Devils Tower is a steep-sided mass of igneous rock that rises above
the surrounding hills and the valley of the Belle Fourche River in
Crook County, Wyo. It is composed of a crystalline rock, classified as
phonolite porphyry, that when fresh is gray but which weathers to
green or brown. Vertical joints divide the rock mass into polygonal
columns that extend from just above the base to the top of the Tower.
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 formations have been only slightly deformed by faulting and
folding. Within 2,000 to 3,000 feet of the Tower, the strata for the
most part dip at 3°-5° towards the Tower. Beyond this distance, they
dip at 2°-5° from the Tower.
The Tower is believed to have been formed by the intrusion of magma
into the sedimentary rocks, and the shape of the igneous mass formed
by the cooled magma is believed to have been essentially the same as
the Tower today. Devils Tower owes its impressiveness to its
resistance to erosion as compared with the surrounding sedimentary
rocks, and to the contrast of the somber color of the igneous column
to the brightly colored bands of sedimentary rocks.