In the Black Hills area the marine Jurassic rocks comprising the Gypsum Spring and Sundance formations represent the upper Bajocian, Bathonian, Callovian, and Oxfordian stages of the Middle and Upper Jurassic. A minor disconformity was developed during the upper Bathonian and a major, regional disconformity during the middle to upper Callovian. Subdivisions of the Sundance formation from base to top are herein defined as the Canyon Springs sandstone member, the Stockade Beaver shale member, the Hulett sandstone member, the Lak member, and the Redwater shale member. Sandy sediments in the lower four members were probably derived locally from the east or southeast, in the upper member mainly from the west. The Gypsum Spring formation represents the first widespread invasion and withdrawal of marine waters during the Jurassic in the Western Interior region. It is Middle Jurassic in age and correlative with the Gypsum Spring and Sawtooth formations farther west. The Canyon Springs sandstone member and the Stockade Beaver shale member represent a widespread marine invasion at the beginning of the Upper Jurassic and are correlative with the lower part of the Suitdance formation of north-central Wyoming, the “Lower Sundance” of central Wyoming, and the Rierdon formation of Montana. The overlying marine Hulett sandstone member is correlative with similar sandstone in the “Lower Sundance” formation in central Wyoming. It is probably present in North Dakota and southeastern Montana. I t is missing in the outcrop in Montana but may be represented there by shale or shaly limestone at the top of the Rierdon formation. It probably passes westward in Wyoming into the upper part of the Twin Creek limestone. The Lak member consists of redbeds, is apparently non-marine, and is correlative at least in part with the Entrada and Preuss sandstones farther west. I t is possibly present in North Dakota but has not been identified in Montana. The Redwater shale member represents the last Jurassic marine invasion in the Western Interior region of the United States. It is mainly or entirely Oxfordian in age and correlative with highly glauconitic sandstone and shale farther west, variously called Swift formation, Stump sandstone, Curtis formation, and “Upper Sundance” formation.