Crossing the San Andreas Fault

Somewhere along this grade, with no sign to mark it, you cross one of the most famous fractures on Earth: the San Andreas Fault. It runs some 800 miles up the length of California, the boundary where two great slabs of the planet — the Pacific Plate and the North American Plate — grind slowly past one another.

This is no abstract line on a map. In January 1857 the southern San Andreas ruptured in the Fort Tejon earthquake, one of the largest ever recorded in California, tearing the ground for nearly 225 miles. The same fault, farther north, produced the 1906 San Francisco quake. Here in the Antelope Valley the fault makes its great westward "Big Bend," and the constant squeeze of that bend is what has heaved up the San Gabriel Mountains to your south.

Watch for the tell-tale signs as you drive: long straight valleys, sudden lines of springs and greener brush, ridges that don't quite line up. You're riding across a slow-motion collision that has been building these mountains — and storing up the next great earthquake — for millions of years.