From abstract: The northern Black Hills bentonite mining district includes parts of Crook County, Wyo., Carter County, Mont., and Butte County, S. Dak. Within this district, many beds of bentonite occur interspersed with sedimentary strata of Cretaceous age that have an average total thickness of about 3,000 feet and consist chiefly of marine shale, marl, and argillaceous sandstone. The bentonite beds occur in formations ranging upward from the Newcastle sandstone to the lower part of the Mitten black shale member of the Pierre shale. Tertiary (?) and Quaternary deposits of gravel, sand, and silt are present on extensive terraces, and deposits of such materials also extend along stream courses in all parts of the district.

A report which includes 200,000 square miles in the southern part of the Williston basin of North Dakota, South Dakota, and Montana. The Devonian rocks consist mostly of marine carbonate, evaporite, and shale beds.

This report studies Devonian rocks in an approximately 200,000-square-mile area between Williston Basin, central Montana, and north-central Wyoming.

From introduction: This report on the geology of the Powder River Basin is one of a series of reports by the Geological Survey on sedimentary basins in the United States.