This subsection comprises the mountains and hills at intermediate elevations on the southwest, or coastal, side of the Peninsular Ranges.  It extends from the Santa Ana Mountains southeastward to the Mexican border.  The northern part of the subsection is bounded on the northwest by the Elsinore Fault Zone.  The southwestern edge of the subsection is near a line through the San Marcos Mountains, Woodson Mountain, Iron Mountain, Sequan Peak, and Otay Mountain.  The climate is hot and subhumid; it is modified moderately by oceanic influence.   MLRA 20d.

Lithology and Stratigraphy.  This subsection contains mostly Mesozoic granitic rocks.  There are some Pre-Cenozoic granitic and metamorphic rocks and some Mesozoic mafic plutonic rocks, also.  There are small areas of Pleistocene sediment and Recent alluvium.

Geomorphology.  This is a subsection of moderately steep to steep mountains and hills with narrow to rounded summits and narrow to broad canyons.  There are some rolling plateau surfaces, too.  The streams run southwestward toward the ocean, but San Luis Rey River initially runs parallel to the trend of the Peninsular Ranges and takes a 90° turn before running to the ocean.  Many of the streams have alluvial plains a few hundred yards wide, or even wider in places.  The subsection elevation range is about 1200 feet to 4000 feet.  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.  Also, there are Rhodoxeralfs on mafic plutonic (diorite and gabbro) rocks and Lithic Xerorthents, Haploxerolls, and Haploxeralfs on other kinds of rocks.  The soils of floodplains and terraces are mostly Typic Xeropsamments, Cumulic Haploxerolls,  and Typic Haploxeralfs.   Most of the soils are leached free of carbonates.  The soils are well drained.  The soil temperature regimes are mostly thermic.  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, Mixed chaparral shrublands, and Coastal sagebrush series.

Characteristic series by lifeform include:

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

Shrublands: California buckwheat series, Chamise series, Chamise - Eastwood manzanita series, Chamise - bigberry manzanita series, Chamise - cupleaf ceanothus series, Eastwood manzanita series, Chamise - mission-manzanita - woollyleaf ceanothus series, Chamise - white sage series, California sagebrush - California buckwheat series, Chaparral whitethorn series, California buckwheat - white sage series, Mixed scrub oak series, Scrub oak series, Scrub oak - chamise series, Scrub oak - chaparral whitethorn series, Scrub oak - birchleaf mountain-mahogany series,  Sumac series, White sage series.

Forests and woodlands: Bigcone Douglas-fir series, Bigcone Douglas-fir - canyon live oak series, Birchleaf mountain-mahogany series, Black oak series, Coulter pine series, Coulter pine - canyon live oak series, Coast live oak series, Engelmann oak series, Tecate cypress stands.

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