Lithology and Stratigraphy
This subsection contains mainly Mesozoic and Tertiary sedimentary rocks and Miocene volcanic rocks. More specifically, there are Triassic, Jurassic, Cretaceous, Eocene, and Miocene marine and Oligocene and Plio-Pleistocene nonmarine sedimentary rocks. The Miocene volcanic rocks are mostly andesite, basalt, and pyroclastic rocks. Marine terraces are narrow and not well represented in the subsection. There is little recent alluvium in the valleys.
This is a subsection of steep mountains with narrow to broad summits and narrow canyons. The mountains trend east-west. The elevation range is from sea-level up to 3111 feet on Sandstone Peak. There many are other peaks and ridges above 2500. Mass wasting and fluvial erosion and deposition are the main geomorphic processes.
The soils are mostly Lithic Xerorthents; Lithic and Calcixerollic Xerochrepts; Lithic, Pachic, and Calcic Pachic Haploxerolls; Typic Argixerolls; and Ultic Palexerolls. Also, there are Fluventic Haploxerolls in recent alluvium. The soils are well drained. Carbonates accumulate in some of the soils. The soil temperature regimes are thermic. Soil moisture regimes are mostly xeric.
The predominant natural plant communities include California sagebrush - California buckwheat series and Mixed sage series at lower elevations and Chamise series and Mixed chaparral shrublands at higher elevations. There is some Coast live oak series.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series.
Shrublands: Bigpod ceanothus series, Black sage series, California buckwheat series, California buckwheat - white sage series, California encelia series, California sagebrush series, California sagebrush - black sage series, California sagebrush series - California buckwheat series, Chamise series, Chamise - black sage series, Chamise - chaparral whitethorn series Coyote brush series, Coast pickle-pear series, Mixed sage series, Mixed scrub oak series, Scrub oak series, Scrub oak - birchleaf mountain-mahogany series, Scrub oak - chamise series, Sumac series, White sage series.
Forests and woodlands: California bay series, California sycamore series, California walnut series, Coast live oak series, Valley oak series.
The mean annual precipitation is about 15 to 25 inches. It is practically all rain. Mean annual temperature is about 54° to 62° F. The mean freeze-free period is about 300 to 350 days.
Runoff is rapid and the streams are generally dry during the summer. There are no natural lakes.