (Kf): Western Montana: gray, fine- to medium-grained, locally conglomeratic sandstone with interbeds of dark gray carbonaceous shale. Local conglomerate, porcellanite, and coal. Central Montana: dark gray, carbonaceous shale interbedded with siltstone and sandstone, chert-pebble conglomerate, and coal. Big Elk Sandstone Member: light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black, clayey shale. Boulder River Sandstone Member: gray to greenish gray, very fine-grained to conglomeratic, glauconitic sandstone. Brackish to nonmarine. Thickness exceeds 2,135 m (7,005 ft) in Lima Peaks region, and exceeds 915m in the Greenhorn, Snowcrest, Gravelly, and Pioneer Mountains of southwestern Montana. Elsewhere thickness as much as 215 m (705 ft).

Telegraph Creek through Carlile Formation: Telegraph Creek, Niobrara, Carlile Formations (Cretaceous) at surface, covers < 0.1 % of this area

Telegraph Creek (Ktc): Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft). Niobrara Formation (Kn): Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft). Carlile Formation(Kca): Dark to light gray, noncalcareous, partly silty or sandy shale with upper zone of white-weathering, calcareous concretions, middle zone of sandy, orange-weathering, calcareous concretions, and basal zone of ferruginous concretions. Marine. Thickness as much as 195 m (640 ft).

Timber Lake and Trail City Members of Fox Hills Formation (Cretaceous) at surface, covers < 0.1 % of this area

Timber Lake Member (Kftt): Yellowish orange to gray, fine- to medium grained, noncalcareous, hummocky-bedded sandstone. Thickness 15–22 m (50–72 ft). Trail City Member (Kftc): Yellowish orange, wavy-bedded siltstone and black shale with calcareous concretion zone. Thickness 10 m (33 ft).

Kootenai through Dinwoody Formation: Kootenai, Morrison, Thaynes, Woodside, and Dinwoody Formations (Cretaceous through Triassic) at surface, covers < 0.1 % of this area

Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft). Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Thaynes Formation: Gray and tan limestone with interbedded siltstone, shale, and tan sandstone. Locally with red shale tongues. Marine. Thickness as much as 290 m (950 ft). Woodside Formation: Maroon and red siltstone, dolomite, and shale. Tidal flat. Thickness as much as 250 m (820 ft). Dinwoody Formation (TRd): Interbedded green siltstone, shale, sandstone, and carbonate that grades eastward into red shale, siltstone, and anhydrite. Nearshore and restricted marine. Thickness as much as 330 m (1,083 ft).

Quartzofeldspathic gneiss (Lower Proterozoic or Archean) at surface, covers < 0.1 % of this area

Quartzofeldspathic gneiss

Greenhorn Formation (Cretaceous) at surface, covers < 0.1 % of this area

Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft).

Cody Formation (Cretaceous) at surface, covers < 0.1 % of this area

Gray to dark gray shale with some sandstone beds. Eldridge Creek Member: greenish gray, glauconitic, fine-grained sandstone. Marine. Thickness as much as 465 m (1,526 ft).

Ferdig and Cone Members of Marias River Formation (Cretaceous) at surface, covers < 0.1 % of this area

Ferdig Member (Kmf): Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft). Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft).

Morrison Formation and Ellis Group (Jurassic) at surface, covers < 0.1 % of this area

Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Ellis Group (Je) - Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft).

Telegraph Creek and Marias River Formations (Cretaceous) at surface, covers < 0.1 % of this area

Telegraph Creek (Ktc): Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft). Marias River Formation (Kmr) - Kevin Member (Kmk): Dark gray shale with calcareous concretions and numerous bentonite beds. Upper part calcareous. MacGowan Concretionary Bed (middle part of member): grayish brown concretionary dolostone and limestone with phosphatic pellets, and gray to black chert pebbles. Marine. Thickness as much as 350 m (1,148 ft). Ferdig Member (Kmf): Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft). Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft). Floweree Member (Kmfl): Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft).

Judith River through Eagle Formation: Judith River, Claggett, and Eagle Formations (Cretaceous) at surface, covers < 0.1 % of this area

Judith River Formation (Kjr): Light brown to light gray, fine- to coarse-grained sandstone with interbeds of gray to black carbonaceous shale, silty shale, and thin coal. Local Parkman Sandstone Member (lower part of formation):yellowish gray to brownish gray and olive green, fine- to medium-grained, cross-bedded sandstone interbedded with yellowish gray, silty shale. Estuarine, brackish, and nearshore marine. Thickness as much as 305 m (1000 ft). Claggett Formation (Kcl): Dark gray to gray shale that weathers brown, with thin, gray sandstone laminae and beds in upper or middle part and calcareous concretions in lower part. Marine. Thickness as much as 170 m (558 ft). Eagle Formation (Ke): Gray or grayish brown, thick-bedded sandstone, sandy shale, and thin interbedded coal. Chert-pebble lag deposit common at top of formation. Virgelle Member (lower part of formation in parts of central Montana): very light gray, gray, or brown sandstone that coarsens upward. Root zone and thin carbonaceous beds at top of member. Delta, coastal plain, tidal flat, and marine shoreface and foreshore. Thickness as much as 150 m (492 ft).

Muddy and Thermopolis Formations (Cretaceous) at surface, covers < 0.1 % of this area

Muddy Formation (Kmd): Gray to brown fine- to medium-grained sandstone with interbedded dark gray locally carbonaceous shale. Deltaic, shoreface, and offshore marine. Thickness as much as 50 m (164 ft). Thermopolis Formation (Kt): Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft).

Blackleaf Formation through Ellis Group: Blackleaf, Kootenai, Mount Pablo, Morrison, Swift, Rierdon, and Sawtooth Formations (Cretaceous and Jurassic) at surface, covers < 0.1 % of this area

Blackleaf Formation (Kbl) - Bootlegger Member (Kbb): Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft). Vaughn Member (Kbv): Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft). Taft Hill Member (Kbt): Black shale, gray bentonitic siltstone, and glauconitic sandstone. Marine. Thickness as much as 180 m (590 ft). Flood Member (Kblf): Upper part: medium gray, cliff-forming fine- to medium-grained chert-rich, quartzose sandstone, middle part medium to dark gray shale or gray siltstone; lower part: yellowish brown, gray, or olive gray fine-grained sandstone and siltstone with carbonaceous laminae and thin films of coal and dark gray silty, bentonitic shale. Sandstone beds thin eastward. Quartzose basal sandstone in southwestern Montana. Nearshore marine, tidal flat, and lagoon. Thickness as much as 230 m (755 ft). Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft). Mount Pablo Formation: Reddish gray and grayish green mudstone and dark gray shale interbedded with gray limestone and sandstone. Gray basal conglomerate or conglomeratic sandstone. Alluvial plain. Thickness as much as 30 m (98 ft). Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Ellis Group (Je) - Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft).

Granitic rock (Tertiary or Cretaceous) at surface, covers < 0.1 % of this area

Granitic rock

Swift Formation (Jurassic) at surface, covers < 0.1 % of this area

Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft).

LaHood Formation: locally includes Table Mountain and Moose Formations in Highland Mountains (Middle Proterozoic) at surface, covers < 0.1 % of this area

LaHood Formation: Dark gray, dark brown, reddish brown, coarse, arkosic conglomerate locally with very large clasts derived from crystalline metamorphic rocks. Grades from mega-clast conglomerate to shale or argillite over short distances. Thickness as much as 3,000 m (9,842 ft). In Highland Mountains - Table Mountain Formation: Very light gray quartzite and argillaceous quartzite. Thickness as much as 530 m (1,740 ft). Moose Formation: Silty argillite and siltite. Thickness as much as 90 m (295 ft).

Quadrant and Amsden Formations (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft).

Mafic phonolite (Tertiary) at surface, covers < 0.1 % of this area

Mafic phonolite

Stillwater complex (Archean) at surface, covers < 0.1 % of this area

Layered ultramafic and mafic rocks from peridotite and pyroxenite at the base (ultramafic series) to gabbro and anorthosite in the upper part (banded series). Maximum exposed thickness in the Beartooth Mountains is about 6,700 m (22,000 ft).

Colgate Member of Fox Hills Formation (Cretaceous) at surface, covers < 0.1 % of this area

White to yellowish, fine- to medium-grained, porous sandstone. Brackish to marine shoreline. Only present near Glendive (eastern Montana) and in several other isolated areas. Thickness 0–40 m (130 ft).

Tyler Formation (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Cameron Creek Member (upper part of formation):red, purple, and brown mudstone and siltstone, and quartzose sandstone with subordinate gray shale and limestone. Bear Gulch Member (middle part of formation, locally): gray, fossiliferous, platy, argillaceous limestone. Stonehouse Canyon Member (lower part of formation): dark gray, carbonaceous mudstone with lenses of sandstone or conglomeratic sandstone. Fluvial and intertidal. Thickness as much as 180 m (590 ft).

Syenite (Tertiary) at surface, covers < 0.1 % of this area

Syenite

Virgelle Formation (Cretaceous) at surface, covers < 0.1 % of this area

Light gray, fine- to coarse-grained, locally titaniferous sandstone. Marine shoreface and foreshore. Thickness as much as 90 m (295 ft).

Fall River Formation (Cretaceous) at surface, covers < 0.1 % of this area

Gray and brown, fine- to medium-grained, quartzose sandstone with thin interbeds of dark gray shale. Nearshore marine. Thickness as much as 70 m (1,214 ft).

Plateau Rhyolite (Quaternary) at surface, covers < 0.1 % of this area

Central Plateau Member: Upper part: rhyolitic flows with abundant phenocrysts dominantly of quartz and sanidine, and lacking plagioclase. Lower part: light gray, dense, fine-grained to aphanitic rhyolitic ash-flow tuff, with angular to rounded phenocrysts of quartz, sanidine, pyroxene, and olivine that make up as much as 25 percent of rock volume.

Kibbey Formation (Mississippian) at surface, covers < 0.1 % of this area

Red, quartzose sandstone, siltstone, and shale, locally with subordinate thin gypsum beds and limestone bed in middle part. Intertidal and subtidal. Thickness as much as 105 m (345 ft).

Snowcrest Range Group: Conover Ranch, Lombard, and Kibbey Formations (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Conover Ranch Formation: Red siltstone and mudstone with minor silty limestone and sandstone beds. Thickness 35 m (115 ft). Lombard Formation: Gray, silty limestone with minor black shale. Thickness 120 m (394 ft). Kibbey Formation: Yellow siltstone, sandstone, and mudstone with minor silt limestone in lower part. Thickness 80 m (262 ft).

Carter Creek through Coberly Formation: Carter Creek, Jens, and Coberly Formations (Cretaceous) at surface, covers < 0.1 % of this area

Carter Creek Formation: Varicolored sandstone, siltstone, and silty mudstone, with subordinate dark shale and lenses of coarse-grained conglomeratic dacitic tuff. Nearshore marine, brackish, and terrestrial. Thickness as much as 2,000 m (6,562 ft). Jens Formation: Dark gray shale, siltstone, siliceous mudstone, volcaniclastic siltstone, and minor fine-grained calcareous sandstone. Marine. Thickness as much as 505 m (1,657 ft). Coberly Formation: Gray limestone and tan, massive sandstone. Intertidal and shallow marine. Thickness as much as 215 m (705 ft).

Quadrant Formation (Pennsylvanian) at surface, covers < 0.1 % of this area

Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m(2,625 ft) in southwestern-most Montana.

Ellis Group: Swift, Rierdon, and Piper or Sawtooth Formations; not all formations present at all locations (Jurassic) at surface, covers < 0.1 % of this area

Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft).

Mylonite (Early Proterozoic) at surface, covers < 0.1 % of this area

Mylonite

Telegraph Creek through Fall River Formation: Telegraph Creek, Niobrara, Carlile, Greenhorn, Belle Fourche, Mowry, Thermopolis, and Fall River Formations (Cretaceous) at surface, covers < 0.1 % of this area

Telegraph Creek (Ktc): Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft). Niobrara Formation (Kn): Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft). Carlile Formation (Kca): Dark to light gray, noncalcareous, partly silty or sandy shale with upper zone of white-weathering, calcareous concretions, middle zone of sandy, orange-weathering, calcareous concretions, and basal zone of ferruginous concretions. Marine. Thickness as much as 195 m (640 ft). Greenhorn Formation (Kgr): Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft). Belle Fourche Formation (Kbf): Gray to black shale with ironstone concretions and numerous bentonite beds. Mosby Sandstone Member (upper part of formation in north- and east-central Montana): brown sandstone locally with chert pebbles, interbedded with gray shale. Big Elk Sandstone Member (lower part of formation in southwest-central Montana): light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black clayey shale. Marine shelf. Thickness as much as 260 m (853 ft). Mowry Formation (Km): Light gray to silvery gray, platy to blocky, siliceous shale and subordinate thin-bedded, gray siltstone or very fine-grained sandstone laminae or beds. Fish scales common in central Montana. Ledge-forming, chert-bearing, fine- to medium-grained sandstone at top throughout west-central Montana. Marine. Thickness 240 m (787 ft). Thermopolis Formation (Kt): Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft). Fall River Formation (Kfr): Gray and brown, fine- to medium-grained, quartzose sandstone with thin interbeds of dark gray shale. Nearshore marine. Thickness as much as 70 m (1,214 ft).

