Mexico’s continental and peninsular area of approximately 2,000,000 km2 between 15°N and 32°30′N comprises dryclimate latitudes north of the Tropic of Cancer, so that about 60 percent of the northern part of the country is desert or semidesert. The main physiographic elements are shown on Figure 1.
The Sierra Madre Occidental and Sierra Madre del Sur are formed principally by a 2,000-m thickness of interbedded extrusive volcanic and pyroclastic rocks, apparently generated by early and mid-Tertiary volcanic emissions through extensive fissures and northwest-trending faults, and middle to late Tertiary granodioritic igneous intrusions. This volcanic sequence overlies a Precambrian metamorphic basement in portions of the states of Sonora, Oaxaca, and Chiapas. Strong fluvial and tectonic erosion has deeply carved both mountain ranges, giving rise to a large number of geohydrologically important river basins (Fig. 2).