Region 4: The Cascade-Sierra Mountains

The Cascades and Sierra Nevada are each the result of distinct volcanic arcs that produced one long span of mountainous terrain. Stretching from Canada all the way down to northern California, the younger Cascade Range was produced by intense volcanism brought on by the continuing subduction of oceanic lithosphere under the North American plate. The frequent ejection of volcanic material contributed greatly to the sediments and minerals present in the area, and this set the stage for the formation of soils that are found almost exclusively in the Pacific Northwest.

Andisols compose most of the Washington area of the Cascades, while Inceptisols dominate the Oregon Cascades. Uniquely identified by the presence of volcanic glass and minerals derived from igneous rocks, many of these soils formed under dense coniferous forests. As in the Rocky Mountains, erosion is frequent in this terrain, making prolonged and deeper soil development difficult.

The Sierra Nevada are located in California, directly south of the Cascades. The Sierra’s distinctive granite originally formed from ancient magma intrusions that have long since cooled and are now heavily weathered (Figure 8.14). Even without active volcanism, the range was able to grow due to exhumation and uplift, and its relief became more pronounced with the formation of the lower Basin and Range region to the east. The Sierra act as a transition zone between the arid climate in the east and the coastal climate to the west, and Alfisols tend to form in this type of zone. Although these soils form throughout the nation, and over a greater area east of the Mississippi River, the unique suborder of Xeralfs make up the majority of the Alfisols in the region. These soils can be forested, or even used as crop and grazing lands.

Figure 8.14: The heavily weathered soils of Temple Crag in the Sierra Nevada consist of carbonate-rich Alfisols and Inceptisol.