Marshall Sandstone consists of a lower Marshall and Napoleon member. The upper member is recognizable only in exposures in the southern part of the basin. The Marshall conformably overlies the Coldwater Shale and conformably underlies the Michigan Formation. The total thickness of the Marshall appears to be vary between 40 and 110 m , the maximum occurring near the basin center in Ionia Co. Thins toward west and north. The upper, or Napoleon Sandstone Member reaches its maximum thickness of 49 to 61 m in the southern part of the basin . Generally, the Marshall is sparsely fossiliferous, sometimes cross-bedded and rippled, very fine- to coarse-grained sandstone of buff, tan, or gray color. The Napoleon is predominantly a medium-grained sandstone with some coarse sand intervals. It is nonfossiliferous and commonly cross-bedded and rippled. The lower Marshall, on the other hand, is mostly fine-grained sandstone with some very fine sand intervals, It is fossiliferous and generally flat-bedded. Age is Osagean.

Source: GNU records (USGS DDS-6; Reston GNULEX).