An outcrop of silt- and mudstone in Jerauld County, S. Dak., has produced a small collection of fossil vertebrates, including Monosaulax sp. and Merychippus sp. These strata, heretofore doubtfully referred to the Oligocene-Miocene and the Pliocene by authors, can now be dated as late Miocene (Barstovian), and are equivalent to the Valentine Formation. The fossil assemblage is named the Wessington Springs local fauna.

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