Measuring Fault Displacement with Piercing Points

Faults like those of the San Andreas fault system cut through preexisting features and carry the two halves apart. By finding those two halves, known as piercing points, we can determine how far they have been displaced. From that data we can start to reconstruct the history of the fault over geologic time. The more piercing points that can be documented, the better we can test models of tectonic plate motion.

Remains of an ancient submarine canyon with a distinctive set of rocks—a granodiorite and an overlying conglomerate—are found in two places that are 180 kilometers apart today: Point Lobos State Reserve and Point Reyes National Seashore on the California coast. Here are photos from both localities showing evidence that they were once together. Nowhere else in the world are both parts of such a dramatic piercing point preserved in beautiful public parks.

I thank Kathleen Burnham for expertly leading the Point Lobos/Point Reyes field trip on which I took these photos.