Geologic units in Broadwater county, Montana

Additional scientific data in this geographic area

Gravel (Quaternary) at surface, covers 19 % 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.

Newland Formation (Middle Proterozoic) at surface, covers 15 % of this area

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

Middle Tertiary sediment or sedimentary rock (Tertiary) at surface, covers 10 % 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).

Gravel (Quaternary and Tertiary) at surface, covers 8 % 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.

Elkhorn Mountains Volcanics (Cretaceous) at surface, covers 7 % 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.

Upper Tertiary sediment or sedimentary rock (Tertiary) at surface, covers 5 % 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).

Greyson Formation (Middle Proterozoic) at surface, covers 4 % 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).

Spokane Formation (Middle Proterozoic) at surface, covers 3 % of this area

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

Granitic rock (Cretaceous) at surface, covers 3 % of this area

Granitic rock

Pilgrim through Flathead Formation: Pilgrim, Park, Meagher, Wolsey, and Flathead Formations (Cambrian) at surface, covers 3 % 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).

Mission Canyon Formation: locally includes Big Snowy Formation in south-central Montana (Mississippian) at surface, covers 2 % 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).

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

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

Middle Greyson Formation (Middle Proterozoic) at surface, covers 2 % 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).

Lower Greyson Formation (Middle Proterozoic) at surface, covers 2 % 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).

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

Andesite

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 2 % 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).

Lodgepole Formation (Mississippian) at surface, covers 1 % 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).

Phosphoria Formation through Big Snowy Group: Phosphoria, Quadrant, and Amsden Formations, and Big Snowy Group (Permian through Mississippian) at surface, covers 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).

Slim Sam Formation (Cretaceous) at surface, covers 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).

Frontier Formation (Cretaceous) at surface, covers 1 % 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).

Upper Greyson Formation (Middle Proterozoic) at surface, covers 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).

Kootenai Formation through Ellis Group: Kootenai, Morrison, and Ellis Group (Cretaceous and Jurassic) at surface, covers 1.0 % 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).

Madison Group: Mission Canyon and Lodgepole Formations; or Castle Reef and Allan Mountain Formations (Mississippian) at surface, covers 0.5 % 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).

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

Monzodiorite

Empire Formation (Middle Proterozoic) at surface, covers 0.4 % 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.

Three Forks and Jefferson Formations (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).

Mowry and Thermopolis Formations (Cretaceous) at surface, covers 0.4 % 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).

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

Dacite

Kootenai Formation (Cretaceous) at surface, covers 0.3 % 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 and Ellis Group (Jurassic) at surface, covers 0.3 % 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).

Piegan Group: Helena and Wallace Formations (Middle Proterozoic) at surface, covers 0.2 % 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).

Diorite (Late Proterozoic) at surface, covers 0.2 % of this area

Diorite

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

Diorite

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

Monzonite

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

Latite

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

Andesite and diorite

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).

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).

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).

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).

Two Medicine Formation (Cretaceous) at surface, covers < 0.1 % 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).

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.

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

Basalt

Amsden Formation (Pennsylvanian and Mississippian) at surface, covers < 0.1 % 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).

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).

Gammon Formation (Cretaceous) at surface, covers < 0.1 % 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).

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

Intrusive rock