Showing posts with label Salinian Terrane. Show all posts
Showing posts with label Salinian Terrane. Show all posts

Thursday, January 30, 2014

Where the Sierra Nevada Rises From the Sea: A Compilation of Posts




The Sierra Nevada of California is one of the great mountain ranges of the world. The soaring granite peaks, the deep glacial valleys, and the towering Sequoia trees represent some of the most beautiful sights to be seen anywhere on the planet. What is less known is that a portion of the Sierra Nevada is present elsewhere in the state. Around 30 million years ago the San Andreas fault system became active and sliced off a portion of the southern Sierra Nevada batholith and carried it northwest for several hundred miles. It now makes up a considerable portion of the Central California coastline, stretching from Big Sur on the south to Bodega Bay on the north. It is in my humble opinion one of the most beautiful and dynamic coastlines to be found anywhere on planet Earth.

I recently finished my latest blog series, and I present here a compilation of the fifteen entries in the series. If you missed any, here is the place to find them!
A sneak peak at the coming series. At this point I hadn't fully appreciated that I was going to be covering the entire Salinian Terrane, so I didn't mention the Sierra Nevada connection in the first post.
With the second post I hit upon the idea of the Sierra Nevada connection with my exploration of Limekiln State Park in the southern part of the Big Sur coast. It is a stunningly rugged stretch of coastal cliffs, and amazingly, the state of California almost shut down this beautiful place.
I punted on this one. I wrote this blog four months earlier about what may be the prettiest cove along the prettiest coast in the world. But the post fit well with the theme and scope of this series, so here it is: one of California's two tidal falls. And gigantic landslides.
A short distance inland from the coast we discover a gem of a state park, Pfeiffer Big Sur. The Sierra Nevada has the Sequoia trees, Big Sur has Coast Redwoods. And both the Sierra Nevada and Big Sur have huge wilderness areas. The wildlands around Big Sur are far less crowded.
Big Sur has some nice beaches, and not all of them are on the main highway. You have to be a bit of a sleuth to find Pfeiffer Beach, but it's worth the effort. And...garnet sand beaches!
One of the lesser known beach parks of the central California coast, Garrapata is a beautiful place that belies its horrendous name ("tick" beach). It has some of the nicest exposures of "Sierran" granite in the region.
Point Lobos was named for the seals and sea lions, not for terrestrial canines. It is another gem along the coast with unique exposures of conglomerates that accumulated in deep underwater canyons that rival Yosemite in their depths.
We finish our journey through Point Lobos and move north onto the Monterey Peninsular. Here it is trees that take up some of the geologic story. Monterey Pines grow naturally only in a few places, mainly on the Peninsula, but have become one of the most widely planted trees in the world. The Monterey Cypress is another unique species in the region.
We take a look at coastline of totally different character as we reach the Half Moon Bay region. There are prominent marine terraces that make for gentle scenery (and apparently great golf courses). There are some nice tidepools in the area, and during the right time of year, the Mavericks hit, the gigantic waves that bring surfers from around the world.
The San Andreas fault looms large in the history of the Central California coastline, but hasn't made an appearance on our journey until now. At Mussel Rock in Daly City, the fault trace moves offshore. The epicenter of the 1906 San Francisco may have been close by. And there is a famous folk song about the cookie-cutter houses on the high, unstable slopes...
Maybe you haven't heard that there is a big bridge that connects the city of San Francisco with the Marin Headlands and the rest of Northern California. It's not likely, but it's possible. There is some interesting geology going on underneath the bridge abutments.
The Marin Headlands expose rocks that were once part of the midocean ridge, the vast planet encircling mountain range that no mountain climber can ever hope to climb. The scenery on the Marin is majestic. And to invaders in World War II, the cliffs would have been deadly.
The Point Reyes Peninsula has wide sandy beaches, sand spits, a bay that may have been a landing for Sir Francis Drake, Tule Elk, and a lighthouse that has to put up with some really rotten weather.
If the Point Reyes Peninsular bears the brunt of violent Pacific Storms, the mountains of the peninsula shelter the lands to the east. Two bays along the San Andreas fault are peaceful and serene, which belies their violent origin.
We wrap up our exploration of the Sierra that rises from the sea with a look at Bodega Head, the site of a classic horror movie, and a horror story with a nuclear reactor as the main character. A reactor that was almost built on top of the San Andreas fault. It's also the northernmost exposure of the Salinian/Sierra Nevada rocks.

