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Subsection M261Fb
Lower Foothills Metamorphic Belt

This subsection is the lower elevation western edge of the Sierra Nevada from the northwest end of the Sierra Nevada southeastward to just beyond the Merced River.  It has a hot and subhumid climate.  MLRA 18d.

Lithology and Stratigraphy.  Mainly Jurassic and Paleozoic marine sedimentary and metavolcanic rocks; some Mesozoic granitic rocks; and minor Eocene and Pliocene basalt and andesite occur in this subsection.  Ultramafic rocks are common along faults.  There is very little Tertiary sedimentary rock and Quaternary alluvium.

Geomorphology.  This subsection is on moderately steep to steep mountains and hills at the western foot of the Sierra Nevada.  All of the northern part and most of the southern part of the subsection is west of the Melones Fault Zone.  West of this fault zone the western slope of the Sierra Nevada does not appear as a plateau, except on a few plutons,  but as a series of northwest to north-northwest aligned ridges that decline in elevation from northeast to southwest.  Major rivers cut across the ridges to form a trellis drainage pattern, although it is not a distinct pattern.  Alluvial fans, floodplains, and terraces are not extensive.  The subsection elevation range is about 300 to 3000 feet.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly Lithic and Ruptic - Lithic Xerochrepts; Mollic and Ultic  Haploxeralfs; and Lithic Xerorthents.  Those on ultramafic rocks are mostly Mollic Haploxeralfs and Lithic Argixerolls.  The soils are well drained.  Bicarbonate weathering and leaching and accumulation of clay in subsoils are the main pedogenic processes.  Soil temperature regimes are mostly thermic.  Soil moisture regimes are xeric.

Vegetation.   The predominant natural plant community is Blue oak series.   Also, there are some Needlegrass grasslands, Chamise series on shallow and rocky soils, and Valley oak series in valleys.

Climate.  The mean annual precipitation is about 20 to 40 inches.  It is practically all rain.  Mean annual temperature is about 52 to 62  F.  The mean freeze-free period is about 225 to 300 days.

Surface Water.   The Feather, Yuba, American, Cosumnes, Mokelumne, Stanislaus, Tuolumne, and Merced Rivers cross this subsection.  Runoff is rapid to these rivers and their tributaries.  All but the larger streams are generally dry during the summer.  There are no natural lakes, but many reservoirs. 


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