This subsection includes the Bighorn Mountains, the northern edge of the San Bernardino Mountains and the Little San Bernardino Mountains.  It is bounded by the Mojave Desert on the north and northeast and by the Coachella Valley on the southwest.  This is the hot and dry eastern end of the Transverse Ranges.  MLRAs 20e, 29f, and 30g.

Lithology and Stratigraphy.  This subsection is mostly Mesozoic granitic rocks and Pre-Cambrian gneiss.   Also, there is some Paleozoic marine sedimentary rock and Quaternary nonmarine sediments.  There is Recent alluvium in Morongo Valley.

Geomorphology.  The Bighorn Mountains are an appendage of the San Gorgonio Mountains, separated from it by the Pipes Canyon fault.  Broad, dissected pediments slope east-northeast across the Bighorn Mountains.  The general trends of these slopes are on the order of 4 to 12%, but the actually gradients to drainage ways are greater.  The Little San Bernardino Mountains, which are separated from the Bighorn Mountains by the Morongo Valley fault, are steep and very steep mountains with narrow canyons.  They trend northwest, parallel to the Mission Creek fault in the San Andreas Fault Zone. The subsection elevation range is from about 500 feet up to 5500 feet.  Mass wasting and fluvial erosion and deposition are the main geomorphic processes.

Soils.  The soils are mostly Typic Xerochrepts, Entic Ultic Haploxerolls, and Mollic Haploxeralfs.  In the Little San Bernardino Mountains there are these plus Lithic Torriorthents and Lithic Camborthids.  On Quaternary nonmarine sediments there are Typic Torripsamments, Typic Haplargids, and Haplic Durargids, and in Recent alluvium, Typic and Mollic Xerofluvents, Typic Xerorthents, and Typic Haploxeralfs.  The soils are well drained.  Soil temperature regimes are mostly thermic, and some soils on north-facing slopes at higher elevations are mesic.  Soil moisture regimes are xeric (nearly aridic) and aridic.  Most of the soils with aridic moisture regimes are in the Little San Bernardino Mountains.

Vegetation.  The predominant natural plant communities are Singleleaf pinyon series, California juniper series, and Mixed chaparral shrublands.  Creosote bush series, some Mojave yucca series, and Joshua tree series, occur in areas with aridic soil moisture regimes.

Characteristic series by lifeform include:
Grasslands: California annual grassland series.
Shrublands: Big sagebrush series, Black bush series, Catclaw acacia series, Chamise series, Chamise - cupleaf ceanothus series, Creosote bush series, Cupleaf ceanothus - fremontia - oak series, Fourwing saltbush series, Joshua tree series, Mixed saltbush series, Mojave yucca series, Mulefat series,  Nolina series, Scalebroom series, Scrub oak series, Shadscale series.
Forests and woodlands: Birchleaf mountain-mahogany series, California juniper series, Curlleaf mountain-mahogany series, Singleleaf pinyon series.

Surface Water.  Runoff is rapid.  All but the larger streams are dry through the summer, and water runs in them only after rain in the Little San Bernardino Mountains.  Natural lakes are absent.