Geologic units in Sully 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).

Eolian Deposits (Quaternary) at surface, covers 0.1 % 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).

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).