The marine Pierre shale, of Late Cretaceous age, on the northern and western flanks of the Black Hills, consists of dark gray shale with some sandy shale and sandstone, and many beds of bentonite. It ranges in thickness from 2,000 to 2,700 ft. In this area the Pierre shale may be divided into several members on the basis of lithologic differences in the shale and the presence of sandy and bentonitic units. In ascending order, these are the Gammon ferruginous member (including the Groat sandstone bed), the Mitten black shale member, and an unnamed upper part that consists of half or more of the formation. This upper part is chiefly dark gray shale but contains at the base a gray silty shale unit and near the top the Monument Hill and Kara bentonitic members. Studies in progress by Cobban reveal at least 16 ammonite zones in the Pierre shale on the western flank of the Black Hills. Appended is selected list of fossil collections from Pierre shale, Gammon ferruginous member.