Greenhorn and Belle Fourche Formations (Cretaceous) at surface, covers < 0.1 % of this area

Greenhorn Formation (Kgr): Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft). Belle Fourche Formation (Kbf): Gray to black shale with ironstone concretions and numerous bentonite beds. Mosby Sandstone Member (upper part of formation in north- and east-central Montana): brown sandstone locally with chert pebbles, interbedded with gray shale. Big Elk Sandstone Member (lower part of formation in southwest-central Montana): light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black clayey shale. Marine shelf. Thickness as much as 260 m (853 ft).

Miner Creek and Cokedale Formations (Cretaceous) at surface, covers < 0.1 % of this area

Livingston Group. Miner Creek: Alternating beds of olive gray tuffaceous siltstone and greenish gray volcaniclastic sandstone, silicified tuff, and bentonite. Sulphur Flats Member (lower part of formation): massive, cross-bedded, poorly sorted sandstone. Inland coastal plain. Thickness as much as 410 m (1,345 ft). Cokedale Formation: Olive gray, tuffaceous siltstone, dusky yellowish green and light gray, volcaniclastic-rich sandstone, grayish brown mudstone, water-laid tuff, bentonite, and carbonaceous claystone; coal in lower part. Transitional inland coastal plain to brackish. Thickness as much as 790 m (2,592 ft).

Mafic intrusive rock (Middle Proterozoic) at surface, covers < 0.1 % of this area

Mafic intrusive rock

Arikaree Formation (Tertiary) at surface, covers < 0.1 % of this area

Greenish gray, fine-grained sandstone with interbedded light gray volcanic ash. Nonmarine to possibly marine-influenced. Thickness as much as 80 m (262 ft). Incudes White River Group - Brule Formation: Pink, sandy, locally bentonitic claystone with siliceous concretions. Nonmarine. Preserved in local areas below Arikaree Formation. Thickness as much as 10 m (33 ft). Chadron Formation: Very light gray, fine-grained sandstone. Nonmarine. Preserved locally with Brule Formation. Thickness as much as 30 m (98 ft).

Basalt: includes basalt of several ages in southwestern Montana (Tertiary) at surface, covers < 0.1 % of this area

Basalt

Three Forks through Bighorn Formation: Three Forks, Jefferson, and Bighorn Formations (Mississippian through Ordovician) at surface, covers < 0.1 % of this area

Three Forks Formation (MDt): Sappington Member (upper part of formation): yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Bighorn Formation (Ob): Yellowish gray to very pale orange crystalline dolomite with local basal sandstone. Marine shelf. Thickness as much as 120 m (394 ft).

Amsden Formation and Big Snowy Group (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft). Big Snowy Group (Mbs) - Heath Formation (Mh): Black, fissile, commonly petroliferous shale with subordinate sandstone, gypsum, and coal. Restricted marine to coastal plain. Thickness as much as 150 m (492 ft). Otter Formation (Mo): Green, grayish green, gray, and subordinate reddish brown shale interbedded with light brown or gray limestone. Open and semirestricted marine. Thickness as much as 150 m (492 ft). Kibbey Formation (Mk): Red, quartzose sandstone, siltstone, and shale, locally with subordinate thin gypsum beds and limestone bed in middle part. Intertidal and subtidal. Thickness as much as 105 m (345 ft).

Middle Greyson Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Greenish gray siltite and dark gray argillite with interbedded bands of light brown, sandy argillite and feldspathic quartzite. Thickness as much as 915 m (3,000 ft).

Shedhorn through Amsden Formation: Shedhorn, Quadrant, and Amsden Formations (Permian through Mississippian) at surface, covers < 0.1 % of this area

Shedhorn Formation (Psh): Orangish brown, very fine- to medium-grained, well-sorted, quartzose sandstone. Nearshore marine shelf. Thickness as much as 70 m (230 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft).

Everts and Virgelle Formations (Cretaceous) at surface, covers < 0.1 % of this area

Everts Formation (Kevt): Light-colored, fine- to medium-grained, lenticular sandstone and medium to light gray or locally greenish, yellowish, or brownish gray mudstone. Shallow marine and brackish. Thickness as much as 425 m (1,395 ft). Virgelle Formation (Kvi): Light gray, fine- to coarse-grained, locally titaniferous sandstone. Marine shoreface and foreshore. Thickness as much as 90 m (295ft).

Mowry and Thermopolis Formations (Cretaceous) at surface, covers < 0.1 % of this area

Mowry Formation (Km): Light gray to silvery gray, platy to blocky, siliceous shale and subordinate thin-bedded, gray siltstone or very fine-grained sandstone laminae or beds. Fish scales common in central Montana. Ledge-forming, chert-bearing, fine- to medium-grained sandstone at top. throughout west-central Montana. Marine. Thickness 240 m (787 ft). Thermopolis Formation (Kt): Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft).

Tyler Formation (Mississippian) at surface, covers < 0.1 % of this area

Cameron Creek Member (upper part of formation):red, purple, and brown mudstone and siltstone, and quartzose sandstone with subordinate gray shale and limestone. Bear Gulch Member (middle part of formation, locally): gray, fossiliferous, platy, argillaceous limestone. Stonehouse Canyon Member (lower part of formation): dark gray, carbonaceous mudstone with lenses of sandstone or conglomeratic sandstone. Fluvial and intertidal. Thickness as much as 180 m (590 ft).

Morrison through Dinwoody Formation: Morrison, Swift, Rierdon, Sawtooth, Thaynes, Woodside, and Dinwoody Formations (Jurassic and Triassic) at surface, covers < 0.1 % of this area

Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Thaynes Formation: Gray and tan limestone with interbedded siltstone, shale, and tan sandstone. Locally with red shale tongues. Marine. Thickness as much as 290 m (950 ft). Woodside Formation: Maroon and red siltstone, dolomite, and shale. Tidal flat. Thickness as much as 250 m (820 ft). Dinwoody Formation (TRd): Interbedded green siltstone, shale, sandstone, and carbonate that grades eastward into red shale, siltstone, and anhydrite. Nearshore and restricted marine. Thickness as much as 330 m (1,083 ft).

Hoppers Formation (Cretaceous) at surface, covers < 0.1 % of this area

Livingston Group. Olive gray and yellowish gray, volcaniclastic sandstone, conglomeratic sandstone, and subordinate olive gray mudstone. Channel fill and floodplain. Thickness as much as 765 m (2,510 ft).

Fort Union Formation (Tertiary) at surface, covers < 0.1 % of this area

No unique unit description on map. Assumed to include undivided Tertiary age members of the Fort Union Formation: Sentinel Butte (Tfsb), Tongue River (Tftr), Linley (Tflc), Ludlow (Tfld), Lebo (Tfle), Ekalaka (Tfe), and Tullock (Tft) members.

Shonkinite and syenite (Tertiary) at surface, covers < 0.1 % of this area

Shonkinite and syenite

Amphibolite (Middle Proterozoic) at surface, covers < 0.1 % of this area

Amphibolite

Lower Greyson Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Greenish gray siltite and dark gray argillite with interbedded bands of light brown, sandy argillite and feldspathic quartzite. Thickness as much as 915 m (3,000 ft).

Big Snowy Group: Heath, Otter, and Kibbey Formations; not all formations present at all locations (Mississippian) at surface, covers < 0.1 % of this area

Heath Formation (Mh): Black, fissile, commonly petroliferous shale with subordinate sandstone, gypsum, and coal. Restricted marine to coastal plain. Thickness as much as 150 m (492 ft). Otter Formation (Mo): Green, grayish green, gray, and subordinate reddish brown shale interbedded with light brown or gray limestone. Open and semirestricted marine. Thickness as much as 150 m (492 ft). Kibbey Formation (Mk): Red, quartzose sandstone, siltstone, and shale, locally with subordinate thin gypsum beds and limestone bed in middle part. Intertidal and subtidal. Thickness as much as 105 m (345 ft).

Mafic intrusive rock (Late or middle Proterozoic) at surface, covers < 0.1 % of this area

Mafic intrusive rock

Marble (Lower Proterozoic or Archean) at surface, covers < 0.1 % of this area

Marble

Scott Peak Formation (Mississippian) at surface, covers < 0.1 % of this area

Dark gray, chert-bearing, fine grained limestone interbedded with subordinate light brown, calcareous, quartzose siltstone to fine sandstone. Marine. Thickness at type section 685 m (2,247 ft).

Telegraph Creek through Belle Fourche Formation: Telegraph Creek, Niobrara, Carlile, Greenhorn, and Belle Fourche Formations (Cretaceous) at surface, covers < 0.1 % of this area

Telegraph Creek (Ktc): Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft). Niobrara Formation (Kn): Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft). Carlile Formation (Kca): Dark to light gray, noncalcareous, partly silty or sandy shale with upper zone of white-weathering, calcareous concretions, middle zone of sandy, orange-weathering, calcareous concretions, and basal zone of ferruginous concretions. Marine. Thickness as much as 195 m (640 ft). Greenhorn Formation (Kgr): Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft). Belle Fourche Formation (Kbf): Gray to black shale with ironstone concretions and numerous bentonite beds. Mosby Sandstone Member (upper part of formation in north- and east-central Montana): brown sandstone locally with chert pebbles, interbedded with gray shale. Big Elk Sandstone Member (lower part of formation in southwest-central Montana): light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black clayey shale. Marine shelf. Thickness as much as 260 m (853 ft).

Chugwater and Goose Egg Formations (Triassic and Permian) at surface, covers < 0.1 % of this area

Chugwater Formation (TRc): Red siltstone, sandstone, and shale with subordinate gypsum and local limestone bed at top. Shallow restricted marine and nonmarine. Thickness as much as 180 m (590 ft). Goose Egg Formation: Red shale and sandstone with some interbeds of anhydrite, gypsum, and limestone. Nearshore marine, marine, and restricted marine. Thickness 30 m (98 ft).

Phosphoria Formation through Big Snowy Group: Phosphoria, Quadrant, and Amsden Formations, and Big Snowy Group (Permian through Mississippian) at surface, covers < 0.1 % of this area

Phosphoria Formation (Pp): Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft). Big Snowy Group (Mbs) - Heath Formation (Mh): Black, fissile, commonly petroliferous shale with subordinate sandstone, gypsum, and coal. Restricted marine to coastal plain. Thickness as much as 150 m (492 ft). Otter Formation (Mo): Green, grayish green, gray, and subordinate reddish brown shale interbedded with light brown or gray limestone. Open and semirestricted marine. Thickness as much as 150 m (492 ft). Kibbey Formation (Mk): Red, quartzose sandstone, siltstone, and shale, locally with subordinate thin gypsum beds and limestone bed in middle part. Intertidal and subtidal. Thickness as much as 105 m (345 ft).

Empire Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Grayish green and pale olive gray argillite and siltite with subordinate thin beds of quartzite and sandy limestone. Thickness as much as 610 m.

Pilgrim through Meagher Formation: Pilgrim, Park, and Meagher Formations (Cambrian) at surface, covers < 0.1 % of this area

Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft).

Phosphoria Formation through Madison Group: Phosphoria, Quadrant, Amsden, Mission Canyon, and Lodgepole Formations (Permian through Mississippian) at surface, covers < 0.1 % of this area

Phosphoria Formation (Pp): Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m(2,625 ft) in southwestern-most Montana. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft). Mission Canyon Formation (Mmc): Gray, massive limestone with chert beds and nodules, and solution breccia zones. Shallow marine. Thickness as much as 520 m (1,706 ft). Lodgepole Formation (Ml): Woodhurst Member (upper part of formation): light gray, well-bedded limestone, typically with much dark chert, interbedded with thinner calcareous mudstone beds. Paine Member (middle part of formation):dark gray, thin-bedded, silty or fossiliferous limestone. Cottonwood Canyon Member (lower part of formation): black shale with basal conglomeratic lag deposit. Shallow marine. Thickness as much as 305 m (1000 ft).

Woodside and Dinwoody Formations (Triassic) at surface, covers < 0.1 % of this area

Woodside Formation: Maroon and red siltstone, dolomite, and shale. Tidal flat. Thickness as much as 250 m (820 ft). Dinwoody Formation (TRd): Interbedded green siltstone, shale, sandstone, and carbonate that grades eastward into red shale, siltstone, and anhydrite. Nearshore and restricted marine. Thickness as much as 330 m (1,083 ft).

Alaska Bench Formation (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Gray and bluish gray, hard, fossiliferous limestone with subordinate interbeds of red mudstone. Marine. Thickness as much as 105 m (345 ft).

Piper Formation (Jurassic) at surface, covers < 0.1 % of this area

Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft).

Upper Greyson Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Greenish gray siltite and dark gray argillite with interbedded bands of light brown, sandy argillite and feldspathic quartzite. Thickness as much as 915 m (3,000 ft).

Rierdon and Piper Formations (Jurassic) at surface, covers < 0.1 % of this area

Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft).

