As the Belle Fourche River cut down into the soft sedimentary rocks, the harder igneous rocks of Devils Tower were also exposed. Initially, then, Devils Tower would have appeared rather short and stubby. It may have resembled the present-day Missouri Buttes, which lie about 4 miles (6 km) west of Devils Tower National Monument. Because the Belle Fourche River is closer to Devils Tower than to the buttes, the river eroded the tower more than the buttes, eventually exposing the lower parts to make it appear more or less the way it appears today.
Weathering caused the upper portion to be more cracked than the lower part. Hence, the upper and lower parts of Devils Tower are noticeably different. The more rugged appearance of the upper third of the tower may also be due to how it cooled. The upper portion lay at a shallower depth and cooled more rapidly than the lower portion, which may have caused the rocks to be more shattered and cracked.