Region 3: The Rocky Mountains
The Rocky Mountains, a series of at least 100 different mountain ranges that stretch some 4800 kilometers (3000 miles) from northern Alberta southward to New Mexico, make up the great western backbone of the North American continent. The history of the uplift that formed the Rockies is complex, but the bulk of mountain building appears to have occurred during the Laramide Orogeny, which experienced its peak activity between 70 to 50 million years ago. Many of this region’s soils, especially those in the Northern and Middle Rocky Mountains (Idaho, Montana, and northeastern Wyoming), are poorly developed and thin because they have not had sufficient time to develop.
Since Entisols are commonly associated with steep slopes and poorly developed soils, it is easy to imagine why this soil type would be abundant in the Rocky Mountain region. The Bighorn Mountains and Wind River Range of Wyoming and the Big Belt Mountains of central Montana host abundant Entisols. These soils frequently have little agricultural value due to their poorly developed nature and rocky settings, but some high valley systems with sufficient water or irrigation resources can be productive (Figure 8.19). However, Entisols are not always directly associated with mountain slopes. The Killpecker Sand Dunes of the Red Desert in southwestern Wyoming provide a stunning example of poorly developed soils periodically disturbed by active and reactivated sand dunes (Figure 8.20). These dunes, which formed from collected glacial sediments, are part of the largest active dune field in the United States.
Figure 8.19: The Entisols of high mountain alpine environments support a remarkable variety of forbs and other plant species.
Figure 8.20: The Killpecker Sand Dunes, Wyoming.
While Inceptisols represent a level of soil development one step above that of Entisols, they are still very poorly developed. Inceptisols are found on reasonably steep slopes and involve parent rock material that is quite resistant to weathering, so they are frequently associated with mountain formations (Figure 8.21). Both the Clearwater and Salmon River mountains of Idaho and the Bitterroot Range, which straddles the Montana and Idaho border, host a high concentration of these soils. Many of the Inceptisols in this region are associated with forestry, rather than crop cultivation. The thin, rocky nature of the soils prevents significant water retention, placing lower limits on timber production.
Figure 8.21: Castle Peak, in Idaho’s White Cloud Mountains, erodes to form weathered Inceptisols.
Aridisols are commonly found in the intermontane Wyoming Basin, due to the influence of a rain shadow effect from the tall mountains to the west. The Red Desert, a high-altitude desert and sagebrush steppe, hosts an abundance of these poorly developed and unstable soils. While many Aridisols are beyond the practicality of common agricultural and economic practices, not all have been left undeveloped. With major irrigation projects such as the Shoshone Project, which irrigates nearly 40,000 hectares (100,000 acres) of crop and grazing land with dammed flood waters from the Shoshone River, large portions of the rain-shadowed Bighorn Basin have proven to be quite productive, yielding soybeans, alfalfa, barley, oats, corn, sugar beets, and pastureland used to support local livestock production.
Andisols are soils formed from volcanic ash and a varied assortment of volcanic ejecta (Figure 8.22). Globally, they are the least common order, making up less than 1% of all soil coverage. Similarly, in the US, they represent a mere 1.7% of total soil coverage. However, in the Rocky Mountain region, they are commonly found in the Clearwater, Coeur D’Alene, and Cabinet Mountains of Northern Idaho. These soils support some of the most productive conifer forests in the United States due to their unique chemical and physical properties. Andisols frequently contain high concentrations of volcanic glass and various weathered iron- and silica-rich material. Andisols have a high capacity for water retention and often fix large amounts of phosphorus, making it unavailable to plants.
Figure 8.22: An example of an Andisol soil. These soils are formed from volcanic materials.
Scattered Alfisols support forests throughout the Rockies of western Montana (Figure 8.23), and the high mountains of Montana and Wyoming also contain rich Mollisols that support rangeland and forest vegetation.
Figure 8.23: An Alfisol soil from the forests of Montana.
