Geology

The San Bernardino National Forest (SBNF) includes parts of, two major geologic-geomorphic provinces of western North America, the Transverse Ranges and Peninsular Ranges provinces.  The San Gabriel and San Bernardino Mountains are part of the eastern Transverse Ranges and the San Jacinto and Santa Rosa Mountains, Thomas Mountain, and Cahuilla Mountain are part of the northern Peninsular Ranges.  The geology of the two provinces is vastly different one from the other.

The Transverse Ranges province boundary south of the San Gabriel Mountains is the Cucamonga fault zone, a major compressional fault zone at the base of the mountains.   East of the San Gabriel Mountains the province boundary is right-laterally offset 15-20 km by the San Jacinto fault and is located in the structurally complex San Gorgonio Pass area.

Due to fundamental differences in all but the youngest geology, the pre-Quaternary geology of the San Bernardino National Forest is discussed in terms of  three rock assemblages, the San Gabriel Mountains assemblage, the San Bernardino Mountains assemblage, and the Peninsular Ranges assemblage.  Although within the same geologic-geomorphic province all but the youngest geology of the San Gabriel and San Bernardino Mountains is markedly different.  Major lateral displacement on the San Andreas fault has juxtaposed the different rock assemblages of the two ranges.  The San Bernardino Mountains rock assemblage underlies the area of the San Gabriel Mountains north of the San Andreas fault in addition to the physiographic San Bernardino Mountains.  San Gabriel Mountains rock assemblage underlies the San Bernardino basin south of the San Bernardino Mountains and extends eastward through the northern part of San Gorgonio Pass.  In contrast to the Transverse Ranges province, geologic-physiographic subdivisions of the Peninsular Ranges province are similar with only modest lateral fault disruption of the geology.

More about San Bernardino National Forest GeologyProspecting