Pg. 2115 (fig. 4), 2116-2119. Coalbank Hill member of Molas formation. Basal member of formation. Includes red mudstones and siltstones containing solution-rounded limestone boulders and chert fragments and overlying structureless mudstones and siltstones. This is "transition zone" mentioned by earlier writers; the "rubble breccia" of upper Redwall and the "residual soil" in lower part of basal member of Naco (Huddle and Dobrovolny, 1952, USGS Prof. Paper 233-D); and lowest member of Molas as subdivided by Wengerd and Strickland (1954). Thickness about 56 feet at type locality [Coalbank Hill]; 13 feet at Stag Mesa; 5 feet at Ouray. Underlies middle unnamed member; unconformably overlies Leadville limestone. Where Leadville is absent, overlies Ouray limestone; contact appears gradational. No fossils. Age defined only by stratigraphic position, post-Leadville, pre-middle Molas. Mississippian-Pennsylvanian boundary probably contained within it or overlying member.
Type area: Coalbank Hill on U.S. Highway 550, 33 mi north of Durango, [San Juan Co.], southwestern CO.

Source: US geologic names lexicon (USGS Bull. 1200, p. 852).