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Subsection M262Bg
San Gorgonio Mountains

This subsection comprises the lower and warmer parts of the San Bernardino Mountains, which are between the southern branch of the San Andreas fault on the south-southwest and the Mojave Desert on the north.  It extends from the Cajon Pass eastward to near the Pipes Canyon fault.  It includes mountains between the Mission Creek fault and the Banning fault on the south.  The climate is hot to temperate and subhumid.  Marine effects on climate are moderate on the south-southwest side and slight on the north and east sides of the mountains.  MLRA 20e.

Lithology and Stratigraphy.  This subsection contains mostly Mesozoic granitic rocks and Pre-Cambrian igneous and metamorphic rocks.  Also, there is some Paleozoic marine sedimentary rock and minor amounts of Pliocene nonmarine sediments.  The mountains are a horst with faults and steep escarpments on the south-southwest, east-northeast, and west-northwest sides.  Quaternary nonmarine sediments and Recent alluvium are small but important components of the subsection.

Geomorphology.  This is a subsection of steep and very steep mountains with narrow to rounded summits and narrow canyons.  The subsection elevation range is from about 1500 feet up to 6000 feet.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly shallow Typic Xerorthents, shallow Entic Haploxerolls, Entic Ultic Haploxerolls, and Mollic Haploxeralfs on granitic and Pre-Cambrian rocks and Lithic and Typic Xerochrepts on Paleozoic marine sedimentary rocks.  On Recent alluvium and Quaternary marine sediments there are Typic and Mollic Xerofluvents and Ultic Argixerolls.  Most of the soils are leached free of carbonates.  The soils are well drained.  Soil temperature regimes are thermic at lower elevations and on south-facing slopes, and some are mesic on north-facing slopes at higher elevations.  Soil moisture regimes are xeric.

Vegetation.  The predominant natural plant communities are Chamise series on shallow and very stony soils, Live oak chaparral shrublands, and Mixed chaparral shrublands at lower elevations and on south-facing slopes; Ponderosa pine series and Mixed conifer series at higher elevations; and Jeffrey pine series on the north side of the mountains.

Climate.  The mean annual precipitation is about 20 to 30  inches.  Most of it is rain at lower elevations and snow at higher elevations.  Mean annual temperature is about 45° to 60°  F.  The mean freeze-free period is about 200 to 250 days.

Surface Water.  Runoff is rapid.  All but the larger streams are dry through the summer.  Natural lakes are absent. 

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