Brule Formation
The Brule Formation was deposited between 33 and 30 million years ago, roughly the Rupelian age (). It occurs as a subunit of the White River Formation in Nebraska, Colorado, North Dakota, South Dakota, and Wyoming.. It is a sequence of fine grained clastic rocks (claystones, mudstones, siltstones) interbedded with freshwater carbonates, volcanic ash (), and sandstone.