Geologic History of San Diego County

The belt of granitic gneisses exposed in the Laguna Mountains essentially divides the county into two zones of plutonic igneous rocks. To the west, the rocks are much more diverse in composition and character, and form what is referred to as the western zone of the Peninsular Ranges Batholith (PRB).

Generalized geologic map of San Diego County showing the approximate distribution of Jurassic gneisses.

1st category in western PRB:
Gabbroic bodies are composed of olivine, pyroxene, and plagioclase, and are the coarse-grained equivalent of basaltic rocks that are found in ocean basins. These appear to be the oldest plutonic rocks in the western zone but attempts to determine their actual crystallization or emplacement ages have been met with very limited success. These rocks weather slowly to form some of our most familiar mountains peaks. Cuyamaca Peak, along with neighboring Middle and North Peaks, Los Pinos Mountain, and Viejas Mountain, are but some of the high-relief features in the county held up by gabbroic underpinnings. Since gabbroic rocks also contain an abundance of iron-bearing minerals, deep reddish soils tend to mark the lower elevations of these monolithic structures.

Cuyamaca Peak (left) and Viejas Mountain (right) are underlain by gabbroic rocks that are more resistant to erosion than the surrounding tonalite.

Mount Poser, another gabbroic body, rises above less resistant tonalite

2nd category in western PRB: Half or more of the western zone is underlain by tonalite, a specific form of granitic rock that has plagioclase feldspar as its main constituent. The remaining minerals include abundant quartz, as well as biotite and hornblende. Radiometric ages for these rocks range from 102 to about 110 million years. Tonalite can be recognized by the grayish soils that develop around similarly colored outcrops and by the low rolling topography that is created by its relatively rapid rate of erosion.
Left: Outcrop of deeply weathered tonalite (grus) surrounding relatively unweathered cores of fresher rock along Sunrise Highway (S-1) and close-up view (right) of fresh rock. Click on right image for larger image view.

3rd category in western PRB:
The remaining igneous rocks in the Peninsular Ranges batholith are granites that are mineralogically similar to the granitic gneisses found along Sunrise Highway. These plutons are easily recognized from the light-colored, bouldery outcrops that dot their slopes and by their topographic profile. They, like the gabbroic plutons, eroded slowly relative to the tonalite and form high topographic features such as Chiquito Peak, Lawson Peak, and Woodson Mountain. The few available radiometric ages for these rocks range from about 120 to about 110 million years. However, the field relationships of some of these plutonic bodies indicate that they are clearly younger than adjacent tonalites and suggests that the emplacement of these two rock types overlapped in time.

Cuyamaca Lake as seen from the top of Stonewall Peak. The abundance of chemically resistant quartz slows the rate of chemical weathering and leaves these rock types high above more easily weathered igneous materials.	A close-up view of granite at Stonewall Peak with gray quartz and abundant whitish feldspar. Click on photo for a larger image view.
Chiquito Peak is a typical example of the higher topographic profile of this granitic rock.

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