The White River Badlands
Volcanic ash and river mud in South Dakota bury a complete mammal fauna, and erosion later cuts it open.
- When
- —
- Duration
- 5.9 million years
- Known to
- ± 1 million years
- Era
- Oligocene, Cenozoic
The White River Group records floodplains and channels on the eastern side of the rising Rockies, blanketed repeatedly by ash from the western eruptions. The fossil yield is exceptional in both quality and quantity — oreodonts, early horses, rhinos, entelodonts, nimravids and turtles, often complete and often in growth series. Because the sequence is thick, continuous and finely dated, it is the reference section for North American land mammal ages, and much of what is known about Oligocene mammal ecology comes from it.