Charles Sherwood Robinson

“ It is unlikely that magma which had ascended from great depths and had just penetrated the resistant Hulett Member of the Sundance Formation, as well as the Lakota and Fall River Formations, would be stopped abruptly by the less resistant shales above. When the magma reached the shale beds, the weight of the column of igneous rock could have exceeded the strength of the shale, causing the magma to flow horizontally. ”

“ The hills in the vicinity and at the base of the Tower are composed of red, yellow, green, or gray sedimentary rocks that consist of sandstone, shale, or gypsum. These rocks, in aggregate about 400 feet thick, include, from oldest to youngest, the upper part of the Spearfish formation, of Triassic age, the Gypsum Spring formation, of Middle Jurassic age, and the Sundance formation, of Late Jurassic age. The Sundance formation consists of the Stockade Beaver shale member, the Hulett sandstone member, the Lak member, and the Redwater shale member. ”

“ The Lak member is typically poorly bedded soft, very fine-grained calcareous sandstone and siltstone with a few thin gray-green sandy shale partings. At the base and near the top of the member may be a few thin (less than 1 inch to 6 inches thick) well-cemented sandstone beds that form small ridges. The sandstone and siltstone grade almost imperceptibly from one to the other. The color ranges from light yellow brown and yellow to red. In the Devils Tower area, shades of yellow and yellowish brown are most common. ”

“ The rocks exposed in the remainder of the Monument are sedimentary; that is, they were formed by the consolidation of fragmental materials derived from other rocks or accumulations of chemical precipitates that were deposited either on the floors of prehistoric seas or near the shores of such seas. ”