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Ecological Sections: Southern California Mountains and Valleys

Subsection M262Bl
San Jacinto Foothills - Cahuilla Mountains

This subsection includes the lower southwestern slopes of the San Jacinto Mountains, hills along the northeast side of the San Jacinto Fault, and hills and valleys around the Cahuilla Mountains on the southwest side of the San Jacinto Fault.  The climate is hot to temperate, and subhumid.  MLRA 20d.

Lithology and Stratigraphy.  This subsection contains mostly Mesozoic granitic rocks, with large areas of Pliocene and Pleistocene nonmarine sediments and Recent alluvium.  Also, there are Pre-Cretaceous granitic, marine (Jurassic) sedimentary, and metamorphic rocks.

Geomorphology.  This subsection is on lower mountain slopes, moderately steep to steep hills, and nearly level to gently sloping alluvial plains in Terwilliger Valley and other small valleys.  There are alluvial fans, terraces, and floodplains on the alluvial plains.  Pliocene and some Pleistocene nonmarine sediments along the San Jacinto Fault northwest of the Cahuilla Mountains have been eroded to badlands.  The subsection elevation range is from about 1500 feet to 4000 feet.  Mass wasting and  fluvial erosion and deposition are the main geomorphic processes.

Soils.  The soils on granitic rocks are mostly shallow Typic Xerorthents, Typic Xerochrepts, and Typic Haploxeralfs at lower elevations and shallow Entic Haploxerolls, Entic Ultic Haploxerolls, and  Mollic Haploxeralfs at higher elevations.   There are Lithic Haploxerolls and Typic Natrixeralfs on other kinds of rocks.  Soils in badlands are Xerorthents.   Haploxerolls predominate in Recent alluvium.  Calcium carbonates have accumulated in some of the soils at lower elevations.  The soils are well drained.  Soil temperature regimes are mostly thermic; and soil moisture regimes are xeric.

Vegetation.  The predominant natural plant communities are Coast Live oak series, California sagebrush - California buckwheat series, and Black sage series at lower elevations southwest of the San Jacinto Fault, and some Chamise series and Red shank - chamise series.

    Characteristic series by lifeform include:

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

    Shrublands: Big sagebrush series, Bigberry manzanita series, Brittlebush series, California buckwheat series, California buckwheat - white sage series, California sagebrush series, California sagebrush - black sage series, California sagebrush - California buckwheat series, Chamise series, Chamise - bigberry manzanita series, Chamise - black sage series, Chamise - white sage series, Chamise - wedgeleaf ceanothus series, Chamise - mission-manzanita - woollyleaf ceanothus series, Red shank series, Red shank - chamise series, Red shank - birchleaf mountain-mahogany series.

    Forests and woodlands: Bigcone Douglas-fir series, Bigcone Douglas-fir - canyon live oak series, Birchleaf mountain-mahogany series, California juniper series, Coulter pine series, Coulter pine - canyon live oak series, Coast live oak series, Parry pine series, Singleleaf pinyon series.
Climate.  The mean annual precipitation is about 10 to 20 inches.  It is practically all rain.  Mean annual temperature is about 50 to 60  F.  The mean freeze-free period is about 200 to 250 days.

Surface Water.  Runoff is rapid, except from nearly level alluvial plains.  All but the larger streams are dry through the summer.  There are no natural lakes. 

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