View of a pinnacle of sedimentary beds of the Brule Formation (Oligocene age) near the Castle Trail parking area in the Big Badlands area of Badlands National Park. The Brule Formation is park of the White River Group of Tertiary age (Eocene to Miocene). The fossiliferous sedimentary rocks formed from the accumulation of terrestrial stream channel and flood plain deposits on an alluvial plain east of the early Rocky Mountains in what is today western South Dakota, western Nebraska, and eastern Wyoming (Martin, 1987). Click here to see a stratigraphic chart for Badlands National Park.