This subsection includes the Puente and Chino Hills, which are northwest of the Santa Ana River, and most of the Santa Ana Mountains, which are southeast of the Santa Ana River.  The climate is hot and subhumid; it is modified moderately by marine influence.  MLRA 20d.

Lithology and Stratigraphy.  This subsection contains mostly Jurassic marine clastic sedimentary, Jurassic volcanic, and Mesozoic granitic rocks.  There is some mafic plutonic rock and small areas of Pleistocene basalt.  The Puente and Chino Hills consist of Miocene marine sedimentary rocks.

Geomorphology.  This is a subsection of steep to very steep mountains with narrow to rounded summits and narrow canyons.  There are some rolling plateau surfaces, also.  The hills northwest of the Santa Ana River are steep.  These hills and the Santa Ana Mountains trend northwest.  The Santa Ana Mountains are bounded on the northeast by a steep escarpment along the Elsinore Fault Zone, and the Puente and Chino Hills are bounded on the south-southwest by the Whittier Fault Zone. The subsection elevation range is from about 300 feet along the Santa Ana River up to 5687 feet on Santiago Peak.  Mass wasting and fluvial erosion are the main geomorphic processes.

Soils.  The soils are mostly shallow Typic Xerorthents, Typic Xerochrepts, and Typic Haploxeralfs on granitic rocks and Lithic Xerorthents, Lithic Haploxerolls, and Typic Haploxeralfs on Jurassic sedimentary and volcanic rocks.  They are Calcic and Pachic Haploxerolls and Typic Argixerolls on Miocene sedimentary rocks.  Rhodoxeralfs and Haploxeralfs predominate on mafic plutonic and volcanic rocks.  Most of the soils, except those in the Puente and Chino Hills, are leached free of carbonates.  The soils are well drained.  Soil temperature regimes are mostly thermic, but some are mesic on north-facing slopes at higher elevations.  Soil moisture regimes are xeric.

Vegetation.  The predominant natural plant communities are Coast live oak series, Chamise series, which is generally on shallow or very stony soils on south-facing slopes, Manzanita chaparral shrublands, and Mixed scrub oak series.  Some Bigcone Douglas-fir series occurs on north-facing slopes, Canyon live oak series occurs in canyons, California sagebrush series occurs on south-facing slopes at lower elevations, and Coulter pine series occurs at higher elevations.

Characteristic series by lifeform include:

Vernal pools:  Santa Rosa Plateau vernal pools.
Grasslands: California annual grassland series, Purple Needlegrass series.

Shrublands: California buckwheat series, California sagebrush - black sage series, California sagebrush - California buckwheat series, Chamise series, Chamise - black sage series, Chamise - white sage series, Chamise - hoaryleaf ceanothus series, Eastwood manzanita series, Hairyleaf ceanothus series, Hoaryleaf ceanothus series, Mixed scrub oak series, Scrub oak series, Scrub oak - chaparral whitethorn series, Scrub oak - chamise  series, Sumac series, White sage series.

Forests and woodlands:  Bigcone Douglas-fir series, Bigcone Douglas-fir - canyon live oak series, Black oak series, Canyon live oak series, Coast live oak series, Coulter pine series, Coulter pine - canyon live oak, Coast live oak series, Engelmann oak series, Interior live oak series, Knobcone pine series, Tecate cypress stands.

Surface Water.  Runoff is rapid.  All but the larger streams are dry through the summer.  Natural lakes are absent, but some of the streams drain to Lake Elsinore on the northeast side of the subsection.