The Geology of
The Rainy ButtesGeologically the buttes are a wondrous example of the power of nature. The caprock of both East and West Rainy are the remains of an ancient riverbed and floodplain, part of the Arikaree formation, dating to 28 million years ago, making them some of the youngest rocks exposed in North Dakota. The Rainy Buttes rise 400 to 500 feet above the surrounding prairie to a height of 3,310 feet above sea level, and share similar sedimentary structure with other buttes in the southwest corner of the state with which they were once connected. With the melting of the ice from the last ice age all of the 500+ feet of intervening land between these buttes was removed down to the level of the fields below. The buttes were islands as the water washed the surrounding land southward. As can be seen by the pattern of deposition in the side profile of the caprock, it is the remnant of an ancient river and animals buried in its mud were protected from decay and were fossilized as silica replaced the calcium in the bone.Geologists surmise that the hard caprock of the buttes is the reason that the buttes remained while the less resistant land surrounding them was removed.
Below the caprock lie sandstones that belong to the White River Formation of the Oligocene Period in the Tertiary Age dating 30-35 million years B.P. The Little Badlands between Schefield and South Heart and the Chalky Buttes near Amidon are also of the White River Formation. The farther you walk down the side of the butte the older the formations until at the bottom you reach the Sentinel Butte Formation of the Paleocene period of the Tertiary Age, which is the type of soil which makes up most of the land around New England. This formation is particularly known for its Petrified Wood deposits.
Embedded in the caprock are fossils that tell a story of the fauna that existed at the time of deposition. Fossils found on the buttes include Oreodont (giant pig), a giant cat (similar to a saber-tooth tiger), and bones of three toed horses, which stood only 18 inches high.
[In part excerpted from a publication titled: The Chadron, Brule and Arikaree Formations In North Dakota by E.Murphy, J. Hoganson and N. Forsman, North Dakota Geological Survey]