Geology of Devils Tower National Monument, Wyoming / A Contribution to General Geology

A steep-sided igneous monolith is described, composed of phonolite porphyry that weathers from gray to green or brown and is divided by vertical joints into polygonal columns. Surrounding hills consist of about 400 feet of sedimentary sandstone, shale, and gypsum, including parts of the Spearfish, Gypsum Spring, and Sundance formations with named members, dipping gently toward the monolith. The report documents minor faulting and folding, terrace and talus deposits, and stratigraphic relations, and interprets the landform as the resistant core of a magma intrusion whose cooled shape largely corresponds to the present exposed mass.