Physical Geography: Tucson, Az, USA
After the War of 1812, the Southwest generally meant Missouri, Arkansas, and Louisiana; after Texas was annexed, it, too, was included. In the wake of the war with Mexico, the Southwest embraced most, but not all, of the territory that was acquired under the Treaty of Guadalupe Hidalgo (1848), including land often considered part of the “West”—i.e., New Mexico, Arizona, and all or parts of Oklahoma, Colorado, Utah, and Nevada, as suited the convenience of the user of the term. It ordinarily excludes California.
Plate tectonics—the shifting of large, relatively thin segments of Earth’s crust—and stream erosion have done the most to create Arizona’s spectacular topography. Specifically, the Pacific Plate and the North American Plate came into contact and created the major tectonic forces that uplifted, wrinkled, and stretched Arizona’s geologic crust, forming its mountain ranges, basins, and high plateaus. Over the course of millennia, rivers and their tributaries have carved distinctive landforms on these surfaces. To Arizona’s two major physiographic divisions, the Colorado Plateau and the Basin and Range Province, geologists add the Transition Zone (or Central Highlands). The northeastern two-fifths of Arizona is part of the scenic Colorado Plateau. Far less rugged than adjacent portions of the plateau in Utah, these tablelands in Arizona consist mainly of plains interrupted by steplike escarpments. Although they are labeled mesas and plateaus, their ruggedness and inaccessibility have been exaggerated. The incomparable Grand Canyon of the Colorado River provides the major exception to what has proved to be an area easily traversed. Forest-clad volcanic mountains atop the plateaus provide the state’s highest points: Humphreys Peak, 12,633 feet (3,851 metres), in the San Francisco Mountains, and Baldy Mountain, 11,403 feet (3,476 metres), in the White Mountains.