Being about 28 million years old, rocks from great distances have been juxtaposed against rocks from very different locations and origins. The hallmark of the San Andreas Fault is the different rocks on either side of it. In San Bernardino and Los Angeles Counties, many of the roads along the fault cut through great mountains of gouge, the powdery, crumbled rock that has been pulverized by the moving plates.
In other places, it is more subtle because the fault hasn’t moved in many years and is covered with alluvium, or overgrown with brush.
#Color me mind plates series
In many places like the Carrizo Plain (San Luis Obispo County) and the Olema Trough (Marin County), the fault is easy to see as a series of scarps and pressure ridges. But communities like Desert Hot Springs, San Bernardino, Wrightwood, Palmdale, Gorman, Frazier Park, Daly City, Point Reyes Station and Bodega Bay lie squarely on the fault and are sitting ducks. And despite San Francisco’s legendary 1906 earthquake, the San Andreas Fault does not go through the city. San Francisco, Sacramento and the Sierra Nevada are on the North American Plate. San Diego, Los Angeles and Big Sur are on the Pacific Plate.
It slices California in two from Cape Mendocino to the Mexican border. The San Andreas Fault is the sliding boundary between the Pacific Plate and the North American Plate. Map copyright by David Lynch ( click to enlarge). The arrows show the directions of relative motion along the fault. Areas to the west (left) of the fault are part of the Pacific tectonic plate. Areas to the east (right) of the fault are on the North American tectonic plate. San Andreas Map: The red line on this map follows the surface trace of the San Andreas Fault across California.
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