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The San Andreas Fault is a rupture on the surface of the earth that marks a separation of two continental plates; the North American and the Pacific. This rupture is at least 65 million years old, the time when North America separated from Europe creating the Atlantic Ocean.

The North American Plate is moving about a half-inch per year to the west, while the Pacific Plate is moving about the same speed but to the northwest, towards Japan and the Aleutian Islands. That means the Pacific Plate’s speed to the west is one-half that of the North American Plate. Because of this the North American Plate is overriding the Pacific Plate, thus creating the San Andreas Fault.

The San Andreas is not a single rupture but many, and the zone is many miles deep. Where that movement shows on the surface we call a fault, and the movement below can show on the surface at more than one place. The San Andreas is therefore a multi-fault system. In Northern California it is comprised of the main San Andreas, the Hayward, the Calaveras and several other faults, the Napa and the Pacific Star among them. One cannot track a fault from the surface unless you have visual evidence showing on the surface of the planet. Because Northern California is heavily forested and has inland bodies of water, faults are not readily visible until they have a recent break. Over time because of erosion, those breaks will be covered by water or vegetation, or even new sediment, again covering evidence of a fault. It is possible that the Pacific Star and Napa Faults are the northern section of the Hayward Fault, as the Hayward Fault disappears under San Pablo Bay and the Napa into forest.

The surface rocks at the winery on which one stands today are 130,000 years old, very young by geologic standards. There is a huge gap in time between that top 6 feet or so and the dark, hard rocks beneath them: about 65 million years! The young rocks are part of a recent wave terrace from an earlier, higher sea level when there was less ice at the polar caps than today. What rocks that used to be between them have been eroded away by wave action over millennia. The fault has been moving many millions of years, most of the time covered over by the ocean or newer sediments, like that young sandstone or soil. Where the geologists were able to see the fault was a break between sections of the dark, old sandstone exposed by ocean waves. The fault has little vertical movement. Each section to the West of a fault break moves northwesterly away from the immediate one on the East side. There are 5 known separate sections of the fault where they have moved. The easternmost follows under Highway One. The next three are in the field, obvious only by the offset of our creek moving it north and by the deep cut bays on the north and south of our peninsula. The sea mostly exposes the Westernmost, where the geologists first saw the fault, and with a little imagination one can easily see it.

The Pacific Star Fault
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