The stratigraphy of the Marshall formation should not be studied as an independent unit. The underlying Coldwater shale can be divided into an eastern sandy facies and a western dolomitic facies. The sandy facies is inseparable from the Marshall formation at most places. Redbeds occur in both the Marshall formation and the upper Coldwater sediments in eastern Michigan. They are unreliable as stratigraphic markers except in local areas.

Neither color nor grain size provides a suitable basis for a division of the Marshall into two members throughout the Michigan basin. The coarse-grained Napoleon sandstone (upper Marshall) is a mappable unit in parts of eastern Michigan but loses its identity westward.

The top of the Marshall grades into the overlying Michigan formation. The Michigan "Stray" sandstones are the result of an interfingering of the Marshall and Michigan deposits and should not be considered to be entirely of Michigan age. In addition to the recognized eastern source of sediments for the Marshall formation, a western source is apparent.