The Basin and Range is a unique topographical region consisting mostly of north- and south-running mountain ranges and valleys. These features formed as a result of extension of the continental crust during the Neogene. This extension, or stretching, caused the crust to break along straight, nearly parallel faults, along which steep mountains rose and flat-bottomed valleys dropped. In the Southwest, valley bottoms can be close to sea level, particularly in southwestern Arizona (in Death Valley, California, the Basin and Range actually has valleys below sea level). The mountains, on the other hand, can top 3000 meters (10,000 feet)! The region’s low valleys are commonly occupied by dry, alkaline lakes with very little vegetation, but in the slightly cooler, higher valleys of southern Arizona and southwestern New Mexico, the soil is more hospitable to plant growth.

With the exception of high mountaintops, a very dry climate dominates the Basin and Range. Aridisols are by far the most common soils, occupying the low- and mid-elevation parts of the basins and adjacent mountain ranges. These soils tend to contain a high amount of calcium carbonate, and are moderately to strongly alkaline; they support drought-resistant plants including sagebrush, Joshua tree, and yucca. While many Aridisols are beyond the practicality of common agricultural and economic practices, not all have been left undeveloped. Many of the region’s Aridisols belong to the suborder Argids, which are rich in clay; with irrigation, these can be productive soils (Figure 7.16). In northwestern Utah, however, the dominant suborder is Salids—these fine-grained and salt-heavy soils are commonly found in depressions and playa lakes, including the extensive Great Salt Lake Desert.

Figure 7.16: The famous chile pepper farms of southern New Mexico produce their crops in Aridisol soils.

Entisols are the next most common type of soil in the Basin and Range. These soils, young and lacking in horizons, are found scattered along floodplains and streams where alluvial sediments are deposited, and river valleys where unconsolidated sediment has been transported from the steep surrounding slopes. In Utah, several areas of highly sandy sediment have been designated as dune land. One of the largest dune fields in the state is Little Sahara, whose sand dunes result from silicate-rich deposits left by the Sevier River during the Pleistocene (Figure 7.17). Strong winds carried the sediment from the river delta to its current location, where it covers an area of about 570 square kilometers (270 square miles).

Figure 7.17: Sand Mountain, a 210- to 245-meter (700- to 800-foot) wall of Entisol sand at Little Sahara in Utah, is a popular location for ATV riding.

Other soil types that appear in the Basin and Range are uncommon. Soils weathered from the region’s mountains are characteristically rocky, shallow, and gravelly. Dry Alfisols occur near the tops of the highest mountains in southern Arizona and New Mexico, and along the Mogollon Rim at the edge of the Colorado Plateau. Freely drained Inceptisols, associated with steep slopes and resistant parent material, occur scattered across mid-elevations in southern Arizona and southwestern New Mexico. These soils are commonly associated with the newly formed soils of mountainous terrain and do not lend themselves well to agriculture due to their poor development. Arid Mollisols, which are fertile but can become dusty and dry during drought conditions, are present at mid-elevations that surround mountains where moisture is slightly higher.

In the Basin and Range, patches of soil have been heavily influenced by existing sedimentary rock material lain down during the uplift of the Rockies and the deposition of Mesozoic sediments. The erosion of exposed Cretaceous marine shales produces Vertisols, soils that experience drastic fluctuations in volume when exposed to water. The only Vertisols found in the Southwest occur in the Basin and Range, in southern Arizona and New Mexico (Figure 7.18). During dry periods, these clayey soils shrink and form wide cracks at the surface; the cracks seal shut again when moisture enters the soil. Because Vertisols shrink and swell so readily, it is extremely difficult—and even dangerous—to build structures or roads on top of them. Expansion of the clay minerals can cause foundations to crack and roads to buckle; millions of dollars are spent every year on repairing damage done by expansive soils. The action of shrinking and swelling within the soil also prevents the formation of distinct horizons.

Figure 7.18: Cracked Vertisol soils near Ash Fork, Arizona.