The crests of the Sierra Madre Occidental, in the western part of Mexico, form the boundary between the west coast States of Sonora and Sinaloa and the interior States of Chihuahua and Durango. These plateau-like crests, carved from gently inclined volcanic rocks, are in contrast to the Sierra Madre Oriental, in eastern Mexico, which consists for the most part of parallel ridges carved from folded sedimentary rocks.
The western Sierra Madre takes form south of the International Boundary by the coalescing of mountain ranges which in southern New Mexico and Arizona are more or less isolated. South of the boundary, the plains between the mountains become narrower, and the volcanic rocks spread out in a broad plateau. The western edge of the plateau, at an elevation of 6000 feet or more, breaks off toward the Gulf of California in lofty escarpments (PL 2), which are trenched by . . .