3. Intrusion of Magma
About 50 million years ago (near the end of the Laramide Orogeny) magma (molten rock) welled up into the older sedimentary rocks; this happened in many places across the northern part of the Black Hills. One of these magma bodies would later become Devils Tower. However, the magma that formed the tower cooled underground, never reaching the surface. Thus, Devils Tower remained buried under more than a mile of sedimentary rock for many millions of years.