The mountains that stand near Glen Canyon are quite unusual. They are known as laccolithic mountains formed where plumes of magma have made their way toward the surface only to run out of pressure before they spilled out on the surface. Instead, the magma pushes the overlying strata into dome-shaped mountains. Navajo Mountain and the Henry Mountains are all examples of volcanoes that never quite made the grade about 20 to 30 million years ago when the magma was first injected. Erosion of the mountains in the intervening years has sometimes carved deeply enough into the domes to reveal the now-solidified magma cores to sculpt sharp peaks and rugged ridges.