Following The Great San Andreas Fault
I have a fascination and love for our mother mountains, the San Gabriels. I’m particularly taken with the violent geology of the San Gabriels. They are renowned as some of the fastest growing mountains in the world, shooting up several feet at a time with each major earthquake, and shedding their growth almost as fast, raining rocks and mud down onto the foothills in each major storm. Whose fault is this violent geographical legacy? The San Andreas Fault, of course.
The San Andreas Fault is actually the crumbling edge where two great continental plates rub and bump against each other. The North American Plate, on which sits the Mojave Desert, is moving southward, while the Pacific Plate, on which we sit, is moving northward. North of us the fault has a bulge where it takes a brief western jog. As the Pacific plate moves north, it hangs up on this bulge, and big wrinkles form. The San Gabriels are but one of the wrinkles.
The San Andreas Fault is a part of our history and culture. It forms our landscape, and affects our future. Yet how many of us have actually seen the San Andreas? My wife and I set out last week to drive along the San Andreas, from Cajon Pass to the Central Valley. We picked up the fault where it crosses the I-15 in the Cajon Pass, and headed west on Highway 138 towards Wrightwood. Wrightwood sits immediately atop the fault. Geologists have focused on Wrightwood, and studies have shown a whopping 14 major quakes in the last 1500 years along this stretch of the fault, about one per century. The last was in 1857, so you can see why seismologists are nervous. At the Big Pine Ranger Station on the north side of the road we observed an exposed fault scarp, where the ground has broken and risen, forming a shear cliff 30 feet high.
Traveling further, and turning right onto the Big Pines Highway, we encountered Jackson Lake, a naturally occurring sag pond. Sag ponds are a defining feature of the San Andreas. They form as the fault gaps, or “sags,” and allow water to collect. These sag ponds are strung along the fault – Elizabeth Lake, Lake Hughes, Quail Lake. Indeed, the entire route along the fault we observed well-watered meadows and thick lines of trees, indicating that the fault traps and holds water much better than the surrounding dry landscape.
As we descended along Big Pines Highway, switchbacking back and forth over the fault, we could clearly see the San Andreas stretching out before us to the northwest. It looked like a jagged tear, filled in with darker green vegetation, stark against the brown floor of the desert by Palmdale, and disappearing into a distant valley, the Leona Valley. As we drove in the distinct trough of the big fault through Valyermo and along the Ft. Tejon Road, we began to readily recognize the sharp, cliff-like scarps along the edges. We also wondered how many homeowners in the many new developments perched on each side of (or sometimes in) the fault understood that in the next “big one” they will be thrown several feet sideways in under a second, and that no structure can absorb that kind of acceleration. Not to be doom-and-gloom, but we also noted that the California Aqueduct crosses the fault several times. It and other aqueducts would surely be severed, cutting the majority of L.A.’s water supplies.
We continued through the Leona Valley to Gorman, and then followed the fault through Frazier Park, and finally down into the Central Valley. It was back roads all the way, and we experienced an amazing array of unspoiled California landscape – mountainous pine forests, desert, meadowlands, wildfire burn zones, coastal oak and white oak forests, high table-lands and finally the flat plains of the Central Valley. It can be done in just a day, and it’s right in our backyard.
It’s a drive worth doing, and with a good map with some topography on it, it’s not hard to follow the world’s most famous fault.