The exposed rocks at Devils Tower National Monument are classified into two general types: igneous and sedimentary. The tower itself is composed of igneous rock formed by cooling and crystallization of once-molten materials. The rocks exposed around the tower are sedimentary-layers of shale, sandstone, silt stone, mud stone, gypsum, and limestone. They were formed by the consolidation of fragmented materials derived from other rocks or the accumulation of chemical precipitates that were deposited on the floors or near the shores of ancient seas. Devils Tower owes its impressive height to the differing rates of erosion of these two rock types-the soft sedimentary rocks have eroded more readily than the hard igneous rock. One of the most striking features of the tower is its polygonal columns, formed as the igneous mass slowly cooled and crystallized. Most of the columns are 5-sided, but some are 4-sided or 6-sided. Numerous cross-fractures in the upper part of the to divide the columns into many small, irregularly-shaped blocks.