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.