GREAT BASIN

also called Great Basin Desert, distinctive natural feature of western North America that is equally divided into rugged, north-south-trending mountain blocks and broad intervening valleys. It covers an arid expanse of about 190,000 square miles (492,000 square km) and is bordered by the Sierra Nevada range on the west, the Wasatch Mountains on the east, the Columbia Plateau on the north, and the Mojave Desert on the south. With the Sonoran, Chihuahuan, and Mojave deserts, the Great Basin forms one of four divisions of the North American Desert. Most of the U.S. state of Nevada, the western half of Utah, and portions of other nearby states lie within its boundaries. The Great Basin is the largest subdivision-consisting of the northern half-of the Basin and Range Province, a physiographic feature extending southward to include southern Arizona, southeastern and central New Mexico, the western tip of Texas, and northwestern Mexico. The mountain ranges of the Great Basin have been likened, in an old survey report, to a group of caterpillars, all crawling irregularly northward. The ranges are from 60 to 120 miles (95 to 190 km) long and 3 to 15 miles (5 to 24 km) wide. The valleys are usually somewhat wider than the ranges and are mostly deserts, with floors 1,000 to 6,000 feet (300 to 1,800 m) above sea level. The ranges have peaks commonly reaching 9,000 feet (2,750 m) or more above sea level, and where this occurs they catch a moderate amount of precipitation and support conifer stands. The western face of the Sierra Nevada otherwise blocks rain-bearing winds off the Pacific Ocean from reaching the Great Basin, forming a rain shadow over the entire region. The annual rainfall of 6 to 12 inches (150 to 300 mm) in the basin supports little more than a sparse desert or semidesert vegetation. The Great Basin is particularly noted for its internal drainage system, in which precipitation falling on the surface leads eventually to closed valleys and does not reach the sea. The Humboldt River of northern Nevada, for example, rises in ranges in the northeast of the state, drains a number of small valleys on its way westward, and ends in a closed basin called Humboldt Sink. The Great Salt Lake lies in the final and lowest catchment basin of western Utah and gathers much of the drainage of the region that has not evaporated or seeped underground en route. The arid Great Basin for a long time thwarted westward cross-country travel to California and hence impeded significantly the development of the western United States. Jedediah Smith (1798-1831), a great explorer of the West, made the first journey across the basin in 1824 but did not document his travels. He was followed by John C. Frmont, who surveyed an eastern swath of the Great Basin in 1846 but did not cross it. The California Gold Rush brought thousands westward in 1848 and 1849, many of them reaching Salt Lake City and then attempting alternate routes across the Great Basin. A survey made in 1867-78 produced the first federally sponsored scientific account of the climate, travel conditions, and resources of the Utah-Nevada region. The ranges and valleys of the Great Basin may be characterized as huge blocks of the Earth's crust that have been uplifted, sunk, and tilted. Enormous cracks, or faults, bound the blocks, and the uplifted parts have been eroded over geologic time, with the debris accumulating over the depressed parts. There are about 30 major fault-bounded blocks, generally 15-30 miles (24-48 km) across, between the Wasatch Range and the Sierra Nevada. In many places volcanic rocks that formed about 30,000,000 years ago have been cut and displaced by the block faults. Furthermore, many of the faults exhibit fresh surfaces, indicating recent movement; there are historical records of earthquakes and constant contemporary micro-earthquakes, indicating that faulting has continued to the present day. Minerals have proved to be the greatest resource of the Great Basin. Much of the nation's gold, magnesite, barite, and mercury are produced in Nevada, which is also among the leading producers of lithium, silver, diatomite, and gemstones. Utah is the leading producer of beryllium ore and is among the leading producers of gold, silver, copper, iron ore, and molybdenum. Most of the population in Utah is located along the western base of the Wasatch Range focusing on Salt Lake City, with sustaining water supplies coming from streams in the mountains and also from wells tapping the great underground water reservoir trapped beneath the adjacent valley. Similarly, on the other side of the Great Basin, a good part of the population of western Nevada, centring on Reno, is found along the east front of the Sierra Nevada, which supplies most of the water for that area.