Rise of the Sierra Madre: A Brief History of the San Gabriel Mountains

USC Libraries

Rise of the Sierra Madre: A Brief History of the San Gabriel Mountains

Last Friday, 41,000 acres of the Angeles National Forest charred by the 2009 Station Firereopened to the public. With summer's return, and with newly accessible trails to explore, Southern Californians will soon flock to the San Gabriel Mountains for fresh air and mountain scenery.

Stretching from the Cajon Pass in the east to the Newhall Pass in the west, the San Gabriel Mountains are something of a topographic anomaly. Whereas most mountain ranges in North America parallel the coasts, the San Gabriels and the other Transverse Ranges, including the San Bernardino, Santa Monica, and Santa Susana, run east-to-west. Geologists credit this crook -- responsible for the prominent jog at Point Conception in the California coastline -- to movement between the Pacific and North American tectonic plates.

About 20 million years ago, the Pacific plate began scraping against the North American plate and broke off a piece of continental crust. Trapped between two plates, the loose block of crust turned 90 degrees clockwise as the Pacific plate dragged it to the northwest. About 5-7 million years ago, part of that block began rising as a mountain range along the Sierra Madre and Cucamonga fault zones, creating the San Gabriels and forming an unusual bend in the otherwise orderly line of coastal mountain ranges.

Since then, the geologically active mountains have been thrust up quickly -- and have been cut back by erosion almost as fast. Much of the Los Angeles metropolis, in fact, sits atop the accumulated sediments washed off the slopes of the San Gabriels.

Today, the mountains trap moist, cool air from the Pacific Ocean and block cismontane Southern California from the hot climate of the continental interior, creating what Carey McWilliams described as "an island on the land." Their rugged slopes have presented a formidable obstacle to travelers and traders, and an existential threat to entire foothill communities, but they have also provided Southern California with a number of productive and recreational uses.