Devils Tower National Monument

Devils Tower in Devils Tower National Monument is a steep sided igneous body that displays spectacular columnar jointing. The igneous rocks of Devils Tower formed about 40 million years ago (Halvorson, 1980) when molten magma intruded the sedimentary rocks of the Black Hills region. Geologists debate whether or not the magma reached the surface as a volcanic eruption. As the magma cooled and hardened, it cracked, forming spectacular columns. Overlying rocks and surrounding sedimentary rocks have been eroded away, allowing Monument visitors a view of these striking joint patterns.