The cliffs here are a good place to become acquainted with the Purisima Formation, a mixture of shale and mudstone with occasional large concretions along certain bedding planes. You can find a few fossils here if you look carefully, but there are better places farther along.

The middle portion of the Fitzgerald Reserve is on the east side of the ridge facing the Half Moon Bay airport. From Airport Road you can see where the San Gregorio fault runs along its base.

The north end of the reserve is where the tidepoolers all go. The fault comes out here, pointing straight at Stinson Beach in Marin County where it merges with the San Andreas fault.

Before you go out on the rocks, turn around and look at the east side of the entrance, which is typical marine terrace sand.

Then go to the west side and see the contrast presented by the Purisima Formation.

The Purisima in the tidepools is sanded by the surf to bring out its tilted bedding layers, which are punctuated by layers of dropstones and shell hash.

Shell hash is pretty much what it sounds like, a layer rich in shell fragments.

Farther along the north side of the beach, the rocks turn rapidly into coarse granite conglomerate, some layers containing outright boulders. It was an extremely vigorous environment that produced rocks like this—perhaps a high sea level and large landslides produced by major earthquakes. Perhaps something like Devils Slide today.

But at some point, be sure and climb to a higher vantage point to see why every geology class in the Bay Area comes here to practice mapping.

It is a magnificant syncline, or folded trough, where fault motion has bent the rock layers and the sea has planed them off into this dramatic arc.

News Daily Newsletter

The Bay

Emma’s Must-Sees

Videos from KQED Live

Donor-Advised Funds

synclinessynclines

Geological Outings Around the Bay: Fitzgerald Marine Preserve