About 50 million years ago, magma was injected into layers of sedimentary rock, forming the Tower one and one-half miles below the earth’s surface. Over millions of years, erosion wore away the surrounding sedimentary rock and exposed the Tower seen today. Geologists agree that it is an igneous intrusion formed from hardened magma, but they differ on its original size and shape. One interpretation holds that it is an irregularly shaped igneous stock formed when magma cooled and crystallized before reaching the surface. Another sees it as the remnant of a mushroom-shaped laccolith, with magma likewise solidifying underground. A third interprets it as a cylinder-shaped volcanic plug, part of the plumbing system that fed a volcano and became plugged when magma solidified below ground. Because erosion has altered the evidence, the correct interpretation may never be known.