The Great Basin is a vast, internally drained region of the Western United States, bounded by the Sierra Nevada and southern Cascade Mountain ranges to the west and the Wasatch Mountains and western rim of the Colorado Plateau to the east. Although less discrete, northern and southern boundaries are generally defined by the drainages of the Columbia and Colorado Rivers. Included are most of Nevada and major portions of Utah, California, and Oregon. Contrary to what the name implies, the Great Basin is composed of not one, but many closed basins or valleys separated by over 100 relatively narrow mountain ranges arranged with a more or less north-south orientation. This geography is characteristic of the larger Basin and Range Province extending from southern Oregon and Idaho to Sonora, Mexico, of which the Great Basin occupies the northern and most elevated third. Valley floor elevations range from 86 m below sea-level in Death Valley, California, to approximately 2,000 m above sea-level in central Nevada. Mountain elevations also vary with the summits of 33 mountain ranges exceeding 3,000 m. The mountains of the Great Basin began forming approximately 17 million years ago when a complex series of normal faults began forming in the earth?s crust in response to continental stretching (Morris and Stubben 1994). Mountains ranges (horsts) and valleys (grabens) formed as blocks of crust were vertically displaced along those faults. As the mountains eroded, the surrounding valleys filled with their debris. The cores of many eastern Great Basin ranges are composed of limestone, dolomite, and quartzite; rocks derived from Paleozoic marine sediments. Younger Mesozoic and Cenozoic volcanics such as granite, tuff, basalt, and breccia are found throughout the Great Basin but dominate in the west and north. The physical and chemical characteristics of valley soils from massive sand dunes to fine-textured playas reveal the diversity of these parent materials.