Quadrant Formation through Big Snowy Group: Quadrant and Amsden Formations, and Big Snowy Group; not all formations of Big Snowy Group present at all locations (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m (2,625 ft) in southwestern-most Montana. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft). Big Snowy Group (Mbs) - Heath Formation (Mh): Black, fissile, commonly petroliferous shale with subordinate sandstone, gypsum, and coal. Restricted marine to coastal plain. Thickness as much as 150 m (492 ft). Otter Formation (Mo): Green, grayish green, gray, and subordinate reddish brown shale interbedded with light brown or gray limestone. Open and semirestricted marine. Thickness as much as 150 m (492 ft). Kibbey Formation (Mk): Red, quartzose sandstone, siltstone, and shale, locally with subordinate thin gypsum beds and limestone bed in middle part. Intertidal and subtidal. Thickness as much as 105 m (345 ft).

Andesite (Cretaceous) at surface, covers < 0.1 % of this area

Andesite

Volcanic rock (Cretaceous) at surface, covers < 0.1 % of this area

Volcanic rock

Diorite (Cretaceous) at surface, covers < 0.1 % of this area

Diorite

Marble (Archean) at surface, covers < 0.1 % of this area

Marble

Schist and gneiss (Lower Proterozoic or Archean) at surface, covers < 0.1 % of this area

Schist and gneiss

Livingston Group: Hoppers, Billman Creek, Miner Creek, and Cokedale Formations (Cretaceous) at surface, covers < 0.1 % of this area

Pale green or gray, volcaniclastic conglomerate and conglomeratic sandstone, dark gray volcanic flows, purple flow breccia, brown and gray tuff, and maroon, green, and gray mudstone and siltstone. Fluvial and alluvial fan. Thickness about 1,000 m (3,280 ft). Note: Mapped as Livingstone Group on printed map, but unit description exists for Livingstone Formation (Klv).

Thaynes through Dinwoody Formation: Thaynes, Woodside, and Dinwoody Formations (Triassic) at surface, covers < 0.1 % of this area

Thaynes Formation: Gray and tan limestone with interbedded siltstone, shale, and tan sandstone. Locally with red shale tongues. Marine. Thickness as much as 290 m (950 ft). Woodside Formation: Maroon and red siltstone, dolomite, and shale. Tidal flat. Thickness as much as 250 m (820 ft). Dinwoody Formation (TRd): Interbedded green siltstone, shale, sandstone, and carbonate that grades eastward into red shale, siltstone, and anhydrite. Nearshore and restricted marine. Thickness as much as 330 m (1,083 ft).

Bighorn through Flathead Formation: Bighorn, Pilgrim, Park, Meagher, Wolsey, and Flathead Formations (Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Bighorn Formation (Ob): Yellowish gray to very pale orange crystalline dolomite with local basal sandstone. Marine shelf. Thickness as much as 120 m (394 ft). Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft).

Groat sandstone bed of Pierre Formation (Cretaceous) at surface, covers < 0.1 % of this area

Gray, ferruginous and glauconitic, fine- to coarse-grained sandstone, siltstone, and sandy to silty gray shale. Offshore marine. Pinches out laterally. Thickness as much as 100 m (328 ft).

Newland and LaHood Formations (Middle Proterozoic) at surface, covers < 0.1 % of this area

Newland Formation (Yn): Dark bluish gray limestone. Thickness as much as 2,000 m (6,562 ft). LaHood Formation (Yla): Dark gray, dark brown, reddish brown, coarse, arkosic conglomerate locally with very large clasts derived from crystalline metamorphic rocks. Grades from mega-clast conglomerate to shale or argillite over short distances. Thickness as much as 3,000 m (9,842 ft).

Eagle through Belle Fourche Formation: Eagle, Telegraph Creek, Niobrara, Carlile, Greenhorn, and Belle Fourche Formations (Cretaceous) at surface, covers < 0.1 % of this area

Eagle Formation (Ke): Gray or grayish brown, thick-bedded sandstone, sandy shale, and thin interbedded coal. Chert-pebble lag deposit common at top of formation. Virgelle Member (lower part of formation in parts of central Montana): very light gray, gray, or brown sandstone that coarsens upward. Root zone and thin carbonaceous beds at top of member. Delta, coastal plain, tidal flat, and marine shoreface and foreshore. Thickness as much as 150 m (492 ft). Telegraph Creek Formation (Ktc): Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft). Niobrara Formation (Kn): Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft). Carlile Formation (Kca): Dark to light gray, noncalcareous, partly silty or sandy shale with upper zone of white-weathering, calcareous concretions, middle zone of sandy, orange-weathering, calcareous concretions, and basal zone of ferruginous concretions. Marine. Thickness as much as 195 m (640 ft). Greenhorn Formation (Kgr): Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft). Belle Fourche Formation (Kbf): Gray to black shale with ironstone concretions and numerous bentonite beds. Mosby Sandstone Member (upper part of formation in north- and east-central Montana): brown sandstone locally with chert pebbles, interbedded with gray shale. Big Elk Sandstone Member (lower part of formation in southwest-central Montana): light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black clayey shale. Marine shelf. Thickness as much as 260 m (853 ft).

Linley Member of Fort Union Formation (Tertiary) at surface, covers < 0.1 % of this area

Reddish to grayish brown conglomerate interbedded with coarse-grained sandstone, siltstone, and mudstone. Alluvial fan deposits derived from Beartooth uplift. Thickness as much as 610 m (2,000 ft).

Marias River and Blackleaf Formations (Cretaceous) at surface, covers < 0.1 % of this area

Marias River Formation (Kmr) - Kevin Member (Kmk): Dark gray shale with calcareous concretions and numerous bentonite beds. Upper part calcareous. MacGowan Concretionary Bed (middle part of member): grayish brown concretionary dolostone and limestone with phosphatic pellets, and gray to black chert pebbles. Marine. Thickness as much as 350 m (1,148 ft). Ferdig Member (Kmf): Gray and dark gray shale with thin siltstone or sandstone beds at the top. Marine. Thickness as much as 125m (410 ft). Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft). Floweree Member (Kmfl): Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft). Blackleaf Formation (Kbl) - Bootlegger Member (Kbb): Dark gray shale interbedded with sandstone, siltstone, and bentonite beds. Marine. Thickness as much as 100 m (328 ft). Vaughn Member (Kbv): Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft). Taft Hill Member (Kbt): Black shale, gray bentonitic siltstone, and glauconitic sandstone. Marine. Thickness as much as 180 m (590 ft). Flood Member (Kblf): Upper part: medium gray, cliff-forming fine- to medium-grained chert-rich, quartzose sandstone, middle part medium to dark gray shale or gray siltstone; lower part: yellowish brown, gray, or olive gray fine-grained sandstone and siltstone with carbonaceous laminae and thin films of coal and dark gray silty, bentonitic shale. Sandstone beds thin eastward. Quartzose basal sandstone in southwestern Montana. Nearshore marine, tidal flat, and lagoon. Thickness as much as 230 m (755 ft).

Paleosol (Quaternary and Tertiary) at surface, covers < 0.1 % of this area

Light gray to very pale orangish gray calcrete and calcareous silt with scattered small pebbles near the base. Northeastern Montana.

Snowcrest Range and Tendoy Groups: Conover Ranch, Lombard, Kibbey, McKenzie Canyon, Mission Canyon, Middle Canyon, and Paine Formations (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Snowcrest Range Group (PAMsr) - Conover Ranch Formation: Red siltstone and mudstone with minor silty limestone and sandstone beds. Thickness 35 m (115 ft). Lombard Formation: Gray, silty limestone with minor black shale. Thickness120 m (394 ft). Kibbey Formation: Yellow siltstone, sandstone, and mudstone with minor silt limestone in lower part. Thickness 80 m (262 ft). Tendoy Group (Mtd) - McKenzie Canyon Formation: Gray, chert-bearing limestone and solution breccia. Marine. Thickness 140 m (460 ft). Mission Canyon Formation: Light gray, chert-bearing limestone and brown dolomitic limestone. Shallow marine. Thickness 180 m (590 ft). Middle Canyon Formation (Mmd): Dark gray, silty limestone with chert beds and nodules. Shallow marine. Thickness at type section 335 m (1,100 ft). Paine Formation: Dark gray, silty limestone with solution breccia zone. Shallow marine. Thickness 230 m (755 ft).

Amphibolite (Archean) at surface, covers < 0.1 % of this area

Amphibolite

Diorite and gabbro (Tertiary) at surface, covers < 0.1 % of this area

Diorite and gabbro

Bighorn Formation (Ordovician) at surface, covers < 0.1 % of this area

Yellowish gray to very pale orange crystalline dolomite with local basal sandstone. Marine shelf. Thickness as much as 120 m (394 ft).

Trachyandesite (Tertiary or Cretaceous) at surface, covers < 0.1 % of this area

Trachyandesite

Intrusive rock (Cretaceous) at surface, covers < 0.1 % of this area

Intrusive rock

Three Forks and Jefferson Formations (Devonian) at surface, covers < 0.1 % of this area

Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft).

Shedhorn Formation (Permian) at surface, covers < 0.1 % of this area

Orangish brown, very fine- to medium-grained, well-sorted, quartzose sandstone. Nearshore marine shelf. Thickness as much as 70 m (230 ft).

Altyn and Waterton Formations (Middle Proterozoic) at surface, covers < 0.1 % of this area

Altyn Formation: White to light gray dolomite interbedded with dolomitic sandstone and cross-bedded sandstone. Thickness as much as 800 m (2,625 ft). Waterton Formation: Gray to tan dolomite and dark gray limestone with chert nodules and minor dolomitic siltite and sandstone. Thickness 200 m (656 ft).

Amphibolite (Lower Proterozoic or Archean) at surface, covers < 0.1 % of this area

Amphibolite

Wolsey Formation (Cambrian) at surface, covers < 0.1 % of this area

Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft).

Schist (Lower Proterozoic or Archean) at surface, covers < 0.1 % of this area

Schist

Quartzite (Archean) at surface, covers < 0.1 % of this area

Quartzite

Sediment (Quaternary and Tertiary) at surface, covers < 0.1 % of this area

Yellowish gray to very pale orange, angular silt and clay-size sediment with lenses of angular and subangular locally derived rock ranging to very large boulder size but generally cobble size and smaller. In some areas granules and pebbles float in the silty matrix. Locally cemented; clasts may be coated with calcium carbonate, especially near the base. Thickness as much as 200 ft.

Monzonite (Tertiary) at surface, covers < 0.1 % of this area

Monzonite

Quartzite (Cambrian, late Proterozoic, or Middle Proterozoic) at surface, covers < 0.1 % of this area

Purple to light gray, fine- to medium-grained massive quartzite that contains sparse floating pebbles of quartz interbedded with and grading downward into fine- to coarse-grained feldspathic quartzite with abundant mud chips, trough and planar crossbeds, and flat laminations.

Diorite (Late Proterozoic) at surface, covers < 0.1 % of this area

Diorite

Diorite (Tertiary) at surface, covers < 0.1 % of this area

Diorite

Judith River and Claggett Formations (Cretaceous) at surface, covers < 0.1 % of this area

Judith River Formation (Kjr): Light brown to light gray, fine- to coarse-grained sandstone with interbeds of gray to black carbonaceous shale, silty shale, and thin coal. Local Parkman Sandstone Member (lower part of formation):yellowish gray to brownish gray and olive green, fine- to medium-grained, cross-bedded sandstone interbedded with yellowish gray, silty shale. Estuarine, brackish, and nearshore marine. Thickness as much as 305 m (1000 ft). Claggett Formation (Kcl): Dark gray to gray shale that weathers brown, with thin, gray sandstone laminae and beds in upper or middle part and calcareous concretions in lower part. Marine. Thickness as much as 170 m (558 ft).

Golden Spike Formation (Cretaceous) at surface, covers < 0.1 % of this area

Volcaniclastic sandstone and conglomerate, chert-pebble conglomerate, andesitic lava, andesitic tuff, and limestone. Fluvial, mudflow, and landslide, locally derived from Elkhorn Mountains Volcanics. Thickness as much as 2,700 m (8,858 ft).

Vaughn Member of Blackleaf Formation (Cretaceous) at surface, covers < 0.1 % of this area

Black carbonaceous shale, light gray, greenish to pinkish bentonitic to tuffaceous siltstone and porcellanite, tuffaceous, arkosic sandstone, thin coal; conglomerate beds in western Montana. Fluvial and coastal delta plain. Thickness as much as 200 m (656 ft).

Slim Sam Formation (Cretaceous) at surface, covers < 0.1 % of this area

Greenish gray, pyroclastic tuff and volcaniclastic-rich sandstone, siltstone, mudstone, and porcellanite. Nonmarine, locally derived from Elkhorn Mountains Volcanics. Thickness as much as 200 m (656 ft).

Lower Maudlow Formation (Cretaceous) at surface, covers < 0.1 % of this area

Purple lava flows and gray, brown, green, and purple sandstone, conglomerate, breccia, mudstone, and tuff. Nonmarine. Thickness 1,525–1,675+ m (5,003–5,495+ ft).

