Tuesday, January 01, 2013

Thornton Beach

San Francisco's Ocean Beach transitions southward into progressively higher bluffs in the vicinity of Fort Funston, which then continue to rise on into Daly City towards Mussel Rock.  About a mile of the bluffs between Fort Funston and the northern portion of Daly City are marked by a spectacular deep-seated landslide complex - characterized by rolling topography and shore-parallel ridges and swales.

My brief research suggests a range of ages for this slide - which is often the case with these features - since some people are referring to the age of the overall feature, whereas others refer to specific events or even to more recent reactivation.  From what I can tell, the landslide dates back hundreds of years, but experienced significant movement in the 1906 earthquake and continued to experience some additional sliding during the past century.  Regardless, the railroad, and later the highway, that was built on the face of the bluffs from San Francisco to Pacifica (and beyond), in the early 1900s, has not survived - except in small pieces and on interpretive signs.

Pacifica - Sharp Park

This part of Pacifica is built at beach level -- perhaps on old backshore dunes.  But just as with the bluffs shown in the previous post, this beach has been subject to significant erosion.  For this and many parts of the California coast - the El Nino of the 1982-83 winter was a tough one.  The community has gone through a few iterations of fortifications and is now perched behind a high seawall with a big rock apron.


It was a blustery day (the photos were taken on December 21st), waves were building, but the tide was not particularly high.  I suspect these were pretty tame conditions and certainly not enough to discourage the fishermen.

Pacifica - Esplanade Avenue

The northern portion of Pacifica is built on the 80' vertical edge of a rapidly eroding marine terrace.  Apartment buildings - some older; some recently remodeled - are lined up along the edge, protected by a discontinuous line of riprap.


I assume there is some controversy about the riprap.  But I suppose that riprap is all there will be in another decade or two.  There certainly won't be any more beach at high tide by then. Temporary protection of private structures or long-term protection of public resources?  It looks like the trade off for more riprap is better access to the little bit that's left?

Mussel Rock

Daly City is where the San Andreas fault heads out to sea.  It arrives from the south by way of a series of fault valleys and elongated lakes and reaches the coast just north of Mussel Rock.  It continues northward offshore past the entrance of the Golden Gate before reconnecting briefly with California at Point Reyes.

There's a large divot where the fault intersects the high bluffs, at least in part due to past landslides.  In the 1960's tidy rows of homes were built along the top edge of the bluffs.  Remarkably, most of them are still there.


Mussel Rock Park is the site of a historic landfill, now closed, but heavily fortified with riprap.  Mussel Rock itself is a large block of Franciscan rocks squeezed out along the fault, and described in the opening passage of McPhee's Assembling California.  As McPhee notes, this is a good (but often foggy) spot to "sit and watch the plates move."  Which I suppose is the attraction of those homes perched at the edge of the precipice, witness to storm waves, landslides, and plate tectonics.