Ancient rivers took millions of years to excavate Devils Tower by carrying away softer sedimentary rocks and leaving behind the hard igneous rock called phonolite. This rock type is found here in northeastern Wyoming and central Montana, but mostly in east Africa. The Tower is still emerging as the Belle Fourche River below continues to wash away the softer sedimentary rocks. Plateaus across the valley, some higher than the Tower’s summit, are eroded layers of the same sediment that once surrounded and covered Devils Tower. Today the Tower stands 867 feet, or 264 meters, high from the visitor center to the summit, and approximately one and one-half vertical miles of rock and sediment have washed away since the Tower formed.