Devils Tower is a steep-sided igneous body and, possibly, an erosional remnant of a volcanic neck. It is made of magma that solidified at a shallow level (about 700 to 3,000 feet) below the surface. Erosion then stripped the overlying layers of rock away. The rock at Devils Tower is about 40 million years old. The rock is called a phonolite based on its mineral composition, which includes anorthoclase, aegirine-augite, and sphene. Devils Tower rises 1,253 feet (382 m) above the nearby Belle Fouche River.
