Judith River/White River Fossils
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Geology of the Judith River and White River formation groupsThe White River group is comprised of Oligocene and Miocene deposits covering an extensive area of South Dakota and Nebraska. About 35 million years ago this area was a broad floodplain, draining from the Rocky Mountains and Black Hills, and teeming with wildlife. Today it is a semiarid region of grasslands interspersed with badlands, culminating in the Big Badlands of South Dakota. The White Rover group consists of the Chadron and Brule formations, which are further divided into members. See the chart below. The Chadron formation erodes into rolling exposures while the Brule erodes into steeper faces (the "Wall", as in Wall Drug). Both formations consist of siltstones, sandstones, and conglomerates (river channels). Rain turns the surface into slippery mud, while just below, the rock remains hard. Similarly, the Judith River group consists of clay, mud, and sand deposits in an ancient floodplain near the Cretaceous Bearpaw sea. Fossils eroding from the White River formations are quickly destroyed by the elements, but those eroding out of the Judith River formations consist of very hard rock. I think there is less mineral replacement in the White River fossils. Below is a timeline showing geologic periods that with emphasis on the Judith River and White River formations.
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