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.
Pressure Ridge, California
Pictures of Tectonic Landforms
Pressure ridges form where lateral motions on a curving fault force rocks into a smaller space, pushing them upward. (more below)
Photo by Paul "Kip" Otis-Diehl, USMC, courtesy of the U.S. Geological Survey
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