The Pawnee Buttes of northeastern Colorado lie within spectacular outcrops of Oligocene (White River Group) and Miocene (Ogallala Group) strata that form part of the Chalk Bluffs, an 150 mile long escarpment near the state line. These deposits were some of the earliest High Plains units to be explored by vertebrate paleontologists (Marsh, 1870; Cope, 1873). Consequently their fossil remains yielded the first information on North American faunas of mid- and late Tertiary time. Many parties subsequently exploited these deposits; their work, and his own investigations, were monographed by Galbreath in 1953. The present paper reviews the literature, a substantial part of the collections, as well as later stratigraphic work to present a revised litho- and biostratigraphy focused on the Pawnee Buttes area of eastern Weld County, Colorado. The oldest post-White River Group unit is the Martin Canyon Formation, the type locality of which is in adjacent Logan County. It contains an early Hemingfordian fauna as at its type section. The overlying Pawnee Creek Formation is restricted to the Weld County outcrops and is here differentiated from the younger Ogallala rocks formerly confused with it. The Pawnee Creek formation yields a latest early to early late Barstovian fauna that partially fills the hiatus between the classic Lower Snake Creek and Valentine faunas of Nebraska. Younger, but still early late Barstovian fauna, occur in the strongly disconformable undifferentiated Ogallala sands and gravels that underpin the High Plains surface. These assemblages have correlatives in the faunas of the lower part of the Valentine Formation of Nebraska.
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