The Last Landfall of the San Andreas Fault, at Point Arena

The San Andreas Fault stretches from the Gulf of California, up through central California, through San Francisco and Pt Reyes, where it leaves land, but it makes another landfall again north of Pt Reyes, at Point Arena, before it finally goes off into the ocean to meet up with the Mendocino Triple Junction (where the last vestige of the Farallon plate is being consumed by the North American plate).

I wanted to go to the place where this happened, to feel the great fault leaving land and heading out to sea, even if there was nothing to be seen. So Dave and I headed to the park north of Pt Arena.

From the day-use parking area, we hiked out a trail labelled "beach access" (which seemed strange, given that the beach was aout 20 feet to our left the whole time and the trail paralleled it) for some fifteen minutes, when we got to a place where suddenly we could see a pronounced change ahead. A line of dunes, incongruously aslant to the normal dune line that followed the beach, came in at an angle, and on the other side of that crest, the land looked different: the plants were different, and the sand was a different color (and, as we discovered as we walked out on it, more firmly packed).

The area immediately adjacent to the dune line contained several small alongated ponds. Dave, who reached the top of the dune first, exclaimed "I think that's a sag pond!" and it was hard to argue the point. In all, the spot seemed like a textbook example of a fault. I'm surprised that it doesn't appear in more geology books.

Several days later, at home with a map, we tried to see whether we had really been in the right place. Roadside Geology of Northern and Central California said that the fault passes into the sea at the mouth of Alder Creek. We were just south of the creek, and couldn't see the creek itself (coming in from the northeast) or the minor road that the map shows just north of the creek, but our view was probably blocked by the dunes on the other side of the sag ponds. We're convinced that we were in the right place.
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