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The Uinta Formation, Uinta Basin, Utah, is the type formation for the Uintan North American Land Mammal Age (NALMA). Lack of a high-resolution stratigraphic section in the Uinta Formation has led to confusion regarding the nature of and position of the contact between Uinta B and Uinta C rocks. Historically, this lithostratigraphic boundary has been defined in various ways, which has also affected our understanding of the early Uintan to late Uintan faunal transition. We describe a new high-resolution stratigraphic section in the upper intervals of the Uinta Formation, in the eastern Uinta Basin of Uintah County. Since 1994, fossil collection in this region by parties from Washington University has resulted in more than 200 productive fossil vertebrate localities, with most of these localities correlated to the new stratigraphic section. The section was measured from the lowest productive fossil mammal locality to the …