Geologic units in Lyman county, South Dakota

Pierre Shale (Upper Cretaceous) at surface, covers 62 % 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).

Eolian Deposits (Quaternary) at surface, covers 24 % 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).

Alluvium (Quaternary) at surface, covers 7 % 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 6 % 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).

Ogallala Group (Early Pleistocene to Miocene) at surface, covers 0.7 % 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).

Colluvium (Quaternary) at surface, covers 0.3 % of this area

Clay to boulder-size clasts forming rubble residuum and talus. Thickness up to 30 ft (9m).

Outwash, Terrace (Pleistocene-Upper Wisconsin) at surface, covers 0.3 % of this area

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

Dam (Unknown) at surface, covers < 0.1 % of this area