Pg. 2745-2750. Keyhole sandstone member of Fall River formation. Generally light-brown to medium-brown, very fine-grained, massively bedded, cross-stratified sandstone, commonly containing iron-cemented concretions ranging from less than an inch to about 1 foot in diameter and, less commonly, gray calcareous concretions ranging from a few inches to nearly 4 feet in diameter. Thickness ranges from 15 to about 80 feet and averages about 30 feet; at type section 31.6 feet. Underlies upper silty unit of Fall River formation with sharp contact; overlies lower silty unit with gradational contact. Present in western South Dakota and northeastern Wyoming. Age is Early Cretaceous.
Type section: in NE/4 SE/4 sec. 21, T. 51 N., R. 66 W., Crook, Co, WY, about 0.25 mi north of Keyhole Dam on Belle Fourche River, Carlile quadrangle. Traced throughout large part of northern Black Hills.

Source: US geologic names lexicon (USGS Bull. 1200, p. 2001).

Is a member of Fall River Formation. Considered a nearshore marine blanket sandstone. Traced in outcrops for more than 60 miles. Modern counterparts are barrier islands of the Texas coast. Age is Early Cretaceous.

Source: GNU records (USGS DDS-6; Denver GNULEX).