Visible Fault Features along the San Andreas Rift Zone in Southern California
The San Andreas Fault Zone is a major structural and physiographic feature in California. Many features of the movement can be seen either on a geologic map or out in the field. In this page, I will show many of these features that are visible along the segment of the San Andreas Fault from Parkfield to the Salton Sea.
A sag pond is an area between two parallel fault zones that has dropped, or sagged, down creating a depression that can fill with water. Other examples of “sag ponds” : San Andreas Lake (the fault’s namesake) near San Francisco, Lost Lake in the Cajon Pass, and Jackson Lake near Big Pines.
Valley formed as a result of two parallel fault lines lifting mountains or ridges alongside a dropped down area, or the result of thousands of years of erosion in a fault zone. Rock within fault zones has been weakened and is therefore easier to erode. These valleys can be quite large, as evidenced by the Owens Valley in eastern California, or similar in appearance to Leona Valley near the Antelope Valley in the Mojave Desert.
Fault scarps can be formed through the surface manifestation of movement that occurred underground along a fault during major earthquakes or continued movement along a specific fault zone over a large period of time.
Rock that has been ground to a fine powder or clay along faults is known as “fault gouge”. These zones of fault gouge can be as wide as 25 feet or as narrow as one inch. With continued movement, solid Granite can be turned into clay as evidenced in many places along the San Andreas Fault Zone.
Movement along a fault can offset drainage features. Many offset streams are visible in the Carrizo Plain segment of the San Andreas Fault. The best example of offset is located at Wallace Creek, in the Carrizo Plain. There, two channels exist because of continued movement. Other smaller examples can be seen in Leona Valley, and along Fort Tejon Road above Littlerock and Pearblossom.
While the section of the San Andreas Fault through the Palmdale area is “locked”, some cracking has occurred along roadways, culverts, and parking lots that cover the fault line. At the Ave S / State 14 Park and Ride lot, some en echelon fractures have been noted. Pavement here is only a few years old.
In Parkfield, the fault is marked in a most amusing way. The bridge here has also been slowly shifted by fault movement since it was built in the 1930’s.