This report describes the stratigraphy and structure of an area of about 5,000 square miles in northeastern Wyoming and adjacent parts of Montana and South Dakota. The area includes the northern end and part of the western side of the Black Hills uplift and the adjoining part of the Powder River Basin.About 11,000 feet of sedimentary rocks ranging in age from Mississippian to early Tertiary are exposed in the area, not including surficial deposits of Tertiary(?) and Quaternary age. The oldest rocks crop out in the southeastern part of the area and consist of 500 to 600 feet of light-gray cherty limestone that makes up the Early Mississippian Pahasapa limestone. Unconformably overlying the Pahasapa limestone is the Minnelusa formation of Pennsylvanian and Permian age. It is 650 to 800 feet thick in drill holes and at outcrops in the southeastern part of the •area, and it consists of interbedded lightgray and pink sandstone, gray sandy limestone and dolomite, some red shale and siltstone, and local beds of gypsum and. anhydrite. The Permian Opeche formation overlies the Minnelusa formation unconformably and comprises 60 to 90 feet of red fine-grained sandstone, siltstone, and silty• shale. Next in order is the Permian Minnekahta limestone, which is composed of light-gray and light purplish-gray thin-bedded limestone about 40 feet thick. The Spearfish formation, 450 to 825 feet thick, BLACK HILLS UPLIFT, WYOMING, MONTANA, SOUTH DAKOTA Weston County. It is composed of dark-gray shale, at places sandy and silty, interbedded with bentonite and numerous limestone and siderite concretions. The formation is divided into several members, which from base to top are the Gammon ferruginous member, 0 to about 1,000 feet thick; the Mitten black shale mem~r, 145 to about 870 feet thick; and the upper part of the Pierre shale including the Monument Hill bentonitic member 150 to about 220 feet thick, and the somewhat younger Kara bentonitic member about 100 feet thick. In the northern part of the area, the Gammon ferruginous member contains a zone of sandstone and sandy shale about 100 feet thick known as the Groat sandstone bed.The Fox Hills sandstone overlies the Pierre shale and is 150 to 200 feet thick. .Yellowish-gray sandstone and gray shale make up the Fox Hills sandstone at most places except in Carter County, Mont., where as much as 100 feet of very light gray cliff-forming sandstone crops out at the top of the formation and is mapped separately as the Colgate member .. Nonmarine sedimentary rocks overlie the Fox Hills sandstone and make up an essentially conformable sequence of sandstone, shale, •and coal beds that includes the Lance formation of Late Cretaceous age, the Fort Union formation of Paleocene age, and the W•asatch formation of Eocene age. The Lance formation is 1,600 feet thick in northern Weston County, Wyo., but thins northward to about 500 feet in Carter County, Mont. The overlying Fort Union formation, 1,500 to about 2,100 feet thick, also thins northward. It is divided into the Tullock member at the base...