This is a little bit of a diversion from my usual topics of gardens and plants, but the recreational aspect here warrants including my geology class field trip along the lines of my Urban Hike series. This is not so much about the makeup of local geology (not about Serpentinite rocks and Radiolarian Chert) as it is about the physical structure of the land.
Back in March, I took a CCSF field class called Geology of San Francisco. I wrote a post with lots of pretty pictures, detailing the underlying layers of rocks that make up San Francisco. (The school Earth Sciences department got hold of it and even put it on their own website. Flattery!) This semester I’m in a class called Environmental Geology. We’re specifically studying anthropogenic changes to the environment and how our manipulations of geology affect the world around us, and how that in turn affects us. By understanding how the land functions, we can make more informed decisions about how we use that land. This is true in construction, to be sure, but it also applies to gardening and agriculture.
On this field trip we started off in SF at Ocean Beach, across the Great Highway from the zoo. We then went south to Thornton Beach State Park in Daly City. After that we visited the spot where the San Andreas Fault hits the ocean at Mussel Rock. Our trip ended with a walk along the beach in Pacifica. Along the way we considered land composition and erosion rates, and how people manage them. We considered severity of natural hazards. We considered how climate change will affect these areas, and assessed what we would recommend if we were the decision-makers for this area.
The California coastline is a dynamic environment. The East Coast of the US is in the middle of a tectonic plate and has very little movement. The beaches are shallow and wide, and you can wade off the coast for quite a ways. The land is low-lying for its entirety (as we’ve tragically been experiencing repeatedly, especially in this last decade of climate change,) and is relatively stable and slow to change. The West Coast, however, is right along a major fault separating tectonic plates. The North American and Pacific plates grind past each other, constantly uplifting the sea floor and changing the shape of the land. Just since I graduated high school and moved to San Francisco, we have come two feet closer to Los Angeles! (One of my classmates last year asked the professor if LA and SF would eventually meet up “a long time from now in, like, 65 years or something.” The diplomatic answer was, “Well, yes and no.”)
Our coastline is rocky and steep. Beaches tend to be narrow, squeezed in between surf and cliffs. In winter, heavy surf washes sand off the beaches. They narrow and many become beaches of pebbles and cobbles. In summer, however, the waves are comparatively gentle. The sand washes back ashore and our beaches widen and become recreational havens. Offshore, the steepness of the underwater topography creates higher breakers than seen on the East Coast, thus the preponderance of surfing around here.
Locally, the cliffs often are loosely-consolidated sandstone, which easily collapses and slides. Older rock has had time to solidify for millions of years and is resistant to erosion, but the local sandstone has been pushed up from the ocean floor on the order of tens of thousands of years, and is much softer. You can carve it with your fingernails. Winter storms and higher surf create the biggest driving forces of erosion for us. With the beaches washed away for the season, the waves pound the bases of the cliffs. Add to that the rain. It not only physically washes down the cliffs, taking the sand with it, but adds tremendous weight to the cliffs as the ground becomes saturated. That there is what you’d call a recipe for landslides.
The sand making up our beaches slowly moves southward with the currents. It comes in part from the cliffs themselves, but the primary source is the rivers. The erosion upstream creates sand that eventually makes it out to sea. Those longshore currents work the sand south and south and south until it reaches a canyon underwater and flows out to sea, where it will eventually solidify and uplift into new mountains and cliffs. This supply of sand is called the Sand Budget. The river-to-canyon cycle of sand is known as a Littoral Cell. A typical West Coast beach is part of a Littoral Cell that’s directly related to, and south of, a river outflow.
We’ve dammed all the rivers in California that bring sediment out to sea. Uh-oh. Where’s the sand supposed to come from to replace what is washing away southward? Good question. Our beaches have gotten narrower and narrower since settlement. Add to that the climate change and melting icecaps raising sea levels. Narrower beaches means the waves reach the cliffs more easily. That means the cliffs erode more readily and more frequently than before. During especially stormy El Niño years, we see some spectacular examples of erosion’s effects. Houses wash into the sea. Pacifica is particularly hard-hit, being nothing but a big seaside shelf of that loose sandstone. Even the past couple of winters we’ve seen yards and homes fall onto the beach, and houses have to be moved back from the cliffs. No coastal area is immune to this, but harder stone erodes much more slowly. In the 1990’s we even saw huge houses go over in the super-duper expensive Sea Cliff development in San Francisco, where people spend millions upon millions of dollars for the privilege of ocean views.
