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Subsection M261Fa
Tuscan Flows

This subsection is a relatively low elevation volcanic plateau at the northeast end of the Sacramento Valley.  The southwesterly sloping plateau is at the extreme southwest end of the Cascade Ranges.  It is included in the Sierra Nevada Foothills section because of ecosystem similarities.  The climate is hot and subhumid.  MLRA 18d.

Lithology and Stratigraphy.  Late Pliocene and Quaternary basalt, andesite, and andesitic lahars dominate this subsection.  Some of the basalt is Holocene.  Small areas of Cretaceous marine and Eocene nonmarine sediments are exposed beneath the flows.  There is some Quaternary alluvium in valleys.

Geomorphology.  This subsection is on a very gently to gently sloping plateau with a few moderately steep to steep volcanic cones on it.  There are several Holocene cinder cones.  Where the plateau is dissected, canyon sideslopes are steep to very steep.  Some of the larger canyons have floodplains and terraces in them.  The subsection elevation range is about 200 to 2000 feet on the plateau surface and up to 3103 on Inskip Hill.  Fluvial erosion is the main geomorphic processes.

Soils.  The soils are mostly Lithic Ruptic - Xerorthentic Xerochrepts, Typic Rhodoxeralfs, and Pachic Argixerolls.  Those on sedimentary rocks are mostly Lithic Xerochrepts and Entic Chromoxererts.  Typic Xerofluvents, Typic Xerochrepts, Mollic Haploxeralfs, Typic Durixeralfs, and Typic Palexeralfs are common on floodplains and terraces.  The soils are well drained.  Bicarbonate weathering and leaching and accumulation of clay and silica in subsoils are the main pedogenic processes.  Soil temperature regimes are thermic.  Soil moisture regimes are xeric.

Vegetation.   The predominant natural plant community is Blue oak series.   Needlegrass grasslands predominate on the Vertisols and shallow soils.  Northern basalt flow and Northern volcanic mudflow vernal pool habitats occur in this subsection.

Climate.  The mean annual precipitation is about 20 to 40 inches.  It is practically all rain.  Mean annual temperature is about 56 to 64  F.  The mean freeze-free period is about 250 to 300 days.

Surface Water.   Streams in this subsection drain to the Sacramento River.  All but the larger streams are generally dry during the summer.  There are no natural lakes. 


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