Includes: Pahasapa Limestone (Mississippian)-White, light-gray to tan, fine- to medium-crystalline limestone and dolomite containing brown to gray chert. Solution features including collapse breccia, sinkholes, and caves are prevalent. Thickness 300-630 ft (91-192 m). Englewood Formation (Mississippian to Dovonian)- Pink, lavender to light-gray, thin- to medium-bedded, finely crystalline, argillaceous, dolomitic limestone. Thickness 30-63 ft (9-19 m).
Variegated, yellow to red, gray to brown, pink to purple, and black, interbedded sandstone, siltstone, shale, limestone, dolomite, calcarenite, chert and brecciated beds. Thickness 394-1,175 ft (120-358 m).
Whitewood Limestone (Ordovician)- Mottled, tan, gray to lavender, fine- to medium-crystalline, sparsely fossiliferous limestone and dolomite. Thickness up to 70 ft (21 m). Winnipeg Formation (Ordovician)- Grat and light-green, fissile shale, and tan, calcareous siltstone, sandy shale, and limestone lenses. Thickness up to 110 ft (34 m). Deadwood Formation (Ordovician to Cambrian)- Variegated, yellow to red, brown, gray, and green, glauconitic, conglomerate, sandstone, shale, dolomitic limestone, and dolomite. Thickness 4-400 ft (1-122 m).
Red sandy shale, siltstone, sandstone, and minor limestone. Interbedded with abundant gypsum. Thickness 328-559 ft (100-170 m).
Clay to boulder-size clasts with locally abundant organic material. Thickness up to 75 ft (23m).
Minnekahta Limestone-Purple to gray, finely crystalline, thin- to medium-bedded limestone with varying amounts of red shale. Thickness 30-50 ft (9-15 m). Opeche Shale- Red siltstone, argillaceous sandstone and shale interbedded with caliche layers. Thickness 85-130 ft (26-40 m).
Gray to dark-gray phylite, slate, and mica schist. Estimated thickness at least 5,000 ft (1,524 m).
Mowry Shale- Black to gray, siliceous, fissile shale and siltstone containing bentonite layers, and sparse sandstone dikes and sills. Thickness 125-250 ft (38-76 m). Newcastle Sandstone- Gray, light-brown to yellow, discontinuously distributed siltstone, claystone, sandy shale, and fine-grained sandstone. Thickness up to 290 ft (88 m). Skull Creek Shale- Dark-gray to blueish-gray shale containing ferruginous, and carbonate concretions. Thickness 150-275 ft (46-84 m).
Includes: Fall River Formation- Variegated brown, red, gray to purple, calcareous, well-sorted, fine-grained sandstone, siltstone, and shale containing mica flakes. Thickness 100-200 ft (30-61 m). Lakota Formation- Yellow, brown, red-brown, gray to black silty shale, pebble conglomerate, and massive to thin-bedded, cross-bedded sandstone. Locally interbedded with fresh-water limestone and bituminous coal beds. Thickness 35-500 ft (11-152 m).
Light- to dark-gray, silceous mica schist and impure quartzite. Differentiated where possible into three primary tongues or lenses. (Xgw1, Xgw2, and Xgw3) Thickness from 1,000 ft to over 5,000 ft (305-1,524 m).
Clay to boulder-size clasts deposited as pediments, paleochannels, and terrace fills of former flood plains. Thickness up to 75 ft (23m).
Medium-gray to dark-greenish-gray phyllite, slate, and biotite schist containing minor chert and amphibolite. Locally intruded by thin metagabbro sills. Thickness 1,000-3,000 ft (305-914 m).
Morrison Formation (Late Jurassic)- Light-gray to green and variegatedred, brown, yellow, or lavender, silceous claystone, shale, and siltstone containing interbedded sandstone and fresh-water limestone lenses. Thickness up to 150 ft (46 m). Unkpapa Sandstone (Late Jurassic)- White, massive to thin-bedded, fine-grained, argillaceous sandstone. May be variegated to banded red, yellow, brown, or lavender. Thickness up to 267 ft (81 m). Sundance Formation (Late to Middle Jurassic)- Greenish-gray, yellow, tan, red to orange, and white, variegated, interbedded, fine- to coarse-grained sandstone, siltstone, clay, and limestone. Thickness 250-350 ft (76-107 m). Gypsum Springs Formation (Middle Jusassic)- Massive white gypsum and minor maroon siltstone and shale. Thickness up to 40 ft (12 m).
Note: see individual descriptions
Light-tan to light-gray stocks and small laccoliths of rhyolite. Contains phenocrysts of oligoclase, quartz, and biotite in a finely crystalline orthoclase or sanidine-quartz groundmass.
Tan to reddish-brown, iron-stained stocks, laccoliths, sills, and dikes of trachyte, quartz trachyte, and alkalic rhyolite. Contains phenocrysts of sanidine, orrthoclase, anorthoclase, aegirine-augite and biotite in a finely crystalline orthoclase-quartz biotite groundmass.
Light-tan quartzite, siliceous schist, and minor chert. Thickness 800-5,000 ft (244-1,524 m).
