Geologic units in Cascade county, Montana

Additional scientific data in this geographic area

Middle Kootenai Formation (Cretaceous) at surface, covers 10 % of this area

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

Adel Mountain Volcanics (Cretaceous) at surface, covers 8 % 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.

Flood Member of Blackleaf Formation (Cretaceous) at surface, covers 7 % 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).

Taft Hill Member of Blackleaf Formation (Cretaceous) at surface, covers 6 % of this area

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

Lower Kootenai Formation (Cretaceous) at surface, covers 6 % of this area

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

Upper Kootenai Formation (Cretaceous) at surface, covers 6 % of this area

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

Bootlegger and Vaughn Members of Blackleaf Formation (Cretaceous) at surface, covers 5 % 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).

Mission Canyon Formation: locally includes Big Snowy Formation in south-central Montana (Mississippian) at surface, covers 5 % 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).

Bootlegger Member of Blackleaf Formation (Cretaceous) at surface, covers 5 % of this area

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

Alluvium (Quaternary) at surface, covers 5 % of this area

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

Morrison Formation and Ellis Group (Cretaceous and Jurassic) at surface, covers 4 % 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).

Ferdig Member of Marias River Formation (Cretaceous) at surface, covers 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).

Two Medicine Formation (Cretaceous) at surface, covers 3 % 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).

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).

Lodgepole Formation (Mississippian) at surface, covers 2 % 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).

Otter Formation (Mississippian) at surface, covers 2 % 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).

Quartzofeldspathic gneiss (Early Proterozoic) at surface, covers 2 % of this area

Quartzofeldspathic gneiss

Gravel (Quaternary) 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.

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).

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

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

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

Granitic rock

Flathead Formation (Cambrian) at surface, covers 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).

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

Floweree Member of Marias River Formation (Cretaceous) at surface, covers 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).

Gravel (Quaternary and Tertiary) at surface, covers 0.9 % 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.

Ferdig and Cone Members of Marias River Formation (Cretaceous) at surface, covers 0.8 % 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).

Cone Member of Marias River Formation (Cretaceous) at surface, covers 0.7 % 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).

Kibbey Formation (Mississippian) at surface, covers 0.7 % 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).

Telegraph Creek Formation (Cretaceous) at surface, covers 0.6 % 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).

Kootenai Formation (Cretaceous) at surface, covers 0.6 % 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).

Amsden Formation (Pennsylvanian and Mississippian) at surface, covers 0.6 % 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).

Blackleaf Formation (Cretaceous) at surface, covers 0.5 % 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).

Vaughn Member of Blackleaf Formation (Cretaceous) at surface, covers 0.5 % 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).

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

Shonkinite

Tyler Formation (Mississippian) at surface, covers 0.4 % 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).

Heath Formation (Mississippian) at surface, covers 0.3 % 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).

Gravel (Tertiary) at surface, covers 0.3 % 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.

Cone and Floweree Members of Marias River Formation (Cretaceous) at surface, covers 0.3 % 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).

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).

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

Mafic phonolite

Neihart Formation (Middle Proterozoic) at surface, covers 0.2 % 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).

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

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

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

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

Pilgrim through Flathead Formation: Pilgrim, Park, Meagher, Wolsey, and Flathead 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). 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).

Two Medicine and Virgelle Formations (Cretaceous) at surface, covers < 0.1 % 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).

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

Metabasite

Eagle and Telegraph Creek 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).

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

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

Syenite

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

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

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

Rhyolite

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).

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).

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

Basalt

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

Amphibolite

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

Diorite

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.

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).

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

Latite

Virgelle and Telegraph Creek Formations (Cretaceous) at surface, covers < 0.1 % 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).

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).

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

Dacite