Geologic units in Brule county, South Dakota

Pierre Shale (Upper Cretaceous) at surface, covers 92 % 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).

Niobrara Formation (Upper Cretaceous) at surface, covers 7 % 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).

Tertiary, undifferentiated (Pliocene to Miocene) at surface, covers 0.5 % of this area

Light greenish-brown and light yellowish-tan to orangish-brown clay, silty clay, and fine sand; light-brown to pink siltstone; green, massive, orthoquartzite conglomerate; and multicolored quartz- and feldspar-rich sand and gravel. Includes equivalents of the Ogallala Group Ash Hollow, Valentine, and Fort Randall Formations. Also includes the Thin Elk, Bon Homme, Herrick, Medicine Root, and "western derived" gravels. Thickness up to 70 ft (21 m).

Carlile Shale (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray to black, silty to sandy shale with several zones of septarian, fossiliferous, carbonate concretions. Contains up to three sandstone units in the upper portion of the formation and sandy calcareous marl at the base. Thickness up to 330 ft (100 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).