Geologic units in Dewey county, South Dakota

Pierre Shale (Upper Cretaceous) at surface, covers 55 % 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).

Fox Hills Sandstone (Upper Cretaceous) at surface, covers 30 % 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).

Alluvium (Quaternary) at surface, covers 8 % of this area

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

Hell Creek Formation (Late Cretaceous) at surface, covers 4 % 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).

Terrace Deposits (Quaternary) at surface, covers 2 % 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).

Eolian Deposits (Quaternary) at surface, covers 0.3 % 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 0.2 % of this area

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

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)

Till, Ground Moraine (Pleistocene-Upper Wisconsin) at surface, covers < 0.1 % 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).