White River Formation

The White River Formation is a Paleogene geologic formation of the central United States, exposed across northeastern Colorado, western Nebraska, the Badlands region of western South Dakota and southeastern Wyoming, spanning the Late Eocene through Early Oligocene, roughly 37.2 to 30.8 million years ago, from the Chadronian through Whitneyan land-mammal ages. It consists mainly of tuffaceous claystone and conglomerate, 230 to 300 meters thick, and is subdivided into the Chadron and Brule Formations, spectacularly exposed in the White River Badlands of South Dakota. It preserves the most complete Late Eocene and Early Oligocene vertebrate fossil record in North America, documenting the transition between the two epochs.