Dark-gray to greenish-gray laccoliths and sills of latite, latitic andesite, and quartz latite. Contains phenocrysts of andesine, oligoclase, biotite, hornblend, and sphene in a finely crystalline andesine-biotite-quartz groundmass.
Light-gray to gray, conglomeratic and feldspathic schist, biotite schist, taconite, and phyllite. Individual conglomerate and fanglomerate tongues from 100-500 ft (30-152 m) thick. Total thickness over 10,000 ft (3,048 m).
Dark-green amphibolite, actinolite schist, and greenstone. Interflow units consists of graphitic schist, chert, and carbonate- and silicate-facies iron-formation. Thickness of individual flows 50-400 ft (244-1,524 m).
Includes: Brule Formation (Oligocene)- White, pink, light-green, and light-brown, massive to thin-bedded, bentonitic claystone, tuffaceous siltstone, and well-bedded, calcareous, tuffaceous quartz sandstone. Thickness up to 150 ft (46 m). Chadron Formation (Eocene)- Upper beds are gray, light-brown to maroon bentonite, claystone, siltstone tuffaceous fine-grained sandstone, and local, silicified carbonate lenses. Basal portion consists of poorly cemented, white, coarse-grained arkose and conglomerate. Thickness up to 160 ft (49 m). Chamberlain Pass Formation (Eocene)- Pale olive to pale red, mottled mudstone containing white, cross-bedded channel sandstone with basal conglomerate. Thickness up to 32 ft (10 m). Slim Buttes Formation (Eocene)- White, grayish- to yellowish-orange,pale-red to pink siltstone, clayey siltstone, bentonitic claystone,medium- to fine-grained sandstone, and conglomerate. Thickness up to 48 ft (15 m).
Dark-gray to black bentonitic shale containing minor limestone lenses, bentonite layers, fossiliferous calcarenite, and large, ferruginous, carbonate concretions. Thickness 150-350 ft (46-107 m).
Gray to greenish-gray sills, laccoliths, and small stocks of phonolite, trachyphonolite, and trachyandesite. Contains phenocrysts of andesine, feldspathoids, aegirine-augite, biotite, and sphene in a finely crystalline plagioclase-biotote-feldsphoid groundmass.
Dark-gray to gray, siliceous biotite phyllite and schist. Thickness greater than 2,500 ft (762m).
Tan to light-gray, conglomeratic siliceous schist, feldspathic schist, and minor marble. Thickness locally over 6,000 ft (1,829 m).
Gray, siliceous mica schist and impure quartzite. Thickness undetermined.
Dark-green sills of amphibolite, actinolite schist, greenstone, and serpentine. Thickness of sills variable, up to 1,000 ft (305 m).
Dark-gray biotite schist, biotite-muscovite schist, pyritic biotite schist, and local massive chert beds. Thickness approximately 2000-4000 ft (610-1,219 m).
Dark-gray to gray, siliceous biotite phyllite, calcareous biotite phyllite, and schist. Minimum thickness 1,500 ft (457 m).
Clay to boulder-size clasts forming rubble residuum and talus. Thickness up to 30 ft (9m).
Light-gray to light-tan marble, phyllite, and calcareous phyllite. Thickness 60-300 ft (18-91 m).
Alkalic basalt, greenstone, and actinolite schist. Includes metamorphosed volcanoclastic rocks and iron-rich schist.
Pink and gray, strongly foliated, medium- to coarse-crystalline, locally pegmatitic, biotite-muscovite granite and gneissic granite.
Banded, dark-green, reddish-brown, and white iron-formation, ferruginous chert, and minor mica schist. Includes three or more ages of oxide-, carbonate-, silicate-, and sulifide-facies iron-formation and interbedded tuffaceous rocks. Thickness 20-500 ft (6-152 m).
Light-gray to light-tan, muscovite schist and muscovite phylite. Thickness approximately 1,000-3,000 ft (305-914 m).
Gray phyillite, mica schist, and biotite-plagioclase schist. Total thickness unknown; approximately 500 ft (152 m) exposed.
Buff and red limy sandstone; some thin limestone beds, solution breccias, and gypsum.
Upper Xgw- Light- to dark-gray, silceous mica schist and impure quartzite.
Dark-green amphibolite and amphibolite schist. Thickness of individual flows 50-200 ft (15-61 m).
Red shale, red siltstone, and white gypsum beds; gypsum beds especially abundant near base.
Light-tan to pink pegmatite
Clay, silt, sand, and gravel in flood plains, fans, terraces, and slopes.
Gray shale, mudstone, marl, calcarenite, and shaly limestone grading upward into light-gray to tan, alternating marl and thin-bedded, fossiliferous limestone. Thickness up to 40 ft (12 m).
Mostly locally derived clasts. Includes some glacial deposits along east flank of Wind River Range. Locally includes some Tertiary gravels.
Minnekahta Limestone - Gray slabby hard limestone. Locally is a member of the Goose Egg Formation. Opeche Shale - Red soft sandy shale. Locally is a member of the Goose Egg Formation.
Pahasapa Limestone - Gray massive dolomitic limestone. Englewood Limestone - Pink slabby dolomitic limestone.