This trip was my father's idea. He had traveled parts of the San Andreas Fault while working on a writing project that would span 20 years, produce five books on the geology of the United States, and earn him the honor of being made a fellow at the Geological Society of America. He had been impressed by the fact that the fault carves a route so flat and smooth and straight that roads can lie on top of it. He was also impressed by the beauty of the landscape that these roads pass through, which the fault in part helps create—narrow, intimate valleys with rock of very different provenance on either side. Arid desert and Joshua trees give way to the live oaks of the Coast Ranges, which in turn give way to vineyards and then to groves of redwoods. In Assembling California, one of my father's five geology books, are two lines that read: "Of the two most direct routes from southern to northern California, always choose the San Andreas Fault. If you have adequate time, it sure beats the hell out of Interstate 5."

From the grapefruit grove we drove northwest—my husband behind the wheel, I in the passenger seat scribbling notes, my father in the back with his maps, navigating us through suburban San Bernardino. Santa decorations and fairy lights twinkled on the lawns of a housing development. The development was right on the fault. We passed an Indian casino, a hospital, a university, a playground, a petroleum pipeline, beehives, a pet emetery—all built right on the fault.

My husband and I were looking for big signs: tilted houses, cracked sidewalks, a huge trench gaping open like a wide mouth. But we soon learned that the land mends itself quickly. Cracks get buried and erased over time. Rather, evidence of the fault is more subtle, found in displaced rock, sag ponds, scarps, offset streams, shutter ridges, linear ridges, shattered ridgetops. The subtlety of the San Andreas Fault is such that people forget that it is there and build upon it—houses, schools, barns, entire towns.

At the eastern edge of San Bernardino, three highways—including the old Route 66—and three railroad lines squeeze together to cut through Cajon Pass, where a saddle in the Transverse Ranges, due to the fault, makes the pass possible. Indians used this pass, as did pioneers and Mormons, and now huge trucks lumber up the grade while scavengers collect bottles at the edge of one of the highways. Cajon Pass is in a punch bowl basin, essentially a hole in the mountains made by fault activity that occurred ten million years ago. The fault mashed up the rock, and erosion sculpted it so that as you stand there you are surrounded by a circle of mountains. My father was mesmerized by the punch bowl. I could tell by looking at him that he was seeing a whole lot more than I could see. My husband and I struggled. By trying, however, I understood clearly that reading land is an art—a transformation of the conceptual into the visual so that you can see back ten million years to watch in fast motion geology in action. I could appreciate that we were riding the fault, that it was lurking beneath us and we were piggybacking it as we made our way up the continent.