This subsection consists of hills and valleys in a graben between the San Jacinto Fault on the northeast and the Elsinore Fault zone on the southwest.  It stretches from the Santa Ana River southeast beyond Perris Valley.  The climate is hot; it is modified somewhat by marine influence. MLRAs 19d and 20d.

Lithology and Stratigraphy.  This subsection contains mostly Mesozoic granitic, gabbroic, marine (Jurassic) sedimentary, and metavolcanic rocks in the hills.  The valleys are filled with Quaternary alluvium.

Geomorphology.  This subsection is on moderately steep to steep hills, pediments, dissected Quaternary nonmarine sediments adjacent to the Elsinore Fault Zone and near the Santa Ana River, and Recent alluvium in Perris Valley and along the Elsinore Fault Zone.  The subsection elevation range is from about 1400 feet to 2600 feet.  Mass wasting and fluvial erosion and deposition are the main geomorphic processes.

Soils.  The soils are mostly shallow Typic Xerorthents and Typic Haploxeralfs on hills and pediments in granitic terrain and  Lithic Haploxerolls, Mollic Haploxeralfs, and Typic Natrixeralfs on other hills.  On Recent alluvium, there are Typic Xerofluvents, Fluventic and Fluvaquentic Haploxerolls.  There are Natric Haploxeralfs, Haplic and Typic Durixeralfs, and Typic Pelloxererts on dissected nonmarine Quaternary deposits and terraces.  Carbonates have accumulated in some of the soils and silica has accumulated in the older soils.  The soils are well drained, except for somewhat poorly drained Fluvaquentic Haploxerolls in Recent alluvium.  Soil temperature regimes are thermic, and soil moisture regimes are xeric.

Vegetation.  The predominant natural plant communities are California sagebrush - California buckwheat series and Needlegrass grasslands.  There is some Black sage series and small areas of Coast live oak series and Chamise series.

Characteristic series by lifeform include:

Vernal pools: San Jacinto Valley  vernal pools.

Grasslands: California annual grassland series, Foothill needlegrass series, Nodding needlegrass series, Saltgrass series.

Shrublands: Bush seepweed series, California buckwheat series, California buckwheat - white sage series, California sagebrush series, California sagebrush - California buckwheat series, Chamise - hoaryleaf ceanothus series, Chamise - white sage series, Fourwing saltbush series, Mixed saltbush series, Scalebroom series, Scrub oak series, White sage series.

Forests and woodlands: Birchleaf mountain-mahogany series, California juniper series, Coast live oak series.

Surface Water.  Runoff is rapid, except from floodplains and lake basins.  All but the larger streams are dry through the summer.   There are sag ponds along the Elsinore Fault Zone, Lake Elsinore being the largest.  Also, there are several reservoirs in the subsection.