The second to the upper most member of Pierre Shale dating to the Maastrichtian Stage of the Upper/Late Cretaceous Period. As with all members of this formation, it was generated from sediments deposited in the Western Interior Seaway of North America.
W. V. Searight described the Mobridge Member as "...a succession of highly calcareous shale, marl, and chalk beds which lies above the gumboforming shales of the Virgin Creek Member and below the noncalcareous shale beds of the Elk Butte Member. The member forms a wide buff band on the outcrop between the dark color of the underlying and overlying beds." It "...consists of medium gray and medium bluish gray to dark gray shale and chalk" and is "...everywhere highly calcareous and more or less chalky."*
*Searight, W. V., 1937, Lithologic stratigraphy of the Pierre Formation of the Missouri Valley in South Dakota