Near the end of the Cretaceous Period, some 75 million years ago (MYA), the landscape of eastern Wyoming and western South Dakota was flat, topped by shales and sandstones deposited within and along the Cretaceous Sea that had recently retreated to the southeast. Successively older Jurassic, Triassic and Paleozoic sedimentary strata lay below this surface veneer, forming a layer cake above the deep Precambrian basement. Then, about 70 MYA, compression within the North American craton forced up the Precambrian core of the Rocky Mountains and, in concert, the Precambrian dome of the Black Hills, which rose through the overlying sediments. In areas surrounding the Black Hills Uplift, fissures developed in the sedimentary layer cake and magma pushed toward the surface, cooling within the Mesozoic and Paleozoic strata before reaching the surface.
One of these pillars of magma would become Devils Tower, which rises above a low ridge of the Bear Lodge Mountains of northeast Wyoming (northwest of the Black Hills). Its summit elevation of just over 5100 feet is almost 1300 feet above the Belle Fourche River, which flows northeastward and then southeastward to join the Cheyenne River east of the Black Hills. This river and its numerous tributaries dissected the Bear Lodge Mountains from a plateau of Mesozoic sediments and, in the process, freed Devils Tower from the layers in which it formed. The vertical columns of the Tower resulted from cooling of its molten rock beneath the surface and its prominence, like other laccolithic formations throughout the American West, attests to the fact that its igneous rock is more resistant to erosion than the encasing Mesozoic sediments. Most of this erosion occurred during the wet climate of the Pleistocene (2.0-0.01 MYA) but the relentless process continues today.
Long a source of human mysticism and the inspiration for America's first National Monument, Devils Tower, like the sediments in which it formed, will eventually erode to a flat surface and be carried off to the sea. Until then, it will loom above the Belle Fourche Valley and stir the imagination of many future generations, perhaps until Close Encounters becomes a reality.
