The San Gabriel Mountains are approximately sixty-six miles long, and
average twenty miles wide. The main axis of the range trends nearly east
and west, and extends from longitude 117 deg.25' to longitude 118 deg.30'. The
widest part of the range is bounded by latitude 34 deg.7' and latitude
34 deg.30'.
The San Gabriel Mountains connect the Sierra Nevada with the Peninsular
Ranges of southern California and Baja California. On the west the San
Gabriels are bordered by the Tehachapi Mountains, which stretch
northeastward to meet the southern Sierra Nevada; to the east, beyond
Cajon Pass, the San Bernardino Mountains extend eastward and then curve
southward to the broad San Gorgonio Pass, from which the San Jacinto
Range stretches southeastward to merge with the Peninsular Ranges.
The rocks comprising the major part of the San Gabriel Mountains
probably were intruded in Late Jurassic times, with severe metamorphic
activity taking place concurrently. A long period of erosion followed
after which deposition took place during much of the Tertiary.
Deformation and uplift beginning in Middle Miocene times resulted in the
formation of east-west-trending faults along both sides of the range. By
repeated movements along these faults the Late Jurassic crystalline
rocks were lifted above late Tertiary and Quaternary sediments and
elevated above the surrounding terrain. Continued uplifts in
post-Pleistocene time together with erosion in Recent times have shaped
the San Gabriel Mountains (Oakeshott, 1937).
The alluvial slopes at the coastal base of the range give way to the
foothills at roughly 1800 feet elevation; whereas the Mojave Desert
merges with the interior foothills at elevations near 4000 feet. The
crest or drainage-divide of the range varies from 6000 to 8000 feet in
elevation, and many peaks are more than 8000 feet high. San Antonio
Peak, the highest peak of the range, rises to an altitude of 10,080
feet. The mountains are characteristically steep and the slopes are
deeply carved by canyons, the larger of which have permanent streams.
The abruptness of the Pacific slope is in many places impressive. The
horizontal distance from the top of Cucamonga Peak, at an elevation of
8911 feet, to the base of the coastal foothills directly to the south,
at 2250 feet, an elevational difference of 6661 feet, is only 3.8 miles.
From the base of Evey Canyon, at 2250 feet, to an unnamed peak to the
northwest with an elevation of 5420 feet, the horizontal distance is 2.1
miles. Because of the steep, rocky nature of many of the slopes and the
lack of soil on them, vegetation may be sparse even at high elevations.
There are few meadows in the mountains.
Because the San Gabriels stand approximately thirty miles from the
Pacific Ocean and are a partial barrier to Pacific air masses sweeping
inland, the desert side and the coastal side of the range differ
climatically. The coastal slope receives much heavier precipitation than
the desert slope. The precipitation, for 1951, of 25.36 inches recorded
at the mouth of San Antonio Canyon on the Pacific slope contrasts with
7.17 inches recorded at Valyermo at the desert base. Nearly all of the
precipitation comes in winter. The higher parts of the range, above
approximately 5000 feet, receive much of their mid-winter precipitation
in the form of snow. Snow often extends down the desert slope well into
the Joshua Tree belt. When there are heavy winter rains the channels of
the usually dry washes are filled with rushing, turbid water. There are
striking differences in temperature between the two sides of the range
and between the lower elevations of the mountains and the higher parts.
For example, in December 1951, the mean temperature at the base of San
Antonio Canyon (2225 feet) at the coastal foot of the range was 55.4 deg.F,
while at Llano (3764 feet) at the desert base it was 43.7 deg.F. In this
same year the December mean for Table Mountain (7500 feet), on the
desert slope, was 33.4 deg.F. The temperature means for July, 1951, at San
Antonio Canyon, Llano, and Table Mountain, were 77.3 deg.F, 82.1 deg.F, and
69.2 deg.F respectively. The weather records for 1951 were used for
illustration because average temperature and average precipitation for
many other years are lacking for most of the weather stations in the
area. There is an important difference in the humidity on the two sides
of the range, but actual data are not available. At certain times,
especially in spring, fog banks moving in from the Pacific Ocean
frequently blanket the coastal base of the mountains and the foothills.
On such days the fog generally "burns off" in the morning, but may
persist into the afternoon or throughout the day. Never in my experience
has fog spilled over the main part of the range far onto the desert
slope, although the fog may push through the lower passes to be
dissipated quickly in the dry desert atmosphere. The obvious differences
in the biota on the two sides of the range are probably due to the
contrasting climates.