Description of the Bioregion Physical Geography

south to northern California where it meets the Sierra Nevada. The Southern Cascades bioregion in California is bounded on the west by the Sacramento Valley and the Kla-math Mountains, and on the east by the Modoc Plateau and Great Basin. The bioregion encompasses the Southern Cas-cades section of Miles and Goudey (1997) and covers approx-imately 4 % (16,740 km2 [6,460 mi2]) of the area of California. Mt. Shasta and Mt. Lassen are located in this bioregion. The Cascades are geologically young and characterized by prominent volcanic peaks (some recently active) that stand above an extensive mainly basaltic plateau. In parts of the cen-tral and southern Cascades, volcanics overlie granitic and metamorphic rocks similar to those of the Klamath Mountains and Sierra Nevada (Oakeshott 1971). Soils are derived from vol-canic material and are classified as Alfisols, Entisols, Incepti