The Rocky Mountains of the Southwest encompass the mountains of central and northwestern Colorado and northeasternmost Utah. The region took form as a result of several orogenic events starting in the Paleozoic, and the present-day Rockies owe their current topography to events that occurred in the Paleogene. The Rockies actually consist of many mountain ranges with high-elevation valleys in between. These valleys are higher than the average elevation of the Colorado Plateau, generally reaching heights of around 1700 to more than 2100 meters (5500 to more than 7000 feet). The Rocky Mountains are home to many of the highest peaks in the continental US, with 54 peaks topping 14,000 feet (approximately 4270 meters)! At the highest elevations, bare rock predominates, but soils have formed below the tree line.

Not surprisingly, the soils of the Rocky Mountains are quite different from those found in surrounding areas of the Southwest. The region’s higher elevation means that water is more abundant, both because temperatures are cooler, which reduces evaporation, and because rainfall and runoff are higher, particularly on the windward sides of the mountains. For this reason, Aridisols, which are widespread elsewhere in the Southwest, are uncommon in the Rocky Mountains, although clay- and carbonate-rich soils are scattered throughout the mountains of northwestern Colorado and northeastern Utah. Alfisols, in contrast, are common at higher elevations and support forested areas with a short, cool growing season. Entisols are uncommon, but do occur in the region’s valleys.

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, and often overlie erosion-resistant bedrock. While Inceptisols represent a level of soil development one step above that of Entisols, they are still very poorly developed. The Rocky Mountains of northern Colorado, especially the Medicine Bow, Front, and Park ranges, host a high concentration of these soils (Figure 7.19). The thin, rocky nature of Inceptisol soil prevents significant water retention, placing lower limits on plant cover.

Mollisols are common throughout lower elevations of the Rockies. These loamy soils are well drained and permeable, containing ample organic matter and a high nutrient content. Valleys that contain these highly productive dark soils naturally support forest, grass, and shrubland. They are used to raise a variety of crops, including hay, wheat, and barley, and also as rangeland for cattle.

Figure 7.19: The Nokhu Crags, part of Colorado’s Never Summer Mountains, erode to form weathered Inceptisols.