The Colorado Plateau is a high region, with an average elevation of 1500 meters (5000 feet). Slopes are relatively gentle across the region, except in mountain ranges, such as the Henry Mountains of Utah, which owe their existence to magmatic intrusion. The geology of the Colorado Plateau comprises mostly Mesozoic-aged sedimentary rocks.

The most common soils on the Colorado Plateau are Aridisols, which occur in the region’s mid-elevations. These dry, coarse soils formed from the weathering of limestones and carbonate parent material deposited in ancient seas. Due to a lack of precipitation that would leach out soluble minerals, Aridisols contain high concentrations of gypsum, carbonates, and salt, which sometimes solidify into caliche—a hard, light-colored layer cemented together by lime. They support drought-resistant vegetation, especially sagebrush steppe ecosystems (Figure 7.12). Irrigation can convert these soils into useable cropland, but they are also managed for livestock grazing.

Figure 7.12: Aridisol soils support the growth of this sagebrush steppe near Utah’s Henry Mountains.

Alfisols are relatively uncommon on the Colorado Plateau. These soils generally form in forested areas as a result of weathering processes that leach minerals from the surface layer into a clay-rich subsoil, where nutrients are retained. Many occur beneath conifer forests in the region’s higher elevations, especially along the Mogollon Rim in Arizona (Figure 7.13) and in the Zuni Mountains in western New Mexico, the southern Chuska Mountains in eastern Arizona, and the Henry Mountains in southcentral Utah.

Figure 7.13: Pale-colored, leached Alfisols are exposed on this path in Coconino National Forest, Arizona.

Mollisols constitute the most fertile soils on the Colorado Plateau, formed where organic matter accumulates beneath prairie grasses and in poorly drained forests (Figure 7.14). These soils are rich in humus, which stores mineral nutrients and contributes to the soil’s high moisture and nutrient content. In the central parts of the Plateau, they are scattered at mid-elevations on the windward sides of mountain ranges where moisture is slightly more abundant. Mollisols are particularly rich in western Colorado, allowing for the cultivation of fruit orchards and other crops. They are also found in a wide band stretching through central Utah along the Plateau’s western margin, where they form some of the state’s most productive and important agricultural soils.

Figure 7.14: A farm in Paonia, Colorado irrigates this field of rich Mollisols with a system of channels.

Entisols, young soils lacking in horizons, are found where erosion and deposition occur faster than the rate of soil formation. In the Colorado Plateau they are common on recent erosional surfaces, especially along the Colorado River and other river courses (Figure 7.15). They also occur as shallow soils on the bedrock of arid uplands. Inceptisols are rare here; these weakly developed soils can be found scattered throughout the region in mountainous areas and on steep slopes.

Figure 7.15: An accumulation of sandy Entisols marks a dry riverbed near Palisade, Colorado.