The argillaceous rocks in the type section of the Morrison formation as described by Waldschmidt and LeRoy may be subdivided on a basis of their most abundant clay mineral. Kaolinite and illite clay mineral zones coincide with the upper unit subdivisions as proposed by Waldschmidt and LeRoy. A thick montmorillonite-rich zone near the center of the formation transects three of the Waldschmidt and LeRoy units. Kaolinite and illite zones are present in the lower unit. The clay minerals are used as indicators to interpret tentatively the environments of deposition for different parts of the Morrison.