Grove Creek through Flathead Formation: Grove Creek, Snowy Range, Pilgrim, Park, Meagher, Wolsey, and Flathead Formations (Cambrian) at surface, covers < 0.1 % of this area

Grove Creek Formation: Shale, limestone, dolomite, and intraformational limestone-pebble conglomerate. Shallow marine. Average thickness 10 m (35 ft). Snowy Range Formation (CAsr): Sage Member (upper part of formation): gray limestone and grayish green shale interbedded with grayish green flat-pebble conglomerate. Dry Creek Member (lower part of formation): greenish gray shale and grayish orange calcareous sandstone. Shallow marine. Thickness as much as 85 m (280 ft). Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft).

Jeffferson through Snowy Range Formation: Jefferson, Maywood(?), Beartooth Butte(?), and Snowy Range Formations in Big Snowy Mountains (Devonian through Cambrian) at surface, covers < 0.1 % of this area

Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Maywood Formation: Thin-bedded dolomitic limestone and dolomite interbedded with calcareous shale. Shallow marine. Thickness as much as 120 m (394 ft). Beartooth Butte Formation: Light gray and grayish red dolomite and limestone breccia and conglomerate, sandy and silty dolomite, and dolomitic siltstone, sandstone, and shale. Isolated outcrops throughout central Montana. Estuarine. Thickness as much as 50 m (164 ft). Snowy Range Formation (CAsr): Sage Member (upper part of formation): gray limestone and grayish green shale interbedded with grayish green flat-pebble conglomerate. Dry Creek Member (lower part of formation): greenish gray shale and grayish orange calcareous sandstone. Shallow marine. Thickness as much as 85 m (280 ft).

Dacite (Cretaceous) at surface, covers < 0.1 % of this area

Dacite

Middle Canyon Formation (Mississippian) at surface, covers < 0.1 % of this area

Dark gray, silty limestone with chert beds and nodules. Shallow marine. Thickness at type section 335 m (1,100 ft).

Shonkinite (Tertiary) at surface, covers < 0.1 % of this area

Shonkinite

Rierdon Formation (Jurassic) at surface, covers < 0.1 % of this area

Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft).

Phosphoria Formation (Permian) at surface, covers < 0.1 % of this area

Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft).

Phosphoria and Park City Formations (Permian) at surface, covers < 0.1 % of this area

Phosphoria Formation (Pp): Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft). Park City Formation: Gray limestone, tan dolomite, and interbedded sandstone, phosphatic shale, and cherty carbonate. Marine shelf. Thickness as much as 115 m (377 ft).

Lombard Formation and Madison Group (Mississippian) at surface, covers < 0.1 % of this area

Lombard Formation: Gray, silty limestone with minor black shale. Thickness 120 m (394 ft). Madison Group (Mm) - Mission Canyon Formation (Mmc): Gray, massive limestone with chert beds and nodules, and solution breccia zones. Shallow marine. Thickness as much as 520 m (1,706 ft). Lodgepole Formation (Ml): Woodhurst Member (upper part of formation): light gray, well-bedded limestone, typically with much dark chert, interbedded with thinner calcareous mudstone beds. Paine Member (middle part of formation):dark gray, thin-bedded, silty or fossiliferous limestone. Cottonwood Canyon Member (lower part of formation): black shale with basal conglomeratic lag deposit. Shallow marine. Thickness as much as 305 m (1000 ft). In Northwest Montana - Castle Reef Formation: Medium to light gray, thick-bedded limestone or dolomite. Sun River Member (upper part of formation): light gray dolomite with thick fossiliferous lenses. Shallow marine. Thickness as much as 300 m (984 ft). Allan Mountain Formation: Dark gray, thinly bedded limestone with thin mudstone and shale partings, and nodular chert. Shallow marine. Thickness as much as 200 m (656 ft).

Jefferson and Bighorn Formations: locally includes Beartooth Butte Formation in central Montana (Devonian and Ordovician) at surface, covers < 0.1 % of this area

Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Bighorn Formation (Ob): Yellowish gray to very pale orange crystalline dolomite with local basal sandstone. Marine shelf. Thickness as much as 120 m (394 ft). In central Montana - Beartooth Butte Formation: Light gray and grayish red dolomite and limestone breccia and conglomerate, sandy and silty dolomite, and dolomitic siltstone, sandstone, and shale. Isolated outcrops throughout central Montana. Estuarine. Thickness as much as 50 m (164 ft).

Tendoy Group: McKenzie Canyon, Mission Canyon, Middle Canyon, and Paine Formations (Mississippian) at surface, covers < 0.1 % of this area

McKenzie Canyon Formation: Gray, chert-bearing limestone and solution breccia. Marine. Thickness 140 m (460 ft). Mission Canyon Formation: Light gray, chert-bearing limestone and brown dolomitic limestone. Shallow marine. Thickness 180 m (590 ft). Middle Canyon Formation (Mmd): Dark gray, silty limestone with chert beds and nodules. Shallow marine. Thickness at type section 335 m (1,100 ft). Paine Formation: Dark gray, silty limestone with solution breccia zone. Shallow marine. Thickness 230 m (755 ft).

Quartzite and amphibolite (Archean) at surface, covers < 0.1 % of this area

Quartzite and amphibolite

Middle Maudlow Formation (Cretaceous) at surface, covers < 0.1 % of this area

Grayish red and grayish green lava flows, varicolored tuff, and bluish green and dark yellowish brown volcanic breccia, tuffaceous volcanic sandstone, mudstone, and minor conglomerate.

Shonkinite (Cretaceous) at surface, covers < 0.1 % of this area

Shonkinite

Basin-fill deposit (Quaternary and Tertiary) at surface, covers < 0.1 % of this area

Interbedded poorly consolidated to unconsolidated, well-bedded to massive silt, sand, and well-rounded cobbles. Ennis Lake area, southwestern Montana.

Purcell Lava (Middle Proterozoic) at surface, covers < 0.1 % of this area

Black to blackish green alkaline basalt flows.

Landslide Creek Formation (Cretaceous) at surface, covers < 0.1 % of this area

Dark gray, conglomeratic, feldspathic, lenticular sandstone interbedded with varicolored dark mudstone, claystone, bentonite, and coal. Fluvial. Thickness as much as 610 m (2,000 ft).

Quartzite (Lower Proterozoic or Archean) at surface, covers < 0.1 % of this area

Quartzite

Metabasite (Early Proterozoic) at surface, covers < 0.1 % of this area

Metabasite

Quartzofeldspathic gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Quartzofeldspathic gneiss

Alkalic intrusive rock (Tertiary) at surface, covers < 0.1 % of this area

Alkalic intrusive rock

Syenite (Cretaceous) at surface, covers < 0.1 % of this area

Syenite

Muddy Formation (Cretaceous) at surface, covers < 0.1 % of this area

Gray to brown fine- to medium-grained sandstone with interbedded dark gray locally carbonaceous shale. Deltaic, shoreface, and offshore marine. Thickness as much as 50 m (164 ft).

Monzodiorite (Cretaceous) at surface, covers < 0.1 % of this area

Monzodiorite

Lennep Formation (Cretaceous) at surface, covers < 0.1 % of this area

Yellow to gray, fine- to medium-grained sandstone with subordinate interbedded gray shale. Nearshore marine. Thickness as much as 185 m (607 ft).

Niobrara through Belle Fourche Formation: Niobrara, Carlile, Greenhorn, and Belle Fourche Formations (Cretaceous) at surface, covers < 0.1 % of this area

Niobrara Formation (Kn): Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft). Carlile Formation (Kca): Dark to light gray, noncalcareous, partly silty or sandy shale with upper zone of white-weathering, calcareous concretions, middle zone of sandy, orange-weathering, calcareous concretions, and basal zone of ferruginous concretions. Marine. Thickness as much as 195 m (640 ft). Greenhorn Formation (Kgr): Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft). Belle Fourche Formation (Kbf): Gray to black shale with ironstone concretions and numerous bentonite beds. Mosby Sandstone Member (upper part of formation in north- and east-central Montana): brown sandstone locally with chert pebbles, interbedded with gray shale. Big Elk Sandstone Member (lower part of formation in southwest-central Montana): light gray, chert-rich sandstone commonly stained dark red, interbedded with thin, dark gray to black clayey shale. Marine shelf. Thickness as much as 260 m (853 ft).

Three Forks through Flathead Formation: Three Forks, Jefferson, Kinnikinic, and Snowy Range Formations, and either Pilgrim, Park, Meagher, Wolsey, and Flathead Formations; or Hasmark, Silver Hill, and Flathead Formations. (Mississippian through Cambrian) at surface, covers < 0.1 % of this area

Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Kinnikinic Formation (Ok): Very light gray, massive quartzite with local lenses of dolomite and dolomitic shale. Marine shelf. Thickness as much as 245 m (804 ft). Snowy Range Formation (CAsr): Sage Member (upper part of formation): gray limestone and grayish green shale interbedded with grayish green flat-pebble conglomerate. Dry Creek Member (lower part of formation): greenish gray shale and grayish orange calcareous sandstone. Shallow marine. Thickness as much as 85 m (280 ft). Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft). Hasmark Formation: Light gray or bluish gray limestone and dark-gray calcareous shale. Shallow marine. Thickness as much as 560 m (1,823 ft). Silver Hill Formation: Brown, green, and very light gray banded calcareous shale interbedded with laminated limestone. Shallow marine. Thickness as much as 120 m (394 ft).

Phyllite (Early Proterozoic) at surface, covers < 0.1 % of this area

Phyllite

Metamorphic rock (Archean) at surface, covers < 0.1 % of this area

Metamorphic rock

Amphibolite (Early Proterozoic) at surface, covers < 0.1 % of this area

Amphibolite

Schist (Early Proterozoic) at surface, covers < 0.1 % of this area

Schist

Travertine (Quaternary) at surface, covers < 0.1 % of this area

(central Montana): White to light grayish pink, typically vuggy and finely crystalline, locally banded limestone.

Augen gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Augen gneiss

Telegraph Creek and Niobrara Formations (Cretaceous) at surface, covers < 0.1 % of this area

Telegraph Creek (Ktc): Thin interbeds of yellowish brown sandstone or siltstone and gray shale. Marine shoreface and offshore. Thickness as much as 200 m (656 ft). Niobrara Formation (Kn): Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft).

Quartzite (Early Proterozoic) at surface, covers < 0.1 % of this area

Quartzite

Cone and Floweree Members of Marias River Formation (Cretaceous) at surface, covers < 0.1 % of this area

Cone Member (Kmc): Calcareous, dark gray shale and thin limestone beds and lenses. Widespread bentonite bed and septarian concretions near base. Shallow marine. Thickness as much as 20 m (66 ft). Floweree Member (Kmfl): Dark gray, noncalcareous shale with interbeds of siltstone. Calcareous concretions and scattered granules of dark gray chert. Shallow marine. Thickness as much as 30 m (98 ft).

Phyllite (Lower Proterozoic or Archean) at surface, covers < 0.1 % of this area

Phyllite

Mylonite (Archean) at surface, covers < 0.1 % of this area

Mylonite

Snaky Canyon and Bluebird Mountain Formations (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Snaky Canyon Formation: Medium gray limestone with yellowish brown chert, interbedded with subordinate thin sandstone beds. Marine. Thickness 245 m (804 ft). Bluebird Mountain Formation: Medium gray to brownish gray quartzite interbedded with yellowish brown, very fine-grained sandstone, gray calcareous siltstone, and medium-gray sandy limestone. Marine. Thickness 275 m (902 ft).

Upper Maudlow Formation (Cretaceous) at surface, covers < 0.1 % of this area

Light gray, volcanic conglomerate and breccia and olive green or dark yellowish brown volcanic sandstone, conglomeratic sandstone, siltstone and mudstone.

Morrison through Chugwater Formation: Morrison, Swift, Rierdon, Piper, and Chugwater Formations (Jurassic and Triassic) at surface, covers < 0.1 % of this area

Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft). Chugwater Formation (TRc): Red siltstone, sandstone, and shale with subordinate gypsum and local limestone bed at top. Shallow restricted marine and nonmarine. Thickness as much as 180 m (590 ft).

Pyroxenite and syenite (Tertiary or Cretaceous) at surface, covers < 0.1 % of this area

Pyroxenite and syenite

Intrusive rock (Tertiary or Cretaceous) at surface, covers < 0.1 % of this area

Intrusive rock

Latite (Cretaceous) at surface, covers < 0.1 % of this area

Latite

Niobrara through Greenhorn Formation: Niobrara, Carlile, and Greenhorn Formations (Cretaceous) at surface, covers < 0.1 % of this area

Niobrara Formation (Kn): Gray to dark gray shale with numerous thin bentonite beds. Chalky aggregates of coccoliths and rhabdoliths in upper part. MacGowan Concretionary Bed (middle part of formation): grayish brown concretionary dolostone and limestone with phosphatic pellets and gray to black chert pebbles. Marine. Lower part noncalcareous. Thickness as much as 9–23 m (30–75 ft). Carlile Formation (Kca): Dark to light gray, noncalcareous, partly silty or sandy shale with upper zone of white-weathering, calcareous concretions, middle zone of sandy, orange-weathering, calcareous concretions, and basal zone of ferruginous concretions. Marine. Thickness as much as 195 m (640 ft). Greenhorn Formation (Kgr): Gray to light gray calcareous shale and shaly marl with thin beds of limestone. Shale contains white to pink calcareous specks. Marine. Thickness as much as 100 m (30–75 ft).

