2. Mountains, Plateaus, and Basins of the Interior West

The western interior offers a variety of huge basins, rug­ged plateaus, deep canyons, and soaring mountains—all of which contribute to some of the nation’s most spectacular terrain. Basin-and-range topography dominates the region between the Sierra Nevada and the Cascades and eastward to the Rocky Mountains and western Texas. Here, relatively low and scattered mountain ranges separate broad and rela­tively flat basins. Many of the basins have interior drainage, or no outward flow. When water flowing into the basins evaporates, salts are left behind to accumulate. Of the many such areas in the American West, Utah’s Bonneville Salt Flats is the best known. Some basins contain large saltwater bodies, such as Utah’s Great Salt Lake and southern California’s Salton Sea. In most basins, though, lakes do not last. They form after a period of rainfall, only to evaporate quickly and disappear, leaving behind a salty encrustation as evidence of their brief existence.

Two huge plateaus occupy the inland Pacific Northwest and the “Four Corners” area of the Southwest. The Columbia Plateau covers portions of eastern Washington, northeastern Oregon, and western Idaho. It is of volcanic origin, formed by magma and lava that poured across the land millions of years ago and accumulated to a depth of up to 6,000 feet (1,829 meters). In addition to its many volcanic features, the plateau offers several other unique landscapes. About one-sixth of its surface is covered by loess, or very fine powderlike material that was deposited by the wind from glacial outwash material during the ice age. This hilly region, the Palouse, contains some of the country’s most fertile soil and best wheat-growing land.

The region also has a number of remarkable features scoured by water erosion. Hells Canyon is a yawning 8,000-foot-(2,438-meter-) deep chasm on the Snake River between northeastern Oregon and western Idaho. It is the deepest river gorge in North America, nearly half a mile (one kilometer) deeper than Arizona’s Grand Canyon. The Columbia Plateau also is the site of what may be the world’s strangest erosional landscape: eastern Washington’s “Scablands.”

(The Scablands are a lunar-like landscape of bare rock. Thousands of years ago, during the late stages of the ice age, a huge lobe of glacial ice dammed today’s Clark Fork River near Sandpoint, Idaho. As water built up behind the barrier, it created ancient Lake Missoula, a water body that extended well into western Montana and reached a depth of about 2,000 feet (610 meters). Ice floats, and eventually, the giant lobe began to rise. This caused an immediate breakup of the ice. The result was an event believed by some scientists to have been the world’s most destructive flood. A torrent of water with a volume estimated to have been 10 times that of the entire world’s river flow was unleashed. Imagine the destruction as the churning water rushed toward the Pacific Ocean at speeds up to 65 miles per hour (105 kilometers per hour)! The force of the raging flood scoured everything in its path, leaving a scablike landscape that is unique to the region.)

The Colorado Plateau, composed of sandstone and limestone layers, is centered on the Southwest’s Four Corners area, where the states of Utah, Colorado, New Mexico, and Arizona meet. Water erosion is primarily responsible for the region’s towering cliffs, many natural bridges and arches, and deep gorges. Eight national parks in southern Utah alone feature landscapes that were formed by water erosion. In northern Arizona, the Colorado River scoured the spectacular Grand Canyon. This gorge, although not the world’s largest, certainly ranks among its best-known and most scenic natural attractions.

The Rocky Mountains extend from northern New Mexico to Montana and as a mountain chain northward into Canada and into Alaska. The Rockies actually are a series of mountain ranges, each of which is recognized by a regional name. The highest points in New Mexico, Colorado, Wyoming, Montana, and Idaho are all Rocky Mountain peaks. Colorado alone has 17 peaks that reach an elevation of more than 14,000 feet (4,267 meters). The state’s highest peak, 14,433-foot (4,399-meter) Mount Elbert, also is the highest elevation in the American Rockies. Mountain glaciers have scoured the jagged terrain for which the mountains are so famous. Colorado’s Rocky Mountain National Park, Grand Teton National Park in Wyoming, and Glacier National Park in Montana all offer marvelous glacier-carved scenery.