The San Andreas is a right lateral transform fault. (If you're on one side of it, the motion on the other side is to the right. It works both ways.) The fault runs 835 miles, from the Mexican border just south of the Salton Sea to Cape Mendocino, in a northwest directional trend. The width of the fault ranges from less than 100 yards to several miles, while its depth is anywhere from 15 to 60 miles. The fault represents the boundary of the North American and the Pacific plates and is one of the best-exposed plate boundaries in the world. Some 30 million years ago, the San Andreas formed after most of another plate, the Farallon, slid under the North American Plate. Somewhere around Santa Barbara, at that time, the Farallon Trench turned into the San Andreas Fault. Since then, it has spread out to the north and the south. Like broken glass, the fault does not consist of one clean line. Rather, it is fractured, with thousands of veins spraying from it and subsidiary faults created by it. In fact, it is called the San Andreas Fault System—a family of faults. When you look at a map with all of California's faults marked on it, the map indicates the San Andreas Fault as one long line with dozens of other minor faults radiating from it like echoes.

Earthquakes will happen on any of these faults, of course, but the one that has shown the most significant movement is the San Andreas (though geologists are now saying that a subsidiary fault, the Hayward, threatens to do more damage to San Francisco than the San Andreas ever has). It moves most commonly in what is known as a stick-slip motion. As the plates strain to move past each other in opposite directions, friction between them holds them together. They lock, but stress accumulates, building until it's so strong it snaps. The snap causes shock waves—earthquakes. Press your hands together so that your index fingers are aligned side by side. In one quick jolt, slide your left hand forward and your right back and you'll get the idea of sudden movement along a right lateral transform fault. The big earthquakes in recorded history along the fault—Fort Tejon in 1857, San Francisco in 1906, Loma Prieta in 1989—moved like this. The land on the Pacific Plate (your left hand) is called Salinia. As a result of earthquakes, hundreds of which occur each day, in the last 15 million to 20 million years Salinia has moved several hundred miles, or an average of 15 to 20 feet per century, toward Alaska.

From Palmdale we headed to Fort Tejon on Elizabeth Lake Road, the road that originally gave my father the idea for this trip. It runs through gentle valleys with prosperous little ranches for cows, horses, llamas, and even ostriches and turkeys. Ducks floated on sag ponds where cows slurped up water. There were churches galore and houses decked out for Christmas. We passed little towns on the San Andreas Fault, towns oblivious to the fact that one side of the valley is going to move thousands of miles.