Tufa (Quaternary and Tertiary) at surface, covers < 0.1 % of this area

Light yellowish gray to light gray, vuggy, partly thinly laminated, locally tuffaceous or silty limestone. Along Snowcrest Range fault, southwestern Montana.

Trachyte (Tertiary) at surface, covers < 0.1 % of this area

Trachyte

Chamberlain Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Dark gray argillite. Thickness as much as 700 m(2,297 ft).

Neihart Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Very light gray and pinkish gray, coarse-grained, well sorted quartzite with subordinate interbedded dark green, micaceous argillite in upper part. Thickness 215 m (705 ft).

Monzonite (Cretaceous) at surface, covers < 0.1 % of this area

Monzonite

Three Forks through Kinnikinic Formation: Three Forks, Jefferson, and Kinnikinic Formations (Mississippian through Ordovician) at surface, covers < 0.1 % of this area

Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Kinnikinic Formation (Ok): Very light gray, massive quartzite with local lenses of dolomite and dolomitic shale. Marine shelf. Thickness as much as 245 m (804 ft).

Pinto Diorite (Early Proterozoic) at surface, covers < 0.1 % of this area

Mottled gray to greenish gray massive, locally gneissic, medium- to coarse-grained diorite.

Pyroxenite (Cretaceous) at surface, covers < 0.1 % of this area

Pyroxenite

Intrusive rock (Tertiary) at surface, covers < 0.1 % of this area

Intrusive rock

Bullion Creek Formation (Paleocene) at surface, covers < 0.1 % of this area

Yellowish-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 200 metres (600 feet).

Granitic rock (Cretaceous or middle Proterozoic) at surface, covers < 0.1 % of this area

Granitic rock

Monzodiorite (Tertiary) at surface, covers < 0.1 % of this area

Monzodiorite

Everts Formation (Cretaceous) at surface, covers < 0.1 % of this area

Light-colored, fine- to medium-grained, lenticular sandstone and medium to light gray or locally greenish, yellowish, or brownish gray mudstone. Shallow marine and brackish. Thickness as much as 425 m (1,395 ft).

Intrusive rock (Late Proterozoic through Archean (age uncertain)) at surface, covers < 0.1 % of this area

Intrusive rock

Granodiorite and granite. (Paleocene and Cretaceous) at surface, covers < 0.1 % of this area

Granodiorite and granite containing biotite and local muscovite (66-54 Ma); composes northern (Bitterroot) lobe of batholith and is ~20 Ma younger than southern (Atlanta) lobe. (Paleocene and Cretaceous Idaho Batholith and Older Cretaceous and Jurassic Intrusive Rocks).

Ultramafic rock (Archean) at surface, covers < 0.1 % of this area

Ultramafic rock

Granitic rock (Silurian) at surface, covers < 0.1 % of this area

Granitic rock

Piegan Group. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Calcareous and dolomitic siltite, quartzite, and subordinate argillite of the Helena and Wallace formations in northern Idaho; south of St. Joe River, correlative rocks include garnet grade feldspathic quartzite and calc-silicate gneiss. (Mesoproterozoic Belt Supergroup).

Morrison through Twin Creek Formation: Morrison, Ellis Group, and Twin Creek Formations (Jurassic) at surface, covers < 0.1 % of this area

Morrison Formation (Jm): Green, gray, or red mudstone and marlstone with subordinate limestone and sandstone beds. Upper part Neocomian age in central Montana and contains carbonaceous black shale and coal. Fluvial, paludal, and lacustrine. Thickness as much as 180 m (590 ft). Ellis Group (Je) - Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Sawtooth Formation: Western Montana: dark gray, platy to shaly, dense limestone with local basal conglomerate. Central Montana: upper calcareous siltstone, middle dark gray shale with thin limestone interbeds, and lower fine-grained sandstone. Three local members Bowes, Firemoon, and Tampico, in descending order. Bowes Member: dark gray to medium gray, calcareous mudstone, limestone, and quartzose sandstone. Firemoon Member: dark to medium gray, limestone and calcareous mudstone. Tampico Member: very light gray, well-sorted quartz sandstone and siltstone, and chert-pebble conglomerate. Shallow marine. Thickness as much as 205 m (673 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft). Twin Creek Formation: Black and gray, calcareous shale and thin-bedded shaly limestone with some beds of yellow sandstone. Marine.

Sphinx Formation (Cretaceous) at surface, covers < 0.1 % of this area

Reddish gray sandstone and conglomerate. Clasts dominantly carbonate and quartzite. Syntectonic alluvial fan. Thickness over1,000 m (3,281 ft).

Rhyodacite (Cretaceous) at surface, covers < 0.1 % of this area

Rhyodacite

Gneiss and amphibolite (Early Proterozoic) at surface, covers < 0.1 % of this area

Gneiss and amphibolite

Basalt (Quaternary or Tertiary) at surface, covers < 0.1 % of this area

Basalt

Wolsey and Flathead Formations (Cambrian) at surface, covers < 0.1 % of this area

Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft).

Undivided surficial deposits (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Mostly alluvium, colluvium, and glacial and landslide deposits. Primarily in Yellowstone area and Bighorn Mountains.

Rhyolite (Tertiary or Cretaceous) at surface, covers < 0.1 % of this area

Rhyolite

Lemhi Group. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Feldspathic fine-grained quartzite, siltite, and subordinate argillite and carbonate-bearing rocks in east-central Idaho; includes Apple Creek, West Fork, Inyo Creek, Big Creek, and Gunsight formations. (Mesoproterozoic Metasedimentary Rocks of Lemhi Subbasin of Belt Basin).

Basalt (Quaternary) at surface, covers < 0.1 % of this area

Basalt

Otter and Kibbey Formations (Mississippian) at surface, covers < 0.1 % of this area

Otter Formation (Mo): Green, grayish green, gray, and subordinate reddish brown shale interbedded with light brown or gray limestone. Open and semirestricted marine. Thickness as much as 150 m (492 ft). Kibbey Formation (Mk): Red, quartzose sandstone, siltstone, and shale, locally with subordinate thin gypsum beds and limestone bed in middle part. Intertidal and subtidal. Thickness as much as 105 m (345 ft).

Jefferson through Flathead Formation: Jefferson, Snowy Range, Pilgrim, Park, Meagher, Wolsey, and Flathead Formations (Devonian through Cambrian) at surface, covers < 0.1 % of this area

Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Snowy Range Formation (CAsr): Sage Member (upper part of formation): gray limestone and grayish green shale interbedded with grayish green flat-pebble conglomerate. Dry Creek Member (lower part of formation): greenish gray shale and grayish orange calcareous sandstone. Shallow marine. Thickness as much as 85 m (280 ft). Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park Formation: Grayish green, fissile, micaceous shale with a few thin beds of calcareous sandstone and thin, gray limestone. Local arkose beds. Offshore marine. Thickness as much as 310 m (1,017 ft). Meagher Formation: Gray to bluish gray limestone and dolomitic limestone, locally mottled and with intraformational conglomerate beds. Shallow marine. Thickness as much as 300 m (984 ft). Wolsey Formation (CAw): Dark green and purplish gray fissile, micaceous shale, thin glauconitic limestone beds and thin, fine-grained sandstone beds. Shallow marine. Thickness as much as 145 m (476 ft). Flathead Formation (CAf): Pinkish gray to light gray sandstone or quartzite. Locally very glauconitic, pebbly, arkosic, or iron-stained. Marine shoreface. Thickness as much as 100 m (328 ft).

Challis intrusive rocks. (Eocene) at surface, covers < 0.1 % of this area

Shallow roots of Challis volcanic field. Older suite of granodiorite and quartz monzodiorite and subordinate diorite, granite, and subvolcanic dacite; includes Jackson Peak, Beaver Creek, Marsh Creek, and Summit Creek stocks (49-45 Ma). Younger suite of granite and minor syenite and subvolcanic rhyolite; includes Sawtooth, Casto, Bungalow, and Lolo Hot Springs plutons (47-43 Ma). (Eocene Challis Magmatic Complex and Related Sedimentary Rocks).

Anorthosite (Middle Proterozoic) at surface, covers < 0.1 % of this area

Anorthosite

Middle Canyon through Kinnikinic Formation: Middle Canyon, McGowan Creek, Three Forks, Jefferson, and Kinnikinic Formations (Mississippian through Ordovician) at surface, covers < 0.1 % of this area

Middle Canyon Formation: Dark gray, silty limestone with chert beds and nodules. Shallow marine. Thickness at type section 335 m (1,100 ft). McGowan Creek Formation: Upper part: dark gray and pale yellowish brown, thin-bedded, calcareous siltstone interbedded with dark gray, silty limestone. Lower part: dark gray, carbonaceous, thin-bedded argillite with interbeds of dark gray siltite, medium gray, fine-grained, conglomeratic sandstone and quartzite, and dark gray, silty limestone. Marine. Thickness as much as 61 m (200 ft). Three Forks Formation (MDt): Sappington Member (upper part of formation):yellowish orange and yellowish gray siltstone. Trident Member (middle part of formation): greenish gray and light olive gray, fossiliferous, calcareous shale that contains interbeds and nodules of fossiliferous, argillaceous limestone. Logan Gulch Member (lower part of formation): yellowish gray and grayish red, argillaceous limestone breccia and shale breccia that may include dolomitic siltstone. Marine to restricted marine with evaporite basins. Thickness as much as 185 m (607 ft). Jefferson Formation (Dj): Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft). Kinnikinic Formation (Ok): Very light gray, massive quartzite with local lenses of dolomite and dolomitic shale. Marine shelf. Thickness as much as 245 m (804 ft).

Mafic intusive rock (Tertiary) at surface, covers < 0.1 % of this area

Mafic intusive rock

Prichard Formation. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Siltite, quartzite, and argillite in northern Idaho; deposited in deep water and contains voluminous ~1470 Ma mafic sills. Includes siltite, quartzite, fine-grained schist, and carbonate rocks structurally above anorthosite near Boehls Butte, northeast of Elk River. (Mesoproterozoic Belt Supergroup).

Snowy Range Formation (Cambrian) at surface, covers < 0.1 % of this area

Sage Member (upper part of formation): gray limestone and grayish green shale interbedded with grayish green flat-pebble conglomerate. Dry Creek Member (lower part of formation): greenish gray shale and grayish orange calcareous sandstone. Shallow marine. Thickness as much as 85 m (280 ft).

Rhyolite flows, tuff, and intrusive igneous rocks (Pleistocene) at surface, covers < 0.1 % of this area

Includes Plateau Rhyolite (age about 0.07 Ma) and interlayered sediments, Mount Jackson Rhyolite (age 0.6 to about 1 Ma), Lewis Canyon Rhyolite (age about 0.9 Ma); and Lava Creek Tuff of Yellowstone Group (age 0.6 to about 1 Ma).

Bluebird Mountain through South Creek: Bluebird Mountain, Railroad Canyon, Surrett Canyon, and South Creek Formations (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Bluebird Mountain Formation: Medium gray to brownish gray quartzite interbedded with yellowish brown, very fine-grained sandstone, gray calcareous siltstone, and medium-gray sandy limestone. Marine. Thickness 275 m (902 ft). Railroad Canyon Formation: Medium gray and brownish black, phosphatic mudstone, shale, limestone, limestone conglomerate, and medium gray sandstone. Marine. Thickness 260 m (853 ft). Surrett Canyon Formation: Dark gray, massive limestone with much scattered white bioclastic debris. Marine. Thickness at type section 67 m (220 ft). South Creek Formation: Dark gray, thin-bedded, chert bearing limestone that alternates with dark gray, argillaceous limestone. Marine. Thickness at type section 91 m (300 ft).

Ravalli Group. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Feldspathic quartzite and subordinate siltite and argillite of the Burke, Revett, and St. Regis formations in northern Idaho; south of St. Joe River, includes correlative garnet-grade quartzite and schist. (Mesoproterozoic Belt Supergroup).

Travertine (Tertiary) at surface, covers < 0.1 % of this area

White to light grayish pink, typically vuggy and finely crystalline, locally banded limestone. Light gray to white, thick-bedded, vuggy limestone.

Lava Creek Tuff (Quaternary) at surface, covers < 0.1 % of this area

Yellowstone Group - Brown, gray, and purple, welded, devitrified ash tuff with quartz, sanidine, and sodic plagioclase phenocrysts.

