4. Regional Uplift and Erosion
About 5 million years ago, the entire Rocky Mountain region (including Utah, Colorado, Wyoming, and Montana) was uplifted high above sea level. As a result, the rivers in the region began to flow aggressively, cutting rapidly downward into the soft sedimentary rocks. Rivers carried the eroded material away and deposited it into the Gulfs of Mexico and California. In the Devils Tower area, the Belle Fourche River and its tributaries removed many layers of softer sedimentary rocks covering the Tower. Hence, the harder igneous rock of Devils Tower was eventually exposed at the surface.