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; 200 to 1000 m) below the surface.Erosion then stripped the overlying lay ers of rock away.evils 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; 200 to 1000 m) below the surface.