Wednesday, January 22, 2014

Where the Sierra Nevada Rises From the Sea: Birds and a Hole in the Head at Bodega


Sea stacks at Bodega Head, with the Point Reyes peninsula in the distance
We are reaching the end of my series on the Central California coastline from Big Sur to Bodega Bay. I've been calling the series "Where the Sierra Nevada Rises From the Sea" for a very specific reason. Much of this section of the coast is composed of granitic rocks that formed in the subduction zone complex that was the origin of the Sierra Nevada batholith, the 400 mile long block of granite that is the backbone of California. The rocks, known as the Salinian Block (or Terrane), have been displaced hundreds of miles northwest by lateral motions along the San Andreas and related fault systems over the last 30 million years or so. Bodega Head is the northernmost exposure of these granitic rocks.

Bodega Head refers to the former island of granitic rock that is now connected to the mainland by a large area of active and stabilized sand dunes. Bodega Bay formed between the head and the mainland, and is nearly closed off on the south side by a sand spit along Doran Beach. A small marina is here and a modest fishing fleet works out of the harbor.
No, this isn't a bird attack at Bodega, but I sure thought about it while I was snapping this shot of Red-wing Blackbirds at San Luis National Wildlife Refuge near Los Banos in the Central Valley.

One may not know Bodega for the geology, but there are those who will remember Bodega Bay as the setting for a rather famous movie about our avian friends, The Birds, Alfred Hitchcock's classic. The original house where many scenes were shot is gone now, but the cypress grove where the house stood can be seen on the sand bar that connects the Head to the mainland. The land is owned today by the University of California, which runs a research facility at the site (occasional tours are offered).

Bodega Head has a horror story that may be the equal of an unexplained attack by our feathered companions. The rock sequence found east of the San Andreas fault is part of the Franciscan Complex, and these rocks are notorious for their slope-failure tendencies. Mass wasting is a way of life for anyone who chooses to build on it. Bodega Head on the other hand is composed of hard granitic rock. No slope failures would be expected there.
The Hole in the Head

So it was that in the 1950s that PG&E looked with great longing at the granite headland and decided that Bodega Head would be an ideal spot for a brand new nuclear power plant. The water they needed was there, there was solid ground to build on in contrast with the lousy slopes east of the fault in the Franciscan Complex. And not that they thought of things this way back then, but the site was fairly isolated from large population centers in case of bad accidents (which we all know NEVER happen with nuclear power plants).

And adjacent to the San Andreas fault! Its not like they didn't know it was there. In 1906, this part of the San Andreas shifted 15 feet or more. So they began digging a hole that would serve as the foundation for the power plant. As the plans and excavations progressed, local opposition began to grow, and ultimately the company somehow realized they were about to put a nuclear reactor practically on top of the San Andreas fault (subsidiary faults were discovered in the excavation pit; a geologist opined that “a worse foundation condition would be tough to envision.”). The pit became known as the "Hole in the Head".

So the nuclear power plant was never built, and Bodega Head remains as barren windswept ridge with beautiful cliffs composed of the northernmost exposures of the granitic rock of the Sierra Nevada. The Hole in the Head filled with seeping groundwater that now supports the local wildlife.

And so ends our little series of a part of the Sierra Nevada mountains that rise from the sea. I hope those of you who've followed it enjoyed the journey. I'll be putting up a compilation of the different posts in the series since it has been a rather off and on project over the last three months!