Lithology and Stratigraphy
This subsection contains mainly Mesozoic granitic and mafic plutonic rocks, Jura-Triassic volcanic rocks, Pre-Cretaceous metamorphic rocks, and Eocene and Oligocene marine and nonmarine and Miocene marine sedimentary rocks. There are many small areas of late Quaternary fluvial deposits.
This is a subsection of moderately steep to steep hills between the mountains of the Santa Ana and Laguna Mountains of the northwest trending Peninsular Ranges and coastal terraces along the southwest edge of the subsection. Many streams crossing from the mountain to the coast have alluvial plains hundreds of yards wide, and there is a broad valley at El Cajon. The subsection elevation range is about 500 to 2000 feet. Mass wasting and fluvial erosion are the main geomorphic processes. Fluvial deposition is an important process on floodplain and alluvial fans in the valleys.
The soils are mostly Lithic Xerorthents; Lithic, Pachic, and Calcic Haploxerolls; Typic Argixerolls; Typic Natrixeralfs; Natric Palexeralfs; and Chromoxererts and Pelloxererts. On granitic rocks, the soils are mostly shallow Typic Xerorthents and Typic Haploxeralfs. On mafic plutonic rocks, they are mostly Mollic and Typic Haploxeralfs and Typic Rhodoxeralfs. Common soils on floodplains, terraces, and alluvial fans are Typic Xeropsamments, Cumulic Haploxerolls, and Typic Haploxeralfs. The soils are well drained. Calcium carbonates accumulate in many of the soils. The soil temperature regimes are thermic. Soil moisture regimes are xeric.
The predominant natural plant communities are California sagebrush - California buckwheat series, and California sagebrush - black sage series, Coast live oak series; and there is some Chamise series, White sage series, Scrub oak series. California sycamore series occurs in riparian areas.
Characteristic series by lifeform include:
- Grasslands: California annual grassland series.
Shrublands: 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, Chamise-mission-manzanita - woollyleaf ceanothus series, Coast prickly - pear series, Mixed sage series, Mixed scrub oak series, Scrub oak series, Scrub oak - chamise series, Sumac series, White sage series.
Forests and woodlands: California sycamore series, Coast live oak series, Engelmann oak series.
The mean annual precipitation is about 12 to 16 inches; It is practically all rain. Summer fog is common. Mean annual temperature is about 56° to 62° F. The mean freeze-free period is about 275 to 350 days.
Runoff is rapid. Many streams that drain from the mountains across this subsection are perennial, but most of the lateral streams are dry though the summer. There are no lakes, but some reservoirs in the subsection.