San Andreas Fault Trail Exploration

Standing on the Pacific Plate, you see Black Mountain (left) Mount Umunhum (center) and Loma Prieta (just right) on the North American plate. Two million years ago, boulders shifted from Loma Prieta to the Pacific plate, and moved northward inch by inch for 23 miles.

San Andreas Lake is a sag pond-one of the features of fault zones. These shallow depressions are places where the earth pulled apart under pressure. There's another sag pond just north of the parking lot. Many are marshy in winter. In the summer they can be harder to spot. Erosion eventually fills in sag ponds.

This boulder is a type of rock called "conglomerate." It's made up of pieces of rock that were eroded from an even more ancient landscape and cemented together into sedimentary rock. The pieces were carried so quickly-- perhaps by water-- they did not break into smaller bits.

In the distance you can see Crystal Springs reservoir, created by damming San Andreas Lake. The lake was first noted by the Portola expedition in 1769. Later it provided much of the water for San Francisco. Its position low in the valley makes it ideal for collecting water. Unfortunately, it lies right on top of the San Andreas Fault.

How can you tell if a feature is a bench caused by earthquake activity or a streambed caused by erosion, such as water running downhill? Look at how the feature is situated. This bench runs perpendicular to the contour of the hill, not parallel with it.

Just behind the bench is a pressure ridge. You can make your own pressure ridges: Place your hand on a table covered by a tablecloth. Push the tablecloth away from you on the table, and watch the fabric fold. The tremendous forces of the Pacific and North American Plates pressing against each other push the land into ridges.

Another good clue to earthquake activity in the landscape is a bench. You might mistake this very flat area at your feet for a road created by people. A bench forms during an earthquake when the ground cracks open. Eventually, weather and erosion fill in the cracks and smooth out the land.

In the woods you'll see more evidence of earthquake activity. This is a reproduction of a fence line that moved five to seven feet during the 1906 earthquake. In Point Reyes, the land slipped over twice as far because it was closer to the quake's epicenter.

Posts topped with yellow bands mark the main fault break from the 1906 earthquake. Posts with white bands mark minor fault breaks. The San Andreas Fault isn't one continuous fault but is made up of many segments. The ground can move at any time along any of the segments.

There can be a heavy flow of water across here in the spring. The earthquake activity in the area grinds the soil into very fine particles, called "fault gouge." The dirt dams the flow of underground water, which backs up and turns the area into a marsh.

You'll see evidence of human activity here. These are all second-growth trees. The original trees were cut down and used to build Palo Alto, as well as rebuild San Francisco after the 1906 earthquake. People lived here, too. Before much was known about earthquakes, a bench looked like a nice, flat spot to build a house!

Trees grow upward, unless something happens to knock them over. The lower part of this trunk dates from the late nineteenth century, while the vertical section dates from the early twentieth century. Apparently, the 1906 earthquake knocked the tree over, and the branches then became the new main trunks.

Another bench marked by posts with white bands indicates that this is an area of minor quake activity.