Faults like the San Andreas fault are rarely perfectly straight, but rather curve back and forth to some degree. When a bulge on one side of the fault is carried against a bulge on the other side, the excess material is pushed upward. (And where the opposite occurs, the ground is depressed in a sag basin
.) The Hector Mine earthquake
of October 1999 created this small "mole track" pressure ridge in the Mojave Desert. Pressure ridges occur in all sizes: along the San Andreas fault, its major bends coincide with mountain ranges like the Santa Cruz, San Emigdio and San Bernardino Mountains.