A distinctive new stratigraphic unit, herein named the Sharps formation, occurs conformably between the underlying Brule formation of the Oligocene White River group and the overlying Monroe Creek formation of the Miocene Arikaree group. The formation is named for exposures near Sharps Corner in Shannon County, southwestern South Dakota, and consists of 390 ft. of massive tan silt and volcanic ash. The basal volcanic ash bed is called the Rockyford ash, as much as 38 ft. thick. Amid the silt beds are scattered small calcareous "potato-ball" concretions, impure gastropod-bearing limestones, clastic and chalcedony dikes, channel sands and gravels, and algal limestones. The formation has been traced eastward 50 mi. or more, and southwestward for 75 mi. to near Harrison, Nebraska. The stratigraphic position of the Sharps formation is similar to that of the basal, channel-fill Gering formation of the Arikaree group in that both underlie the Monroe Creek formation directly, but the lithologic characteristics are dissimilar. The age of the Sharps formation appears to be very early Miocene, based on a vertebrate fauna that includes 17 genera and 25 species.