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Ecological Sections: Southern California Mountains and Valleys

Subsection M262Bd

San Gabriel Mountains

Photo of San Gabriel Mountain range in Southern California mountain and valley ecosystem

This subsection comprises the lower and warmer parts of the San Gabriel Mountains, which are between the San Andreas fault on the north-northeast and the Los Angeles and Fontana Plains on the south.  It is bounded by the Soledad River valley on the northwest and the San Fernando Valley on the southwest and extend eastward to the Cajon Pass. The climate is hot to temperate, and subhumid.  MLRA 20e.

Lithology and Stratigraphy.  This subsection contains mostly Mesozoic granitic rocks and Pre-Cambrian anorthosite.  Also, there is some Pre-Cretaceous Pelona schist.

Geomorphology.  This is a subsection of steep and very steep mountains with narrow to rounded summits and narrow canyons.  The mountains trend east-west, but hills along the San Andreas fault trend west-northwest.  The subsection elevation range is from about 500 feet up to 6000 feet.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly Lithic and shallow Typic Xerorthents, shallow Entic Haploxerolls,  and Calcic Haploxerolls.  Most of the soils, but not all, are leached free of carbonates.  The soils are well drained.  Soil temperature regimes are mostly thermic, and some mesic on north-facing slopes at higher elevations.  Soil moisture regimes are xeric.

Vegetation.  The predominant natural plant communities are Chamise series and Chamise - hoaryleaf ceanothus series, which are generally on shallow and very stony soils.   Live oak chaparral series and Mixed chaparral shrublands also occur.  Ponderosa pine series with some Bigcone Douglas-fir series and Bigcone Douglas-fir - canyon live oak series occurs on north-facing slopes, Jeffrey pine series occurs on the north-northeast side of the mountains, and California juniper series occurs on hills along the San Andreas fault.

    Characteristic series by lifeform include:

    Grasslands: Beaked sedge series, Bur-reed series, California annual grassland series, Tufted hairgrass series.

    Shrublands: Big sagebrush series, Bitterbrush series, Black bush series, Black sagebrush series, California buckwheat series, California buckwheat series, Chamise series, Chamise - bigberry manzanita series, Chamise - white sage series, Chamise - black sage series, Chamise - hoaryleaf ceanothus series, Chaparral whitethorn series,  Cupleaf ceanothus - fremontia - oak series, Fourwing saltbush series, Hairyleaf ceanothus series, Hoaryleaf ceanothus series, Hop-sage series, Mixed saltbush series, Mixed scrub oak series, Mountain whitethorn series, Mulefat series, Nolina series, Parry rabbitbrush series, Scalebroom series, Scrub oak series, Scrub oak - chaparral whitethorn series, Shadscale series, White sage series.

    Forests and woodlands: Birchleaf mountain-mahogany series, California juniper series, Coulter pine series, Coulter pine - canyon live oak series, Bigcone Douglas-fir series, Bigcone Douglas-fir - canyon live oak series, Canyon live oak series, Coast live oak series, Curlleaf mountain-mahogany series, Ponderosa pine series, Singleleaf pinyon series.
Climate.   The mean annual precipitation is about 20 to 30  inches.  Most of it is rain.  Mean annual temperature is about 45 to 60  F.  The mean freeze-free period is about 200 to 275 days.

Surface Water.  Runoff is rapid.  All but the larger streams are dry through the summer.  Natural lakes are absent, except along the San Andreas fault.   There are some sag ponds along the San Andreas fault. 

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