Alder Creek is just north of Point Arena, and except for the small Shelter Cove segment is the northernmost appearance of the San Andreas fault on land. The view is northward. In 1906 ruptures were noted on both sides of the brown rocks at the center. On the left (west) is the Pacific plate, on the right (east) the North American plate. More specifically, the rocks on the west belong to the Gualala block and consist mostly of sedimentary rocks whereas the eastern rocks are older rocks of mixed types, mostly metamorphic, belonging to the Franciscan group. The bluish area next to the brown block appears to be fault gouge, the finely ground material produced after thousands of years of fault movement. It occurs at countless locations along the San Andreas fault.
This area was closely studied after the 1906 earthquake, and some noteworthy photographs were included in the Lawson Report of 1908. A wooden bridge across Alder Creek to the right of this view collapsed. Its replacement survived until floods took it out in 1964, and state Route 1 was moved inland several hundred meters. The roadway leading to this point (and visible at bottom) is closed to vehicles now and gradually disintegrating; guidebooks telling you to look for a "coastal access" sign are no longer correct. The road is the left turn at the 22.48 mile post on Route 1.
The sides of the fault moved almost 5 meters in 1906. Trenching studies have found evidence of an earlier earthquake here, with roughly the same displacement, some time in the 1600s.
