Geologic units in Corson county, South Dakota

Hell Creek Formation (Late Cretaceous) at surface, covers 38 % of this area

Tan to brown, light- to dark-gray, "somber beds" of shale. Interbedded with brown to red carbonaceous shale, gray and brown bentonitic silty shale, and gray, brown and yellow siltstone, sandstone, and claystone-pebble conglomerate. Thickness 260-600 ft (79-183 m).

Fox Hills Sandstone (Upper Cretaceous) at surface, covers 32 % of this area

Bluish-green to green, white to dark-gray, and yellow to tan, carbonaceous and iron-stained, cross-bedded, very fine- to coarse-grained, glauconitic sandstone and siltstone. Interbedded with gray and green to brown shale and siltyshale. Thickness up to 200 ft (61 m).

Pierre Shale (Upper Cretaceous) at surface, covers 11 % of this area

Blue-gray to dark-gray, fissile to blocky shale with persistent beds of bentonite, black organic shale, and light-brown chalky shale. Contains minor sandstone, conglomerate, and abundant carbonate and ferruginous concretions. Thickness up to 1,000 ft (305 m).

Ludlow Formation (Paleocene) at surface, covers 7 % of this area

White, tan, yellow, and gray, cross-bedded, fine- to medium-grained,silty sandstone interbedded with locally bentonitic, gray siltstone, claystone, and sandy to silty claystone. Characterized by uranium-bearing lignite beds and "clinker" beds fromed by burning coalseams. Thickness up to 420 ft (128 m).

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

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

Terrace Deposits (Quaternary) at surface, covers 4 % of this area

Clay to boulder-size clasts deposited as pediments, paleochannels, and terrace fills of former flood plains. Thickness up to 75 ft (23m).

Cannonball Formation (Paleocene) at surface, covers 2 % of this area

Gray and tan siltstone, sandy to silty claystone, and fine-grained, calcareous clayely to silty sandstone, and abundant, round to lenticular carbonate concretions. Thickness up to 180 ft (55 m).

Eolian Deposits (Quaternary) at surface, covers 1 % of this area

(loess and sand dune) Silt to medium-grained sand. Deposited as sand sheets and barchan, linear, and dome-like dunes and as veneer on uplands. Thickness up to 300 ft (91m).

Landslide Deposits (Quaternary) at surface, covers 0.5 % of this area

Landslide, slump, and collapsed material composed of chaotically mixed boulders and finer grained rock debris. Thickness up to 180 ft (55m).

Till, Ground Moraine (Pleistocene-Upper Wisconsin) at surface, covers 0.2 % of this area

Heterogeneous, clay with silt to boulder-size clasts of glacial orgin. A geomorphic feature that is characterized by smooth, rolling terrain. Composite thickness of all Upper Wisconsin till may be up to 300 ft (91 m).

Ogallala Group (Early Pleistocene to Miocene) at surface, covers < 0.1 % of this area

Includes: Ash Hollow Formation- White, tan, and gray, well-cemented, calcareous sandstone and silty limestone often referred to as "mortar beds". Thickness 90-250 ft 27-76 m) Valentine Formation- Gray, unconsolidated, fine- to coarse grained, fluvial siltstone, channel sandstone, and gravel derived from western sources. Thickness 175-225 ft (53-69 m). Fort Randall Formation- Pink and gray claystone with interbedded sandstone. Also includes green to gray orthoquartzite, bentonitic clay, and conglomerate. Thickness up to 130 ft (40 m).

Outwash, Terrace (Pleistocene-Upper Wisconsin) at surface, covers < 0.1 % of this area

Heterogeneous clay to gravel of glaciofluvial orgin. Thickness up to 60 ft (18m)

Pierre Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray shale; marine offshore sediment; as thick as 700 metres (2,300 feet).

Niobrara Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

White to dark-gray argillaceous chalk, marl, and shale. Weathers yellow to orange. Contains thin, laterally continuous bentonite beds, chalky carbonaceous shale, minor sand, and small concretions. Thickness up to 150 ft (46 m).

Hell Creek Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Gray sand, silt, clay, and sandstone; river sediment; as thick as 150 metres (500 feet).

Fox Hills Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Olive-brown sand, shale, and sandstone; marine shoreline and off-shore sediment; as thick as 120 metres (400 feet).