The Chadron Formation is a Late Eocene geologic unit of the northern Great Plains, found in North Dakota, South Dakota, northwestern Nebraska and eastern Wyoming, named for the town of Chadron, Nebraska, and dating to roughly 37 to 34 million years ago. It is composed mainly of fine-grained clastic rock with secondary tuff and sandstone, overlies the Fox Hills Formation, the Yellow Mounds Paleosol and the Pierre Shale, and is overlain by the Brule Formation, together with which it makes up the larger White River Formation; it was mapped by Carl Vondra in the 1950s. The formation carries important White River Fauna material and gives its name to the Chadronian North American land-mammal age.
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