Geologic units in Hughes county, South Dakota

Pierre Shale (Upper Cretaceous) at surface, covers 100 % 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).

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).

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)

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

Dam