Geologic units in McPherson 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).

Pierre Formation (Upper Cretaceous) at surface, covers < 0.1 % of this area

Dark-gray shale; marine offshore sediment; as thick as 700 metres (2,300 feet).