Geologic units in Weston county, Wyoming

Lance Formation (Upper Cretaceous) at surface, covers 25 % of this area

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

Fort Union Formation - Tullock Member (Paleocene) at surface, covers 18 % of this area

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

Pierre Shale (Upper Cretaceous) at surface, covers 13 % of this area

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

Fort Union Formation - Lebo Member (Paleocene) at surface, covers 9 % of this area

Dark-gray clay shale and concretionary sandstone.

Cloverly and Morrison Formations or Inyan Kara Group (Upper Jurassic-Lower Cretaceous) at surface, covers 7 % of this area

Cloverly Formation (North and South Wyoming) - Rusty sandstone at top, underlain by brightly variegated bentonitic claystone; chert-pebble conglomerate locally at base. Cloverly Formation (Northeast Wyoming - Hartville Uplift) - Rusty to light-gray sandstone containing lenticular chert-pebble conglomerate interbedded with variegated bentonitic claystone. Morrison Formation (North and South Wyoming) - Dully variegated claystone, nodular limestone, and gray silty sandstone. In southern Yellowstone and Jackson Hole areas the presence of Morrison is questionable. Morrison Formation (Northeast Wyoming) - dully variegated siliceous claystone, nodular white limestone, and gray silty sandstone. Inyan Kara Group (Northeast - Black Hills) - Rusty to light-gray sandstone containing lenticular chert-pebble conglomerate interbedded with variegated bentonitic claystone. Includes Fall River and Lakota Formations.

Alluvium and Colluvium (Pleistocene-Holocene) at surface, covers 4 % of this area

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

Greenhorn Formation and Belle Fourche and Mowry Shale (Lower Cretaceous) at surface, covers 4 % of this area

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

Newcastle Sandstone and Skull Creek Shale (Lower Cretaceous) at surface, covers 4 % of this area

Newcastle Sandstone - Gray sandstone and sandy shale containing some bentonite and coal. Skull Creek Shale - Black soft fissile shale.

Fox Hills Sandstone (Upper Cretaceous) at surface, covers 3 % of this area

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

Spearfish Formation (Permian-Upper Triassic) at surface, covers 3 % of this area

Red shale, red siltstone, and white gypsum beds; gypsum beds especially abundant near base.

Minnekahta Limestone and Opeche Shale (Permian) at surface, covers 2 % of this area

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.

Sundance and Gypsum Spring Formations (Middle Jurassic-Upper Jurassic) at surface, covers 2 % of this area

Sundance Formation (Js) - Greenish-gray glauconitic sandstone and shale, underlain by red and gray nonglauconitic sandstone and shale. Gypsum Spring Formation - Interbedded red shale, dolomite, and gypsum. In north Wyoming wedges out south in T. 39 N.

Niobrara Formation and Carlile Shale (Upper Cretaceous) at surface, covers 2 % of this area

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

Minnelusa Formation (Lower Pennsylvanian-Permian) at surface, covers 2 % of this area

Buff and red limy sandstone; some thin limestone beds, solution breccias, and gypsum.

Landslide deposits (Pleistocene-Holocene) at surface, covers 1 % of this area

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

Mowry Shale (Lower Cretaceous) at surface, covers 0.6 % of this area

Silvery-gray hard siliceous shale containing abundant fish scales and bentonite beds.

Greenhorn Formation and Belle Fourche Shale (Upper Cretaceous) at surface, covers 0.4 % of this area

Greenhorn Formation - Light-colored limestone, marl, and limy sandstone interbedded with gray concretionary shale. Belle Fourche Shale - Black soft bentonitic concretionary shale. Carlile Shale (Kcl) - Dark-gray sandy shale; Sage Breaks Member at top; Turner Sandy Member in middle.

Gravel, pediment, and fan deposits (Pleistocene-Holocene) at surface, covers 0.4 % of this area

Mostly locally derived clasts. Includes some glacial deposits along east flank of Wind River Range. Locally includes some Tertiary gravels.

Pahasapa and Englewood Limestones (Upper Devonian-Lower Mississippian) at surface, covers < 0.1 % of this area

Pahasapa Limestone - Gray massive dolomitic limestone. Englewood Limestone - Pink slabby dolomitic limestone.

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

Dark-gray sandy shale; Sage Breaks Member at top; Turner Sandy Member in middle.

Minnelusa Formation (Permian to Pennsylvanian) at surface, covers < 0.1 % of this area

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).

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

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

Inyan Kara Group (Early Cretaceous) at surface, covers < 0.1 % of this area

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).

Mowry Shale, Newcastle Sandstone, and Skull Creek Shale (Early Cretaceous) at surface, covers < 0.1 % of this area

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).

Minnekahta Limestone and Opeche Shale (Permian) at surface, covers < 0.1 % of this area

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).

Madison Group (Mississippian to Devonian) at surface, covers < 0.1 % of this area

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).

Spearfish Formation (Triassic to Permian) at surface, covers < 0.1 % of this area

Red sandy shale, siltstone, sandstone, and minor limestone. Interbedded with abundant gypsum. Thickness 328-559 ft (100-170 m).

Morrison Formation, Unkpapa Sandstone, Sundance Formation, and Gypsum Springs Formation (Late Jurassic to Middle Jusassic) at surface, covers < 0.1 % of this area

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).

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

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).