Rock Planes of Arid Regions

Abstract

MONG the most characteristic elements of the arid landscape are far-spreading lowland plains which incline gently away from the larger mountain masses. In I897 McGee' demonstrated that such surfaces in the southwestern United States and northern Mexico consisted in part of remarkably smooth rock planes. He attributed them to the erosive action of sheet floods, vast expanses of moving water formed when a flooded canyon discharged upon the lowland, or when very heavy rains fell directly upon its nearly level surface. To a typical example he applied the term pediment. American writers have often followed McGee in speaking of these rock planes as pediments, although few have accepted his theory of their origin. Other investigators, both in America and abroad, have sought to explain these rock planes as a product of wind erosion or deflation.2 Lawson3 advanced the theory that such a plane could be produced by progressive retreat of the adjacent mountain front due to weathering processes alone, especially rock disintegration under the influence of insolation. Bryan,4 Davis,5 and others have to a large extent followed Lawson,' although both attribute minor roles to lateral planation by streams.