Bighorn through Park Formation: Bighorn, Red Lion, Pilgrim, and Park Formations (Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Bighorn Formation (Ob): Yellowish gray to very pale orange crystalline dolomite with local basal sandstone. Marine shelf. Thickness as much as 120 m (394 ft). Red Lion Formation: Gray limestone with siliceous lamina and black calcareous shale interbedded with thin limestone. Shallow marine. Thickness as much as 115 m (377 ft). Pilgrim Formation: Gray, commonly mottled limestone that may contain intraformational limestone conglomerate. Shallow marine. Thickness as much as 180 m (590 ft). Park City Formation: Gray limestone, tan dolomite, and interbedded sandstone, phosphatic shale, and cherty carbonate. Marine shelf. Thickness as much as 115 m (377 ft).

Upper Missoula Group. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Feldspathic quartzite and subordinate siltite and argillite. Includes Mount Shields, Bonner, Striped Peak, and Libby formations in northern Idaho and possibly correlative quartzite north of Salmon and in the Meadow Creek metamorphic sequence east of Elk City. (Mesoproterozoic Belt Supergroup).

Helena Formation (Middle Proterozoic) at surface, covers < 0.1 % of this area

Northwestern Montana (formerly “lower Wallace”): cycles of basal white quartzite or intraclast beds overlain by couplets of green siltite and argillite, capped by dolomite beds. Calcite pods and ribbons (molar tooth structure) common. West-central Montana: gray to dark gray limestone and dolomitic limestone with siltite partings. Thickness as much as 2,000 m (6,562).

Tinguaite (Tertiary or Cretaceous) at surface, covers < 0.1 % of this area

Tinguaite

Hell Creek Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Gray sand, silt, clay, and sandstone; river sediment; as thick as 150 metres (500 feet).

Diorite (Tertiary or Cretaceous) at surface, covers < 0.1 % of this area

Diorite

Huckleberry Ridge Tuff of Yellowstone Group (Pliocene) at surface, covers < 0.1 % of this area

Lavender to gray-brown welded rhyolite tuff.

Glacial deposits. (Pleistocene) at surface, covers < 0.1 % of this area

Till and outwash consisting of gravel, sand, silt, and clay. Formed by valley glaciers at higher elevations and by the Cordilleran ice sheet in northern Idaho. Includes deposits of several glacial episodes. Includes rock glacier deposits and some modern alluvium derived from reworked till and outwash. (Quaternary Sediments).

Andesite and diorite (Cretaceous) at surface, covers < 0.1 % of this area

Andesite and diorite

Metamorphic rocks. (Paleoproterozoic and Archean) at surface, covers < 0.1 % of this area

Pend Oreille granodiorite gneiss near Priest River (~2650 Ma); schist, gneiss, and subordinate quartzite along North Fork Clearwater River; Kelly Creek granite gneiss northeast of Pierce (~2670 Ma); orthogneiss in Pioneer Mountains (~2600 Ma and 695 Ma); granite gneiss, granite, schist, and amphibolite of Green Creek complex in Albion Mountains (~2600 Ma); granite gneiss in Beaverhead Mountains east of Leadore (~2450 Ma); and marble, quartzite, schist, and amphibolite of uncertain age near Henrys Lake. (Mesoproterozoic to Archean Basement Rocks).

Fort Union Formation (Paleocene) at surface, covers < 0.1 % of this area

(Northwest, Southwest, and Central Wyoming) - Brown to gray sandstone, gray to black shale, and thin coal beds. (East Wyoming) - Light-colored massive sandstone, drab shale, and thick coal beds.

Tonalitic orthogneiss and foliated granodiorite. (Cretaceous) at surface, covers < 0.1 % of this area

Orthogneiss (primarily tonalite) and foliated granodiorite (~90 to ~80 Ma). Includes early phases of the Idaho batholith, migmatite, intrusions along major structures, and plutonic rocks of uncertain age along the Salmon River northwest of Salmon. (Paleocene and Cretaceous Idaho Batholith and Older Cretaceous and Jurassic Intrusive Rocks).

Swauger and Lawson Creek formations. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Quartzose medium- to coarse-grained quartzite of Swauger Formation and siltite and quartzite of overlying Lawson Creek Formation in east-central Idaho. (Mesoproterozoic Metasedimentary Rocks of Lemhi Subbasin of Belt Basin).

Oldest gneiss complex (Early Archean) at surface, covers < 0.1 % of this area

Chiefly layered granitic gneiss, locally migmatitic. Local masses of quartzite, metagraywacke, iron-formation, and other metasedimentary rocks and amphibolite and felsic gneiss thought to be volcanic; metasedimentary rocks in Beartooth Mountains contain detrital zircon dated at more than 3,400 Ma. Inclusions show evidence of granulite-facies metamorphism prior to 2,800 Ma. Mueller and others (1982) suggest that large areas in Beartooth Mountains were invaded by Late Archean granite (age about 2,800 Ma). Bighorn Mountains--Dates of metamorphism 3,000+ Ma.

Pierre Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray shale; marine offshore sediment; as thick as 700 metres (2,300 feet).

Dinwoody Formation (Triassic) at surface, covers < 0.1 % of this area

Interbedded green siltstone, shale, sandstone, and carbonate that grades eastward into red shale, siltstone, and anhydrite. Nearshore and restricted marine. Thickness as much as 330 m (1,083 ft).

Absaroka Volcanic Supergroup: Sunlight Group (Eocene) at surface, covers < 0.1 % of this area

Including Trout Peak Trachyandesite, Wapiti Formation (andesitic volcaniclastic rocks), Crescent Hill Basalt, and Mount Wallace Formation (felsic and mafic volcaniclastic rocks).

Gneiss, schist, and quartzite. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Kyanite and sillimanite gneiss, schist, quartzite, and amphibolite that is metamorphosed lower part of Prichard Formation (Hauser Lake Gneiss) and underlying quartzite of Gold Cup Mountain in the Priest River metamorphic complex. (Mesoproterozoic Belt Supergroup).

Willwood Formation (Eocene) at surface, covers < 0.1 % of this area

Variegated claystone, shale, and sandstone; some lenticular gold-bearing quartzite conglomerate.

Challis Volcanic Group. (Eocene) at surface, covers < 0.1 % of this area

Dacite, andesite, and rhyolite tuffs and flows and subordinate basalt and latite flows; covers large area in south-central Idaho. Includes Absaroka Volcanic Group near Henrys Lake and scattered volcanic rocks in eastern and northern Idaho. (Eocene Challis Magmatic Complex and Related Sedimentary Rocks).

Sedimentary rocks (Mississippian). (Mississippian) at surface, covers < 0.1 % of this area

Marine limestone of Mississippian carbonate banks and turbiditic sandstone, mudstone, and conglomerate of Antler flysch trough; includes Madison and Lodgepole formations and Chesterfield Range Group of southeastern Idaho; McGowan Creek Formation, White Knob Limestone, and overlying carbonate bank of Lost River Range; and Copper Basin Group of Pioneer Mountains. Includes poorly dated Salmon River assemblage east of Stanley, consisting of argillite, siltstone, calcareous sandstone, and limestone. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Metabasite (Archean) at surface, covers < 0.1 % of this area

Metabasite

Amsden Group: Alaska Bench and Tyler Formations (Pennsylvanian and Mississippian) at surface, covers < 0.1 % of this area

Alaska Bench Formation (PAMab): Gray and bluish gray, hard, fossiliferous limestone with subordinate interbeds of red mudstone. Marine. Thickness as much as 105 m (345 ft). Tyler Formation (PAMt): Cameron Creek Member (upper part of formation):red, purple, and brown mudstone and siltstone, and quartzose sandstone with subordinate gray shale and limestone. Bear Gulch Member (middle part of formation, locally): gray, fossiliferous, platy, argillaceous limestone. Stonehouse Canyon Member (lower part of formation): dark gray, carbonaceous mudstone with lenses of sandstone or conglomeratic sandstone. Fluvial and intertidal. Thickness as much as 180 m (590 ft).

Rhyolite (Pliocene and Miocene). (Pliocene and Miocene) at surface, covers < 0.1 % of this area

Rhyolite tuffs and flows of Heise volcanic field (6.6-4.5 Ma; includes Blacktail Creek, Walcott, Conant Creek, and Kilgore tuffs), and rhyolite domes and flows of Magic Reservoir area (6.6-3 Ma). (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Lower Missoula Group. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Siltite, argillite, and dolomitic siltite in northern Idaho; includes Snowslip and Shepard formations, rocks previously mapped as upper member of Wallace Formation, and correlative garnet-grade phyllite and schist south of St. Joe River. Also includes schist, micaceous quartzite, and calc-silicate rocks in lower part of Meadow Creek metamorphic sequence east of Elk City, and argillite, siltite, phyllite, and calc-silicate rocks south of Lost Trail Pass. (Mesoproterozoic Belt Supergroup).

Ellis Group through Chugwater Formation: Swift, Rierdon, Piper, and Chugwater Formations (Jurassic and Triassic) at surface, covers < 0.1 % of this area

Swift Formation (Jsw): Orangish brown, glauconitic, flaggy-bedded, commonly fossiliferous, fine-grained sandstone or sandy coquina with subordinate dark gray shale interbeds; chert pebbles common. In west-central and northwestern Montana, a dark gray, noncalcareous, micaceous shale forms the lower part of the formation, commonly with a basal chert-pebble conglomerate or conglomeratic sandstone as much as 3 m (10 ft) thick. Shallow marine. Thickness as much as 70 m (230 ft). Rierdon Formation (Jr): Gray, locally fossiliferous limestone that may contain floating grains of quartz sand, interbedded with greenish gray limy shale. Lagoonal and marine shelf. Thickness as much as 105 m (344 ft). Piper Formation (Jp): Upper part: red mudstone and gypsum; middle part: gray shale, limestone, and dolomite; lower part: red mudstone and gypsum. Marine and restricted coastal evaporite basins. Thickness as much as 75 m (246 ft). Chugwater Formation (TRc): Red siltstone, sandstone, and shale with subordinate gypsum and local limestone bed at top. Shallow restricted marine and nonmarine. Thickness as much as 180 m (590 ft).

Sedimentary rocks (Permian and Pennsylvanian). (Permian and Pennsylvanian) at surface, covers < 0.1 % of this area

Marine phosphorite, shale, and chert of Phosphoria Formation, fine-grained sandstone and mudrock of Wells, Quadrant, Amsden, and Shedhorn formations, and fine-grained sandstone, carbonaceous mudstone, and limestone of the Snaky Canyon Formation and Sun Valley and Oquirrh groups. Located in south-central and eastern Idaho. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Tonalite, granodiorite, and quartz diorite. (Cretaceous) at surface, covers < 0.1 % of this area

Tonalite, granodiorite, and quartz diorite, typically hornblende-bearing; includes the Payette River tonalite (~90 Ma) along western border zone of the Atlanta lobe, and the ~99 Ma Croesus pyroxene-biotite quartz diorite south of Hailey. Also includes granodiorite with potassium feldspar megacrysts that is typically hornblende-bearing and foliated (~90 Ma in central Idaho and ~100 Ma in northernmost Idaho) and early mafic phases of the Bitterroot lobe (~70 Ma). (Paleocene and Cretaceous Idaho Batholith and Older Cretaceous and Jurassic Intrusive Rocks).

Cody Shale (Late Cretaceous) at surface, covers < 0.1 % of this area

(Northern Yellowstone area) - Gray to brown shale and siltstone. (North and South Wyoming) - Dull-gray shale, gray siltstone, and fine-grained gray sandstone.

Sedimentary rocks (Permian to Mississippian). (Permian to Mississippian) at surface, covers < 0.1 % of this area

Only shown in Henrys Lake area and in cross section. Consists of Ms and PlPs (PPAs). (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Bighorn Dolomite, Gallatin Group, Gros Ventre Formation, Snowy Range Formation, Pilgrim Limestone, Park Shale, Meagher Limestone, Wolsey Shale, Flathead Sandstone, Whitewood Dolomite, and Winnipeg and Deadwood Formations (Middle Cambrian-Upper Ordovician) at surface, covers < 0.1 % of this area

Bighorn Dolomite (Thrust Belt and North Wyoming) - Gray massive cliff-forming siliceous dolomite and locally dolomitic limestone. Bighorn Dolomite (Northern Yellowstone area) - Light-gray massive siliceous dolomite. Gallatin Limestone or Group (Thrust Belt) - Gray and tan limestone. Gallatin Limestone or Group (North Wyoming) - Blue-gray and yellow mottled hard dense limestone. Gallatin Group-Snowy Range Formation (Northern Yellowstone area) - Medium-gray limestone and underlying greenish-gray shale. Gallatin Group-Pilgrim Limestone (Northern Yellowstone area) - Blue-gray and yellow mottled hard limestone. Gros Ventre Formation (Thrust Belt) - Greenish-gray micaceous shale. Gros Ventre Formation (North Wyoming) - Soft green micaceous shale (Upper and Middle Cambrian Park Shale Member), underlain by blue-gray and yellow mottled hard dense limestone (Middle Cambrian Death Canyon Limestone Member), and soft green micaceous shale (Middle Cambrian Wolsey Shale Member). Park Shale (Northern Yellowstone area) - Green micaceous soft shale. Upper part may be Late Cambrian. Meagher Limestone (Northern Yellowstone area) - Blue-gray and yellow mottled hard limestone. Wolsey Shale (Northern Yellowstone area) - Green micaceous soft shale. Flathead Sandstone (Northern Yellowstone area and North Wyoming) - Dull-red quartzitic sandstone. Whitewood Dolomite (Northeast Wyoming) - Buff massive fossiliferous dolomite. Winnipeg Formation (Northeast Wyoming) - Pink to yellow siltstone and shale. Deadwood Formation (Northeast Wyoming) - Red and brown quartzitic sandstone.

