Strike-slip faults like the San Andreas fault are rarely perfectly straight, but rather curve back and forth to some degree (see the three types of fault). When a concavity on one side of the fault is carried against another on the other side, the ground between sags in a depression or basin. (And where the opposite occurs, the ground rises in a pressure ridge.) Where the ground surface of the sag basin falls below the water table, a sag pond appears. This example is from the San Andreas fault just south of the Carrizo Plain near Taft, California. The two sag ponds lie in a larger rift, a linear valley. Sag basins can be quite large; the San Francisco Bay is an example.
Sag basins can also form along faults with part normal and part strike-slip motion, where the blended stress called transtension operates. They may be called pull-apart basins.
Other sag ponds are shown in the San Andreas fault tour, the Hayward fault gallery and the Oakland geology tour.