The Los Angeles Plain subsection consists of the Los Angeles Plain and the San Fernando Valley and includes the Verdugo Mountains, San Rafael Hills and Palos Verdes Hills in southern California. The Los Angeles Plain, which is the largest part of the subsection is south of the Santa Monica and San Gabriel Mountains and west of the San Jose and the Puente Hills. The climate is hot and subhumid; it is modified by marine influence greatly on the Los Angeles Plain and moderately in the San Fernando Valley. The subsection is known by the descriptor: MLRA 19d.
Lithology and Stratigraphy
This subsection contains chiefly of Quaternary alluvium. The Verdugo Mountains and San Rafael Hills are geologically similar to the San Gabriel Mountains; they are mostly Pre-Cambrian gneiss and Mesozoic granitic rocks. The Palos Verdes Hills are principally Miocene sedimentary rocks.
This is a subsection of nearly level floodplains and terraces and very gently to gently sloping alluvial fans. There are small areas of marine terraces, but they are relatively inextensive compared to fluvial terraces. Steep mountains and moderately steep hills are small but important parts of the subsection. Dunes are present along the coast north of the Palos Verdes Hills, and sand has spread across Quaternary terraces behind those dunes. The subsection elevation range is from sea-level to about 1000 feet on the Los Angeles Plain, slightly higher in the San Fernando Valley, and up to 3077 feet in the Verdugo Mountains. Fluvial erosion and deposition are the main geomorphic processes. Mass wasting is important in the mountains, and wind is an important geomorphic agent along the coast.
The soils are mostly Typic Xerorthents and Typic and Mollic Haploxeralfs. Soils in the Verdugo Mountains and San Rafael Hills are shallow Typic Xerorthents, Lithic Haploxerolls, Typic and Calcixerollic Xerochrepts, and Typic Haploxeralfs. Soils on Miocene sedimentary rocks are shallow Typic Xerorthents, Calcic and Pachic Haploxerolls, Typic Argixerolls, and Chromoxererts and Pelloxererts. The soils in dune sand are Typic Xeropsamments. The soils are well drained. Carbonates accumulate in some of the soils. Soil temperature regimes are thermic, and soil moisture regimes are xeric.
Characteristic series by lifeform include:
- Duneland: Sand-verbena - beach bursage series, Dune lupine-goldenbush series.
- Saltmarsh: Cordgrass series, Ditch-grass series, Pickleweed series.
- Grassland: California annual grassland series.
- Shrubland: 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, Coast pickle-pear series, Coyote brush series, Mixed sage series, Sumac series, White sage series.
- Forest and woodland: California sycamore series, California walnut series, Coast live oak series.
The mean annual precipitation is about 12 to 20 inches; it is practically all rain. Summer fog is common. Mean annual temperature is about 58° to 64° F. The mean freeze-free period is about 300 to 350 days.
The Los Angeles River is the largest stream on the Los Angeles Plain. It drains the San Fernando Valley and much of the San Gabriel Mountains. Most of the streams are dry through the summer. There are no lakes or ponds, other than temporary ponding behind dunes.