The unit is dominated by mudrock that accumulated in offshore and marine environments of the Cretaceous North American Inland Sea.

[4] The subunits and intertonguing formations (in italics) in each basin, in stratigraphic order, are: The Mancos Shale was first named by Charles Whitman Cross and C.W.

Lee had traced the unit north into the Grand Mesa area, defining it as all marine shale between the Dakota and the Mesaverde.

Knowlton found that the Mancos Shale could be divided into biostratigraphic layers corresponding closely to formations of the Colorado Group further east.

By 1944, Rankin had concluded that most of the formations of the Colorado Group could be identified as lithostratigraphic members of the Mancos Shale as well.