Pg. 95-101, pl. 1. Basin Ridge group. Thick series of volcanic rocks, flow breccias, tuffs, and rhyolites. The various flows thin toward the south. Thick red and white tuff-breccia and variolitic andesite breccia are the most persistent lenses. Agglomerate, which thickens southward with thinning of flows, is well exposed toward southern end of Basin Ridge. Volcanics consist of lower variolitic andesite member and upper red and white member. Near Basin Ridge gap, lower member comprises augitic variolitic andesite breccia, augitic andesite variolite, variolitic tuff, and hornblendic andesite flow variolite -thickness, 1,000 to 1,150 feet; upper member consists of gray porphyry flow, red breccia, agglomerate, red and white tuff-breccia, and hornblende andesite -thickness, 640 to 715 feet. [Uppermost in group is Basin rhyolite (new).] Mapped on plate 1 as Denver Basin group. Age is Tertiary.
Underlies northern two-thirds of Basin Ridge, central CO, extending from a point about 2 mi southwest of Como, into sec. 10, T. 11 S., R. 76 W., South Park, Park Co. Two outliers, structurally and lithologically similar to Basin Ridge volcanics, form hills approximating in area 1 sq mi each, in sec. 7, T. 9 S., R. 76 W.; sec. 12, T. 9 S., R. 77 W. ; and secs. 29 and 32, T. 8 S., R. 76 W.

Source: US geologic names lexicon (USGS Bull. 1200, p. 231).