The Big Snowy group consists of a middle and upper Mississippian series of shales, sandstones, limestones, and evaporites. Recently drilled deep wells and new interpretations of older wells in Montana and the Dakotas yield information which makes possible further considerations on the subsurface extensions, correlations, and lithology of the Big Snowy sediments in the Williston basin. A lateral tracing of persistent lithologic units, the recognition of the removal of certain units by Mississippian erosion, and the addition of a basal unit not present in outcrop areas make possible satisfactory correlation of the subsurface Big Snowy in the Williston basin.

Paleogeographic studies reveal the influence of the ancestral Sweetgrass arch on the character of clastic sediments, in the Big Snowy group, and indicate the possibility of favorable reservoir conditions in untested areas adjacent to that positive element.