Sedimentary rocks (Paleocene and Cretaceous). (Paleocene and Cretaceous) at surface, covers < 0.1 % of this area

Fluvial conglomerate and sandstone of the Beaverhead Formation northwest of Dubois. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Sedimentary rocks (Cretaceous). (Cretaceous) at surface, covers < 0.1 % of this area

Marine and deltaic sandstone and shale of Cordilleran foreland basin (includes Gannett Group, Frontier, Mowry, Kootenay, and Thermopolis formations) in eastern Idaho thrust belt. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Basalt (Pliocene and Miocene). (Pliocene and Miocene) at surface, covers < 0.1 % of this area

Flows and cinder cones of olivine tholeiite basalt, and shallow basalt intrusives (~15-3 Ma); includes basalt in Owyhee County and southwest of Twin Falls, basalt of Weiser (basalt to andesite), basalt of Cuddy Mountain (alkali basalt and picro-basalt) north of Cambridge, basalt in Mount Bennett Hills north of Gooding, andesite at Square Mountain near Magic Reservoir, and Cub River diabase sill east of Preston. Includes gabbro at depth in cross section D-D’-D”. (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Absaroka Volcanic Supergroup: Washburn Group (Eocene) at surface, covers < 0.1 % of this area

Includes Sepulcher Formation (andesitic and dacitic volcaniclastic rocks), Lamar River Formation (andesitic lava and volcaniclastic rocks), and Cathedral Cliffs Formation (light-colored andesitic volcaniclastic rocks).

Sedimentary rocks (Devonian to Ordovician). (Devonian to Ordovician) at surface, covers < 0.1 % of this area

Marine dolostone and limestone and sandstone in east-central and southeastern Idaho (Summerhouse, Kinnikinic, and Swan Peak sandstones, Fish Haven, Laketown, Jefferson, Three Forks, and Darby formations) and deep-water carbonaceous mudrocks of Phi Kappa, Trail Creek, and Milligen formations east of Ketchum. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Sentinel Butte Formation (Paleocene) at surface, covers < 0.1 % of this area

Grayish-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 200 metres (600 feet).

Sedimentary rocks (Devonian to Cambrian). (Devonian to Cambrian) at surface, covers < 0.1 % of this area

Only shown in Henrys Lake area and in cross section. Consists of OCs (OCAs) and DSOs. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Pierre Shale (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray concretionary marine shale; contains several bentonite beds.

Rhyolite (Pleistocene). (Pleistocene) at surface, covers < 0.1 % of this area

Rhyolite tuffs, flows, and domes; includes Yellowstone Group (2.0-0.6 Ma) and isolated domes on Snake River Plain and north of Soda Springs (less than 2.0 Ma). (Quaternary to Eocene Continental Volcanic and Intrusive Rocks).

Syenitic intrusive rocks. (Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Syenite, quartz syenite, alkali-feldspar granite, and subordinate gabbro (500-485 Ma). Includes Beaverhead, Arnett Creek, Deep Creek, and Yellowjacket plutons southeast and west of Salmon. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Absaroka Volcanic Supergroup: Sunlight Group - Wapiti Formation (Eocene) at surface, covers < 0.1 % of this area

Andesitic volcaniclastic rocks.

Phosphoria through Lodgepole Formation: Phosphoria, Quadrant, Amsden, Heath, Otter, Kibbey, Mission Canyon, and Lodgepole Formations (Permian through Mississippian) at surface, covers < 0.1 % of this area

Phosphoria Formation (Pp): Gray and tan dolomite, cherty limestone, bedded chert, siltstone, sandstone, and gray to black phosphatic and dolomitic shale. Marine shelf and offshore marine. Thickness as much as 255 m (837 ft). Quadrant Formation (PAq): Very light gray, yellowish or pinkish, well-sorted sandstone or quartzite, locally interbedded with subordinate limestone beds. Marine. Generally, thickness as much as 140 m (460 ft), but as much as 800 m(2,625 ft) in southwestern-most Montana. Amsden Formation (PAMa): Red shale, light gray limestone, and cherty and sandy limestone. Coastal plain or marine. Thickness as much as 180 m (590 ft). Heath Formation (Mh): Black, fissile, commonly petroliferous shale with subordinate sandstone, gypsum, and coal. Restricted marine to coastal plain. Thickness as much as 150 m (492 ft). Otter Formation (Mo): Green, grayish green, gray, and subordinate reddish brown shale interbedded with light brown or gray limestone. Open and semirestricted marine. Thickness as much as 150 m (492 ft). Kibbey Formation (Mk): Red, quartzose sandstone, siltstone, and shale, locally with subordinate thin gypsum beds and limestone bed in middle part. Intertidal and subtidal. Thickness as much as 105 m (345 ft). Mission Canyon Formation (Mmc): Gray, massive limestone with chert beds and nodules, and solution breccia zones. Shallow marine. Thickness as much as 520 m (1,706 ft). Lodgepole Formation (Ml): Woodhurst Member (upper part of formation): light gray, well-bedded limestone, typically with much dark chert, interbedded with thinner calcareous mudstone beds. Paine Member (middle part of formation):dark gray, thin-bedded, silty or fossiliferous limestone. Cottonwood Canyon Member (lower part of formation): black shale with basal conglomeratic lag deposit. Shallow marine. Thickness as much as 305 m (1000 ft).

Alluvium and Colluvium (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Clay, silt, sand, and gravel in flood plains, fans, terraces, and slopes.

Lamprophyre (Tertiary) at surface, covers < 0.1 % of this area

Lamprophyre

Slope Formation (Paleocene) at surface, covers < 0.1 % of this area

Grayish-brown and yellowish-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 100 metres (300 feet).

Fort Union Formation - Tongue River Member (Paleocene) at surface, covers < 0.1 % of this area

Thick beds of yellow sandstone interbedded with gray and black shale and many coal beds.

Jefferson Formation (Devonian) at surface, covers < 0.1 % of this area

Birdbear Member (upper part of formation): light to medium gray, sucrosic dolomite. Lower part of formation: grayish black, commonly petroliferous dolomite or limestone that may be interbedded with light gray quartzite. Marine. Thickness as much as 520 m (1,706 ft).

Madison Group, Darby Formation, Three Forks and Jefferson Formations (Upper Devonian-Upper Mississippian) at surface, covers < 0.1 % of this area

Madison Limestone or Group (Thrust Belt, North Wyoming and Northern Yellowstone area) - Group includes Mission Canyon Limestone (blue-gray massive limestone and dolomite), underlain by Lodgepole Limestone (gray cherty limestone and dolomite). Darby Formation (Thrust Belt and North Wyoming) - Yellow and greenish-gray shale and dolomitic siltstone underlain by fetid brown dolomite and limestone. Three Forks Formation (Northern Yellowstone area) - Pink, yellow, and green dolomitic siltstone and shale. Jefferson Formation (Northern Yellowstone area) - Massive siliceous dolomite.

Granite gneiss (Late Archean) at surface, covers < 0.1 % of this area

Layered to massive, locally migmatitic; metasedimentary and metavolcanic rocks locally common. Includes Webb Canyon Gneiss in Teton Range.

Augen gneiss. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Biotite granite and granitic augen gneiss near Elk City and northwest of Salmon (~1380 Ma). Abundant alkali feldspar and high iron content indicate A-type granite composition. (Mesoproterozoic Intrusive Rocks).

Niobrara Formation and Carlile Shale (Upper Cretaceous) at surface, covers < 0.1 % of this area

Niobrara Formation (Kn) - Light-colored limestone and gray to yellow speckled limy shale. Carlile Shale (Kcl) - Dark-gray sandy shale; Sage Breaks Member at top; Turner Sandy Member in middle.

Gneissic and schistose metasedimentary rocks. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Biotite-rich paragneiss and schist, and subordinate feldspathic quartzite and calc-silicate rocks. Probably metamorphosed western facies of Lemhi subbasin. Characterized by igneous suite of Yam and Yag. Includes Elk City metamorphic sequence and metamorphic rocks east of Moscow, along Salmon River northwest of Salmon, and in Pioneer Mountains. (Mesoproterozoic Metasedimentary Rocks of Lemhi Subbasin of Belt Basin).

Fort Union Formation - Tullock Member (Paleocene) at surface, covers < 0.1 % of this area

Soft gray sandstone, gray and brown carbonaceous shale, and thin coal beds.

Fox Hills Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Olive-brown sand, shale, and sandstone; marine shoreline and off-shore sediment; as thick as 120 metres (400 feet).

Kinnikinic Formation (Ordovician) at surface, covers < 0.1 % of this area

Very light gray, massive quartzite with local lenses of dolomite and dolomitic shale. Marine shelf. Thickness as much as 245 m (804 ft).

Pierre Shale (Upper Cretaceous) at surface, covers < 0.1 % of this area

Blue-gray to dark-gray, fissile to blocky shale with persistent beds of bentonite, black organic shale, and light-brown chalky shale. Contains minor sandstone, conglomerate, and abundant carbonate and ferruginous concretions. Thickness up to 1,000 ft (305 m).

Alluvium (Quaternary) at surface, covers < 0.1 % of this area

Clay to boulder-size clasts with locally abundant organic material. Thickness up to 75 ft (23m).

Sedimentary rocks (Jurassic). (Jurassic) at surface, covers < 0.1 % of this area

Marginal marine and marine sandstone and limestone; includes Nugget, Twin Creek, Preuss, Morrison, and Stump formations in eastern Idaho thrust belt. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Three Forks, Jefferson, and Beartooth Formations and Bighorn Dolomite (Middle Ordovician-Upper Devonian) at surface, covers < 0.1 % of this area

Three Forks Formation (Northern Yellowstone area) - Pink, yellow, and green dolomitic siltstone and shale. Three Forks Formation (North Wyoming) - Yellow and greenish-gray shale and dolomitic siltstone. Jefferson Formation (Northern Yellowstone area) - Massive siliceous dolomite. Jefferson Formation (North Wyoming) - Fetid brown dolomite and limestone. Bighorn Dolomite (Northern Yellowstone area) - Light-gray massive siliceous dolomite. Bighorn Dolomite (North Wyoming) - Gray massive cliff-forming siliceous dolomite and locally dolomitic limestone. Beartooth Butte Formation - Red sandstone, limy siltstone, and limestone. Occurs only in the Beartooth Mountains.

Fox Hills Sandstone and Bearpaw Shale (Upper Cretaceous) at surface, covers < 0.1 % of this area

Fox Hills Sandstone (Kfh) - Light-colored sandstone and gray sandy shale containing marine fossils. Bearpaw Shale - Dark-greenish-gray shale containing thin gray sandstone partings.

Greenhorn Formation and Belle Fourche and Mowry Shale (Lower Cretaceous) at surface, covers < 0.1 % of this area

Greenhorn Formation - Light-colored limestone, marl, and limy sandstone interbedded with gray concretionary shale. Belle Fourche Shale - Black soft bentonitic concretionary shale. Mowry Shale (Kmr) - Silvery-gray hard siliceous shale containing abundant fish scales and bentonite beds.

Amphibolite. (Mesoproterozoic) at surface, covers < 0.1 % of this area

Amphibolite formed from metamorphism of mafic sills; includes rocks north- west of Salmon (~1380 Ma) and east of Elk City. (Mesoproterozoic Intrusive Rocks).

Fort Union Formation - Lebo Member (Paleocene) at surface, covers < 0.1 % of this area

Dark-gray clay shale and concretionary sandstone.

Hell Creek Formation (Late Cretaceous) at surface, covers < 0.1 % of this area

Tan to brown, light- to dark-gray, "somber beds" of shale. Interbedded with brown to red carbonaceous shale, gray and brown bentonitic silty shale, and gray, brown and yellow siltstone, sandstone, and claystone-pebble conglomerate. Thickness 260-600 ft (79-183 m).

Lance Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

(North Wyoming) - Thick-bedded buff sandstone and drab to green shale; thin conglomerate lenses. (South and Northeast Wyoming) - Brown and gray sandstone and shale; thin coal and carbonaceous shale beds.

Quartz monzonite of North Yellowstone (Late Archean) at surface, covers < 0.1 % of this area

Quartz monzonite of North Yellowstone

Tensleep Sandstone and Amsden Formation (Upper Mississippian-Permian) at surface, covers < 0.1 % of this area

Tensleep Sandstone (North Wyoming) - White to gray sandstone containing thin limestone and dolomite beds. Permian fossils have been found in the topmost beds of the Tensleep at some localities in Washakie Range, Owl Creek Mountains, and southern Bighorn Mountains. Amsden Formation (North Wyoming) - Red and green shale and dolomite; at base is brown sandstone. Tensleep Sandstone (Sorth Wyoming) - White to gray sandstone containing thin limestone and dolomite beds. Amsden Formation (South Wyoming) - Red and green shale and dolomite; at base is persistent red to brown sandstone.

