This subsection includes alluvial fans in San Gorgonio Pass; basin floor in the Redlands area, between the San Andreas and San Jacinto Faults; and a basin that is separated from the Los Angeles Basin by the Chino,  Puente, and San Jose Hills on the west side of the subsection.  It is just south of the San Gabriel and the San Bernardino Mountains.  The climate is hot and subhumid; it is moderated by marine influence.  MLRA 19d.

Lithology and Stratigraphy.  This subsection contains mostly Quaternary nonmarine sediments and Recent alluvium.  There are some Mesozoic granitic rocks and older sedimentary and metamorphic rocks in the Jurupa Mountains and other hills that are included in this subsection.

Geomorphology.  This subsection is on very gently to gently sloping alluvial fans, mostly from the San Gabriel and San Bernardino Mountains, and some floodplain along the Santa Ana River and its larger tributaries.  A few areas of hills are included in this subsection.  The subsection elevation range is from about 600 feet up to 2400 feet.  Fluvial erosion and deposition are the main geomorphic processes.

Soils.  The soils are mostly Typic Xeropsamments, Typic Xerorthents, Fluventic Haploxerolls, and Typic Haploxeralfs.  There are Haplic and Typic Durixeralfs on old alluvial fans.  Carbonates have accumulated in some of the soils and silica has accumulated in the older soils.  The soils are well drained.  Soil temperature regimes are thermic.  Soil moisture regimes are xeric.

Vegetation.  The predominant natural plant communities are California sagebrush - California buckwheat series and Needlegrass grasslands.

Characteristic series by lifeform include:

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

Shrublands: Brittlebush series, Bush seepweed series, California buckwheat series, California buckwheat - white sage series, California sagebrush series, California sagebrush - California buckwheat series, California sagebrush - black sage series, Chamise series, Chamise - California buckwheat - white sage series, Fourwing saltbush series, Mixed saltbush series, Scalebroom series, White sage series.

Forests and woodlands:  California juniper series, California sycamore series.

Surface Water.  Runoff is rapid, even from the alluvial fans.  All but the larger streams are dry through the summer.   Water runs in the Santa Ana River throughout the year.  Natural lakes are absent.