Named as the upper, dark-colored part of Sepulcher Formation of Washburn Group of Absaroka Volcanic Supergroup for Fortress Mountain. Type area is Bighorn Peak, MT in Montana folded belt province. Also recognized at Sepulcher Mountain, Yellowstone National Park, Yellowstone province, WY. Is mostly dark-gray coarse alluvial facies andesitic volcaniclastic rocks containing abundant hornblende and lesser amounts of pyroxene and biotite. Divisible into an upper and lower dark-gray cliff-forming, crudely bedded, poorly sorted conglomerate, and a middle unit of light-gray poorly resistant conglomerate with varicolored clasts in an ashy matrix. Tuffaceous sandstone, siltstone, mudstone, and petrified trees occur in each unit. Thickens from 800 to 1,400 ft, and grades eastward and upward from fine-grained alluvial facies to near-vent coarse alluvial facies. Extends north of Park into the northern Gallatin Range. Is probably equivalent to Hyalite Peak Volcanics. Fossil leaves in member are probably early middle Eocene.

Source: GNU records (USGS DDS-6; Denver GNULEX).