About 1,157 to 1,188 m (3,800-3,900 ft) of marine Upper Cretaceous rocks, chiefly shale, crop out around the northern flank of the Black Hills uplift. The formations are, from oldest to youngest: Mowry Shale, Belle Fourche Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, Pierre Shale, and Fox Hills Sandstone. Formally named members are applied to parts of the Greenhorn Formation and Pierre Shale, and all of the Carlile Shale. Ammonites are especially diverse and well preserved in calcareous concretions in the Belle Fourche Shale and Greenhorn Formation; 12 genera (19 species) are known from the Belle Fourche, and 16 genera (18 species) from the Greenhorn. The Carlile and Pierre shales contain well-preserved, but less diverse faunas. Only two ammonite genera are known from the Mowry Shale, and none is recorded from the Niobrara Formation and Fox Hills Sandstone.