Geologic units in Park county, Montana

Additional scientific data in this geographic area

Absaroka Volcanics Supergroup (Tertiary) at surface, covers 23 % 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.

Fort Union Formation (Tertiary and Cretaceous) at surface, covers 20 % 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.

Quartzofeldspathic gneiss (Archean) at surface, covers 10 % of this area

Quartzofeldspathic gneiss

Schist and gneiss (Archean) at surface, covers 5 % of this area

Schist and gneiss

Dacite (Tertiary) at surface, covers 4 % of this area

Dacite

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.

Billman Creek Formation (Cretaceous) at surface, covers 3 % 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).

Gravel (Quaternary and Tertiary) 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.

Gneiss and amphibolite (Archean) at surface, covers 2 % of this area

Gneiss and amphibolite

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

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

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.

Schist (Archean) at surface, covers 2 % of this area

Schist

Judith River through Eagle Formation: Judith River, Claggett, and Eagle Formations (Cretaceous) at surface, covers 2 % 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).

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 2 % 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).

Miner Creek and Cokedale Formations (Cretaceous) at surface, covers 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).

Madison Group: Mission Canyon and Lodgepole Formations; or Castle Reef and Allan Mountain Formations (Mississippian) at surface, covers 1 % 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).

Three Forks through Bighorn Formation: Three Forks, Jefferson, and Bighorn Formations (Mississippian through Ordovician) at surface, covers 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).

Sedan Formation (Cretaceous) at surface, covers 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).

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

Latite

Cody and Frontier Formations (Cretaceous) at surface, covers 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).

Hell Creek Formation (Cretaceous) at surface, covers 1 % 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).

Hoppers Formation (Cretaceous) at surface, covers 0.9 % 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).

Mowry through Fall River Formation: Mowry, Thermopolis, and Fall River Formations (Cretaceous) at surface, covers 0.9 % 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).

Eagle and Telegraph Creek Formations (Cretaceous) at surface, covers 0.8 % 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).

Morrison Formation and Ellis Group (Jurassic) at surface, covers 0.7 % 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).

Diorite and gabbro (Tertiary) at surface, covers 0.7 % of this area

Diorite and gabbro

Mylonite (Early Proterozoic) at surface, covers 0.6 % of this area

Mylonite

Pilgrim through Flathead Formation: Pilgrim, Park, Meagher, Wolsey, and Flathead Formations (Cambrian) at surface, covers 0.6 % 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).

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).

Quartzite and amphibolite (Archean) at surface, covers 0.5 % of this area

Quartzite and amphibolite

Quadrant and Amsden Formations (Pennsylvanian and Mississippian) at surface, covers 0.5 % 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).

Landslide Creek Formation (Cretaceous) at surface, covers 0.4 % 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).

Grove Creek through Flathead Formation: Grove Creek, Snowy Range, Pilgrim, Park, Meagher, Wolsey, and Flathead Formations (Cambrian) at surface, covers 0.4 % 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).

Frontier Formation (Cretaceous) at surface, covers 0.3 % 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).

Cody Formation (Cretaceous) at surface, covers 0.3 % 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).

Phosphoria Formation through Madison Group: Phosphoria, Quadrant, Amsden, Mission Canyon, and Lodgepole Formations (Permian through Mississippian) at surface, covers 0.2 % 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).

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.

Telegraph Creek Formation (Cretaceous) at surface, covers 0.2 % 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).

Phosphoria through Amsden Formation: Phosphoria, Quadrant, and Amsden Formations (Permian through Mississippian) at surface, covers 0.2 % 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).

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).

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).

Kootenai Formation through Ellis Group: Kootenai, Morrison, and Ellis Group (Cretaceous and Jurassic) 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). 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).

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

Granitic rock

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

Alkalic intrusive rock

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

Basalt

Sediment or sedimentary rock (Tertiary) at surface, covers < 0.1 % of this area

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

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

Intrusive rock

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.

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.

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.

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.

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

Diorite

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

Intrusive rock

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).

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

Diorite

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

Monzodiorite

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

Syenite

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

Syenite

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).

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).

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).

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.

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

Andesitic volcaniclastic rocks.

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.

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.

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

Lavender to gray-brown welded rhyolite tuff.

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

Quartz monzonite of North Yellowstone

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.

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.

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).

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

Till and outwash of sand, gravel, and boulders.

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

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

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.

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.