United States

Still farther east rise the two highest mountain chains in the conterminous United States—the Cascades and the Sierra Nevada. Aside from elevation, geographic continuity, and spectacular scenery, however, the two ranges differ in almost every important respect. Except for its northern section, where sedimentary and metamorphic rocks occur, the Sierra Nevada is largely made of granite, part of the same batholithic chain that creates the Idaho Rockies. The range is grossly asymmetrical, the result of massive faulting that has gently tilted the western slopes toward the Central Valley but has uplifted the eastern side to confront the interior with an escarpment nearly two miles high. At high elevation glaciers have scoured the granites to a gleaming white, while on the west the ice has carved spectacular valleys such as the Yosemite. The loftiest peak in the Sierras is Mount Whitney, which at 14,494 feet (4,418 metres) is the highest mountain in the conterminous states. The upfaulting that produced Mount Whitney is accompanied by downfaulting that formed nearby Death Valley, at 282 feet (86 metres) below sea level the lowest point in North America.

Mount Whitney, California.© Index Open

The Cascades are made largely of volcanic rock; those in northern Washington contain granite like the Sierras, but the rest are formed from relatively recent lava outpourings of dun-coloured basalt and andesite. The Cascades are in effect two ranges. The lower, older range is a long belt of upwarped lava, rising unspectacularly to elevations between 6,000 and 8,000 feet (1,825 and 2,440 metres). Perched above the “low Cascades” is a chain of lofty volcanoes that punctuate the horizon with magnificent glacier-clad peaks. The highest is Mount Rainier, which at 14,410 feet (4,392 metres) is all the more dramatic for rising from near sea level. Most of these volcanoes are quiescent, but they are far from extinct. Mount Lassen in northern California erupted violently in 1914, as did Mount St. Helens in the state of Washington in 1980. Most of the other high Cascade volcanoes exhibit some sign of seismic activity.

The Western Intermontane Region

The Cordillera’s two main chains enclose a vast intermontane region of arid basins, plateaus, and isolated mountain ranges that stretches from the Mexican border nearly to Canada and extends 600 miles from east to west. This enormous territory contains three huge subregions, each with a distinctive geologic history and its own striking topography.

Forested slopes of the Beartooth Mountains, Mont., in the northern Rocky Mountains.© John Elk

The Colorado Plateau, nestled against the western flanks of the Southern Rockies, is an extraordinary island of geologic stability set in the turbulent sea of Cordilleran tectonic activity. Stability was not absolute, of course, so that parts of the plateau are warped and injected with volcanics, but in general the landscape results from the erosion by streams of nearly flat-lying sedimentary rocks. The result is a mosaic of angular mesas, buttes, and steplike canyons intricately cut from rocks that often are vividly coloured. Large areas of the plateau are so improbably picturesque that they have been set aside as national preserves. The Grand Canyon of the Colorado River is the most famous of several dozen such areas.

West of the plateau and abutting the Sierra Nevada’s eastern escarpment lies the arid Basin and Range subregion, among the most remarkable topographic provinces of the United States. The Basin and Range extends from southern Oregon and Idaho into northern Mexico. Rocks of great complexity have been broken by faulting, and the resulting blocks have tumbled, eroded, and been partly buried by lava and alluvial debris accumulating in the desert basins. The eroded blocks form mountain ranges that are characteristically dozens of miles long, several thousand feet from base to crest, with peak elevations that rarely rise to more than 10,000 feet, and almost always aligned roughly north–south. The basin floors are typically alluvium and sometimes salt marshes or alkali flats.