The 1,500 ft. or so of Carboniferous rocks above the Madison group in the Big Snowy Mountains of central Montana are here divided into 6 lithically and faunally intergrading formations. They are believed to represent an almost continuous depositional sequence of mainly shallow-water sediments that formed near the center of an episodically subsiding marine basin during Late Mississippian and Early Pennsylvanian time. Marginal overlaps and locally discordant relations, attributable to intervals of accelerated subsidence of shoaling, are likely to be prevalent along the now mainly buried shelf-edges of this irregular basin. As a name for the entire intergrading sequence, the Big Snowy group of Scott is expanded and redefined to include, in ascending order, not only the Kibbey sandstone, Otter formation, and Heath formation of his original report, but also 3 overlying formations - the newly named Cameron Creek formation, the Alaska Bench limestone, and the newly named Devils Pocket formation. The Charles formation, which Seager added to the base of the Big Snowy group in 1942, was properly reassigned to the underlying Madison group by Sloss in 1952.