GEOLOGY
The rocks exposed in the Devils Tower National Monument may be divided on the basis of their origin into two general types; igneous and sedimentary. The Tower itself is composed of igneous rock; that is, rock formed directly by cooling and crystallization of once molten materials. 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. These rocks, which crop out around the igneous mass, are layers of shale, sandstone, siltstone, mudstone, gypsum, and limestone. Devils Tower owes its impressiveness to the differing rates of erosion of these rock types—the soft sedimentary rocks erode more easily than the hard igneous rock—and to the contrast of the somber color of the igneous column to the brightly colored bands of sedimentary rock that surround its base.