On our field trip, we saw first-hand how this erosion affects the land and us. At Ocean Beach in SF, erosion has been cutting the beach back. In that post from earlier in the year you can see areas where the road and parking lots have literally fallen over the edge. One way to try to slow the erosion is through riprap. Riprap is a pile of rocks, basically. Giant boulders (or pieces of road and concrete, and sections of brick walls – whatever is available) are piled against the cliffs to slow the undercutting from the waves. However, that means the cliffs are now unavailable to erode into sand for the beaches, further narrowing the beaches for us and those south of us and increasing the chances of collapse! Ahh, irony.
The California Coastal Commission, who regulates how we deal with such things, denied SF’s request to rebuild the riprap walls along our county’s coast. In fact, they made us take down what had been built before. The next method to deal with the erosion encroaching on our $220M sewage treatment plant a couple dozen yards from the surf (brilliant plan that was) is to rebuild the beaches and dunes. This is called beach nourishment. Sand is brought in from elsewhere and piled up on the beaches to add to the sand budget. We get ours offshore from dredging the shipping channels, but in Florida they actually get their new sand from Alaska, if you can believe it. Talk about a carbon footprint! This method is a little experimental for us, and it’ll be interesting to see how the new sand rides out this first winter. They piled it high and steep, and hope it does the trick. If we have to spend more to constantly replace the sand than if we just build a new treatment plant, I’ll be curious to see how the politics play out on this. The environmental protection world is watching us to see how it works!
At Thornton Beach, we viewed cliffs and historic landslides from above. Damn, what a beautiful coastline! The prevailing plant is actually an invasive species – Highway Iceplant (Carpobrotus edulis) – that is a fast-growing succulent groundcover planted extensively to control erosion. Again with the irony, the plant has shallow roots and holds tons of water, and the weight actually increases the likelihood of landslides. Incidentally, as the edulis in its name reveals, it is edible. Leaves and fruits can be eaten (by us), and can be used to make a lovely tart jam. The plant’s fast growth and extreme size (50 foot diameter from a single root system) means it chokes out light for native seedlings and steals water from endemic species. Go harvest some and make some jam right now, and do your part to reduce its impact. 🙂
Thornton Beach was actually a recreational beach not that long ago. Big storms brought significant landslides, and CalTrans eventually had to give up on maintaining a road through here. From the top of the cliffs we could clearly see both landslide erosion and the effects of rain cutting channels and rivulets into the soft sandstone.
At Mussel Rock we got to see the actual line of demarcation between these two tectonic plates. Standing where we were on the soft sandstone cliffs, we were on the NA plate. Looking across to Mussel Rock itself, we are seeing harder green stone on the Pacific plate. Cool! Houses above perch right at the lip of the cliffs, and you can see terrace upon terrace of former landslides. Seriously. Homes on sandstone cliffs, ON the San Andreas Fault, over a beach-less shoreline. Even before the development was completed, eleven of those homes had to be relocated. Yet, people bought what was left. Not for me, thanks. It’s understood that any remediation efforts are purely temporary, since the Earth will absolutely win here. To protect the homes for the short-term, the Coastal Commission has allowed drainage control here. Multiple culverts divert the downflow of water, and walls made of caged rocks (gabions) try to reduce the runoff, and help to filter sediment out of runoff and keep it on the hillside. Below the seaside paths we could see former gabions that had slid down the hill. Obviously not a permanent fix. The weight of the houses themselves adds to the pressure on the slopes, much like the winter rains. Those houses will eventually be more beachfront than they are now, and I suspect we’ll get to see the devastating (but absolutely expected) spectacle in our own lifetimes.
Check out the California Coastal Records Project website if you want to see historic photos to compare changes on your part of the coast! They’ve photographed, literally, every foot of the coastline.
Our final destination was the town of Pacifica. Once a sleepy little fishing village of summer vacation homes, its proximity and easy access to SF (it’s faster to drive there from my work than to drive home) made it become a large and popular suburb of over 100,000 people. Pacifica is a popular spot for surfers. It’s also a popular spot to collapse into the sea, if you’re a sandstone cliff. Homes have been moved back from the edge many times over the decades. Even a couple of years ago we watched houses go over. The debate is on as to how to deal with that. Do we invest public funds to protect private property? There are strong arguments to both sides, but the reality is that it is a guaranteed losing battle. Or, at least a very very expensive one.
But then again, isn’t everything temporary? Even the mighty Rockies are already on a path to oblivion, when it comes down to it. As a society, we have to decide what’s worth the effort.