Anyone who has watched the annual Rose Bowl, from Pasadena, California, has surely noticed the majestic wall of the San Gabriel Mountains that looms north of the city. Stretching for almost 69 miles east to west, this range, like most of the other Transverse Ranges, is fault-block in nature, its steep edge facing south and its more gentle flank dipping northward.
The west end of the San Gabriels is at Weldon Canyon, where Interstate 5 enters the mountains and their east end is at Cajon Pass, traversed by Interstate 15. The San Andreas Fault runs along their northeastern flank, which is bounded by the Mojave Desert; the Santa Clara River Valley aligns their northwestern edge while the Los Angeles Basin stretches south from the range. The highest peak in the San Gabriel Mountains is Mt. San Antonio (also known as Mt. Baldy) which rises to just over 10,000 feet north of Ontario, California; six other peaks exceed 9000 feet in elevation and many more have summits above 8000 feet. Tributaries of the Santa Clara River drain the relatively dry, northern flank of the range while the watershed of the San Gabriel River covers most of the southern flank, which receives most of the annual precipitation.
During the Miocene Period, some 25 million years ago, rifting began to develop across northwest Mexico, producing the Sea of Cortez. This rift extends the East Pacific Rise and connects with the San Andreas Fault; as the Sea has opened (and the Baja has swung westward), tectonic forces have lifted the San Gabriels and other ranges of Southern California. Of course, the process continues today, triggering earthquakes throughout the region.
