Geologic History of San
The Western Plutonic Belt
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.
These rocks can be generally divided into three categories, each of which
can be recognized from a combination of their topographic profile and
general soil or outcrop color.
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
|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