The Pennsylvanian and Permian (?) sediments in west-central and northwestern Colorado crop out in an area of nearly 2300 square miles. Deposited in a trough between the Front Range landmass and Uncompahgre landmass, they consist mainly of red beds and coarse clastic sediment. Some gypsum and marine limestone occurs in the lower part of the sequence.
The Belden shale, composed of dark shale and limestone, is ranked as a formation instead of a member. It replaces the Weber shale as applied to the basal Pennsylvanian in Colorado and is Des Moines in age. The Maroon formation, which rests on the Belden, consists of several thousand feet of red beds. Much, if not all, of it is Des Moines in age. All the gypsum in the lower Maroon is of Des Moines age. The term Weber quartzite is restricted in west-central Colorado to a white sandstone overlying the Maroon formation. The overlying widely distributed State Bridge formation is composed mainly of fine-grained, red, clastic sediment which attains a thickness of more than 5000 feet along the axis of the trough. It is believed to correlate with the Permian Phosphoria formation of northwestern Colorado.