Frontier Formation and Mowry and Thermopolis Shales (Lower Cretaceous-Upper Cretaceous) at surface, covers < 0.1 % of this area

Frontier Formation (Kf) - Gray sandstone and sandy shale. In Northern Yellowstone area, Yellowish- to medium-gray sandstone; tuffaceous and carbonaceous in lower part. Mowry Shale (Kmr) - Silvery-gray hard siliceous shale containing abundant fish scales and bentonite beds. Thermopolis Shale - Black soft fissile shale; Muddy Sandstone Member at top.

Chugwater and Goose Egg Formations (Permian-Upper Triassic) at surface, covers < 0.1 % of this area

Chugwater Formation (North Wyoming) - Red siltstone and shale. Alcova Limestone Member in upper middle part in north Wyoming. Thin gypsum partings near base in north and northeast Wyoming. Goose Egg Formation - Red sandstone and siltstone, white gypsum, halite, and purple to white dolomite and limestone. Chugwater Group or Formation (South Wyoming) - Red shale and siltstone containing thin gypsum partings near base. Group includes Popo Agie Formation (red shale and red, yellow, and purple siltstone; lenses of lime-pellet conglomerate), Crow Mountain Sandstone (red and gray, thick bedded), Alcova Limestone, and Red Peak Formation (red siltstone and shale). Chugwater Formation includes as members all the units listed above. Includes overlying Jelm Formation in Shirley and Seminoe Mountains and at northern end of Laramie Basin. Jelm Formation - Red sandstone.

Sedimentary rocks and sediments. (Oligocene and Eocene) at surface, covers < 0.1 % of this area

Fluvial and lacustrine deposits; includes conglomerate, sandstone, and shale near Salmon; Pass Creek Gravel, Wet Creek Gravel, and Donkey Fanglomerate north of Mackay; Medicine Lodge beds of southern Beaverhead Mountains; and isolated deposits in northern Idaho. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).

Absaroka Volcanic Supergroup: Thorofare Creek and Sunlight Groups (Eocene) at surface, covers < 0.1 % of this area

Thorofare Creek Group - Light-colored volcaniclastic strata, andesite lava flows, and dark-brown breccia; Sunlight Group - including Trout Peak Trachyandesite, Wapiti Formation (andesitic volcaniclastic rocks), Crescent Hill Basalt, and Mount Wallace Formation (felsic and mafic volcaniclastic rocks).

Thermopolis through Kootenai Formation: Thermopolis, Fall River, and Kootenai Formations (Cretaceous) at surface, covers < 0.1 % of this area

Thermopolis Formation (Kt): Dark gray to black shale with subordinate siltstone beds. Middle part of formation contains one or more thin sandstone beds with chert pebble horizons where Muddy Sandstone is not present. Offshore marine. Thickness as much as 305 m (1,000 ft). Fall River Formation (Kfr): Gray and brown, fine- to medium-grained, quartzose sandstone with thin interbeds of dark gray shale. Nearshore marine. Thickness as much as 70 m (1,214 ft). Kootenai Formation (Kk, Kku, Kkm, Kkl): Red, maroon, and olive gray mudstone, tan or gray siltstone, calcareous concretions, limestone beds, and several prominent sandstone beds that include the Greybull Member (top of the formation in south-central Montana) and the Sunburst Member (middle to lower part of the formation in northwest-central Montana), both dominantly quartzose sandstone. Basal Cutbank or Pryor Conglomerate. Alluvial plain with local marine influence in the north. Thickness as much as 335 m (1,099 ft).

Mesozoic and Paleozoic rocks (Upper Mississippian-Lower Cretaceous) at surface, covers < 0.1 % of this area

Mesozoic and Paleozoic rocks (North Wyoming). Shown in small areas of complex structure. East Flank of Absaroka Range - Dinwoody Formation, Phosphoria Formation and related rocks., Tensleep Sandstone, and Amsden Formation (Lower Triassic through Upper Mississippian). East flank of Bighorn Mountains - Cloverly, Morrison, Sundance, Gypsum Spring, Chugwater and Gypsum Spring Formations (Lower Cretaceous through Permian). Mesozoic and Paleozoic rocks (South Wyoming). Shown in small areas of complex structure. South side of Granite Mountains north of Green Mountain - Nugget Sandstone, Chugwater and Goose Egg Formations, Tensleep Sandstone, and Amsden Formation (Jurassic? through Upper Mississippian). South flank of Ferris Mountains--Nugget Sandstone and Chugwater and Goose Egg Formation (Jurassic? through Permian). Northeast flank of Seminoe Mountians--Cloverly, Morrison, Sundance, Chugwater, and Goose Egg Formations (Lower Cretaceous through Permain). West flank of Sierra Madre--Chugwater, Goose Egg, Casper, and Fountain Formations (Upper Triassic through Middle Pennsylvanian). East Flank of Laramie Mountains--Cloverly, Morrison, Sundance, Chugwater, and Goose Egg Formations, and, east of fault in T. 19 N., Casper Formation (Lower Creatceous through Middle Pennsylvanian). Nugget Sandstone (JTRn) (South) - Gray to dull-red, massive to coarsely crossbedded quartz sandstone.

Mesaverde Formation or Mesaverde Group (Upper Cretaceous) at surface, covers < 0.1 % of this area

Mesaverde Formation (north Wyoming) - Light-colored massive to thin-bedded sandstone, gray sandy shale, and coal beds. In Jackson Hole locally contains gold-bearing quartzite conglomerate. North of North Fork Powder River east of the Bighorn Mountains, consists solely of the Parkman Sandstone Member. Mesaverde Group (South Wyoming) - Includes Almond Formation, Ericson Sandstone, Rock Springs and Blair Formations in Rock Springs uplift; Almond Formation (white and brown soft sandstone, gray sandy shale, coal and carbonaceous shale), Pine Ridge Sandstone (light-gray sandstone and thin coal beds), and Allen Ridge (gray sandstone, shale, and thin coal beds) and Haystack Mountains (gray marine sandstone and shale) Formations in Rawlins uplift; Pine Ridge Sandstone (light-gray sandstone and thin coal beds) and Rock River Formation (soft sandstone and sandy shale) in Laramie Basin.

Ludlow Formation (Paleocene) at surface, covers < 0.1 % of this area

White, tan, yellow, and gray, cross-bedded, fine- to medium-grained,silty sandstone interbedded with locally bentonitic, gray siltstone, claystone, and sandy to silty claystone. Characterized by uranium-bearing lignite beds and "clinker" beds fromed by burning coalseams. Thickness up to 420 ft (128 m).

Glacial deposits (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Till and outwash of sand, gravel, and boulders.

Sedimentary rocks (Triassic). (Triassic) at surface, covers < 0.1 % of this area

Red mudrock and fine-grained sandstone and marine limestone; includes Dinwoody, Woodside, Thaynes, and Ankareh formations in eastern Idaho thrust belt. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Intrusive igneous rocks (Eocene) at surface, covers < 0.1 % of this area

Felsic and mafic igneous bodies; the larger are mainly felsic.

Ludlow Formation (Paleocene) at surface, covers < 0.1 % of this area

Grayish-brown and yellowish-brown silt, sand, clay, sandstone, and lignite; river, lake, and swamp sediment; as thick as 100 metres (300 feet).

Terrace Deposits (Quaternary) at surface, covers < 0.1 % of this area

Clay to boulder-size clasts deposited as pediments, paleochannels, and terrace fills of former flood plains. Thickness up to 75 ft (23m).

Frontier Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

(Thrust Belt) - White to brown sandstone and dark-gray shale; oyster coquina in upper part; coal and lignite in lower part. (North and South Wyoming) - Gray sandstone and sandy shale.

Mowry and Thermopolis Shales (Lower Cretaceous) at surface, covers < 0.1 % of this area

Mowry Shale (Kmr) - Silvery-gray hard siliceous shale containing abundant fish scales and bentonite beds. Thermopolis Shale - Black soft fissile shale; Muddy Sandstone Member at top.

Cloverly, Morrison, Sundance, and Gypsum Spring Formations (Middle Jurassic-Lower Cretaceous) at surface, covers < 0.1 % of this area

Cloverly Formation - Rusty sandstone at top, underlain by brightly variegated bentonitic claystone; chert-pebble conglomerate locally at base. Morrison Formation - Dully variegated claystone, nodular limestone, and gray silty sandstone. In southern Yellowstone and Jackson Hole areas the presence of Morrison is questionable. Sundance Formation (Js) - Greenish-gray glauconitic sandstone and shale, underlain by red and gray nonglauconitic sandstone and shale. Gypsum Spring Formation - Interbedded red shale, dolomite, and gypsum. In north Wyoming wedges out south in T. 39 N.

Madison Limestone, Darby or Three Forks, Jefferson, and Beartooth Butte Formations, and Bighorn Dolomite (Middle Ordovician-Upper Mississippian) at surface, covers < 0.1 % of this area

Madison Limestone or Group - Group includes Mission Canyon Limestone (blue-gray massive limestone and dolomite), underlain by Lodgepole Limestone (gray cherty limestone and dolomite). Darby Formation - Yellow and greenish-gray shale and dolomitic siltstone underlain by fetid brown dolomite and limestone. Three Forks Formation - Yellow and greenish-gray shale and dolomitic siltstone. Jefferson Formation - Fetid brown dolomite and limestone. Beartooth Butte Formation - Red sandstone, limy siltstone, and limestone. Occurs only in the Beartooth Mountains.

Gallatin Limestone, Gros Ventre Formation and equivalents, Flathead Sandstone, and Cambrian Rocks (Middle Cambrian-Upper Cambrian) at surface, covers < 0.1 % of this area

Gallatin Limestone (North Wyoming) - Blue-gray and yellow mottled hard dense limestone. Gros Ventre Formation (North Wyoming) - Soft green micaceous shale (Upper and Middle Cambrian Park Shale Member), underlain by blue-gray and yellow mottled hard dense limestone (Middle Cambrian Death Canyon Limestone Member), and soft green micaceous shale (Middle Cambrian Wolsey Shale Member). Flathead Sandstone (North Wyoming) - Dull-red quartzitic sandstone. Cambrian Rocks (South Wyoming) - South flank of Granite Mountains--Blue-gray and yellow mottled hard dense limestone interbedded with soft green micaceous shale; dull-red quartzitic sandstone at base. On and south of Rawlins uplift--Glauconitic quartzitic sandstone.

Fox Hills Sandstone (Upper Cretaceous) at surface, covers < 0.1 % of this area

Light-colored sandstone and gray sandy shale containing marine fossils.

Sedimentary rocks associated with Basin and Range extension. (Quaternary, Pliocene, and Miocene) at surface, covers < 0.1 % of this area

Fluvial, fan, and lacustrine deposits and intercalated volcanic rocks of the Basin and Range Province (~16-2 Ma); consolidated to weakly consolidated sandstone, siltstone, arkose, conglomerate, mudstone, tuffaceous sediment, basalt, basaltic tephra, and rhyolite tuff. Includes deposits of Lake Idaho (Idaho Group) in western Snake River Plain and Salt Lake Formation deposited in Basin and Range Province of east-central Idaho. (Quaternary to Eocene Continental Sediments and Sedimentary Rocks).

Alluvial-fan deposits. (Quaternary) at surface, covers < 0.1 % of this area

Gravel and subordinate sand and silt deposited at mouths of canyons; largest fans are in Basin and Range Province in east-central and southeastern Idaho. (Quaternary Sediments).

Windermere Supergroup. (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Upper part consists of shallow marine and fluvial quartzose sandstone, and minor siltite, shale, and limestone of the Brigham Group of southeast Idaho, Wilbert Formation of east-central Idaho, and quartzites of unknown affinity west of Borah Peak. More highly metamorphosed equivalents are quartzite, metaconglomerate, metasiltite, calc-silicate marble, and schist of Gospel Peaks successions C and D in central Idaho; mature quartzite, biotite schist, and minor calc-silicate rocks of Syringa metamorphic sequence east of Moscow; and schist and quartzite in the Albion Range (Elba Quartzite, schist of Upper Narrows, quartzite of Yost, schist of Stevens Spring, quartzite of Clarks Basin, schist of Mahogany Peaks, and Harrison Summit Quartzite). Lower part consists of diamictite, immature sandstone, and bimodal volcanic rocks related to continental rifting. Includes Pocatello Formation (700-665 Ma), formation of Leaton Gulch near Challis, Shedroof Conglomerate in extreme northwest Idaho, and metamorphic equivalents (schist, marble, calc-silicate rocks, metaconglomerate, and ~686 Ma metavolcanic rocks) of Gospel Peaks successions A and B in central Idaho. (Paleocene to Neoproterozoic Sedimentary and Igneous Rocks of the Cordilleran System).

Landslide deposits (Pleistocene-Holocene) at surface, covers < 0.1 % of this area

Locally includes intermixed landslide and glacial deposits, talus, and rock-glacier deposits.