Lithology and Stratigraphy
This subsection contains mostly Tertiary sedimentary rocks and Quaternary alluvium. More specifically, there are Cretaceous, Eocene, Miocene, and Pliocene marine and Oligocene and Plio-Pleistocene nonmarine sedimentary rocks. Miocene volcanic rocks also occur on and around Conejo Mountain, between the Simi Hills and the Oxnard Plain.
This is a subsection of steep mountains; moderately steep to steep hills; and nearly level to gently sloping floodplains, terraces, and alluvial fans. The Santa Susana Mountains, Oak Ridge, and Simi Valley are oriented east-west. There are eroded Tertiary sedimentary rocks, or badlands, in both the Santa Susana Mountains and the Simi Hills. The subsection elevation range is about 200 to 3750 feet. Mass wasting and fluvial erosion and deposition are the main geomorphic processes.
The soils are mostly Lithic and shallow Typic Xerorthents; Calcixerollic Xerochrepts; and Lithic, Typic, Pachic, and Calcic Pachic Haploxerolls. Fluventic Haploxerolls are common in recent alluvium. There are Typic Argixerolls, Mollic Haploxeralfs, and Abruptic Durixeralfs on terraces and old alluvial fans. The soils are well drained. Carbonates accumulate in some of the soils. Soil temperature regimes are thermic, and soil moisture regimes are mostly xeric.
The predominant natural plant communities include California sagebrush series, Mixed sage series, Chamise series, Mixed scrub oak series, and Coast live oak series. Valley oak series is common on recent alluvial plains, and there is some Bigcone Douglas-fir - canyon live oak series on north-facing slopes at higher elevations. There are small areas of California walnut 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 sagebrush series, California sagebrush - black sage series, California sagebrush series - California buckwheat series, Chamise series, Chamise - black sage series, Coyote brush series, Mixed sage series, Mixed scrub oak series, Purple sage series, Scrub oak series, Scrub oak - chamise series, White sage series.
Forests and woodlands: California bay series, California sycamore series, California walnut series, Coast live oak series, Bigcone Douglas-fir - canyon live oak series, Valley oak series.
The mean annual precipitation is about 16 to 20 inches. It is practically all rain. Mean annual temperature is about 52° to 62° F. The mean freeze-free period is about 275 to 325 days.
Runoff is rapid and all streams are generally dry during the summer. There are no natural lakes.