Driving Through the Most Dangerous Plate Boundary in the World: Geology's Junk Drawer on the Marin Headlands

We continue our drive through the most dangerous plate boundary in the world (past tense, since it became inactive 20 million-plus years ago), and we've arrived at the Marin Headlands, one of the most beautiful coastlines in the world, and a marvelous exposure of what I've called geology's junk drawer, the accretionary wedge of a subduction zone. The rocks of the wedge are called the  Franciscan Complex, and it's appropriate, given the chaotic nature of the rocks

We are taking a little detour from our drive from Point Reyes, and tooling about the headlands on Conzelman Drive. But even before that, I didn't want to miss a gem of a national monument tucked away in the depths of the headlands, Muir Woods.

Muir Woods preserves one of the small remnants of old-growth Redwood forest left in California. More than 95% of the original forests have been cut down, and the northern Coast Ranges have been deeply altered, and the ecosystems shredded. The native range of the Coast Redwoods and the extent of the Franciscan Complex correspond closely, but I don't think the relationship is necessarily biological. The trees need the climate afforded by the proximity of the Pacific Ocean, and the Franciscan Complex is a young rock unit that formed parallel to the same coastline.

As pointed out in the previous post of the series, the Marin Headlands are somewhat unique, having a higher than usual percentage of sea-floor ribbon chert exposures and the underlying pillow basalts of the ocean crust. As we start up the Conzelman Road, we get some marvelous views of the Golden Gate and the orange bridge that crosses it (see the top photo). In the parking areas people use to stare at the bridge, there are marvelous exposures of the chert and basalt.

The chert and basalt have been metamorphosed to some extent and deeply deformed, but the rocks still show much of their original nature. In some of the roadcuts, the chert can be seen lying directly on the sea-floor basal. Geologists have figured out that the sequence preserves about 100 million years of deposition (from 200 to 100 million years ago). This is an extraordinarily long record of slow travel of the plate across the Pacific Ocean.

The scenery is extraordinary along the road. The Golden Gate rises out of the sea in bold cliffs, and though the headlands have been altered by road-building and military activity, the region today exudes beauty and serenity.
 
Wait, what? Military activity on these beautiful cliffs? Why?

When World War II was being fought, San Francisco Bay was one of the most important strategic strongholds for the entire Pacific Theater of operations. There was the harbor itself through which much of the fleet operated, factories and ammunition depots, and the inland agricultural areas. That and a huge civilian population. The hills above the Golden Gate were studded with huge cannons and guns that could wreak devastation on any enemy flotilla that tried to enter the harbor. Below is Battery Mendell, just north of the Point Bonita Lighthouse.

The Marin Headlands region is a stunningly beautiful place with a fascinating geological story. The area is protected as part of the Golden Gate National Recreation Area, which is more or less equivalent to a national park. It is certainly a popular destination, but it's usually possible to find some quiet corners, especially if you are willing to hike a little. In our next post, we'll head down to the beaches of the Marin.

On the Most Inaccessible Mountain Range on the Planet: Walking the Mid-Ocean Ridge in the Marin Headlands

What is the most inaccessible mountain range on planet Earth? The Himalaya? The southern Andes? Some mountain range in the Antarctic wastes? Nah. The most inaccessible mountain range is so difficult to visit that only a handful of humans have ever even laid eyes on it. It lies at the bottom of the sea, and is called the Mid-ocean ridge. It dwarfs the other major mountain ranges around the planet, extending for around 45,000 miles, enough to nearly circle the planet twice.

The Mid-ocean Ridge is not a high mountain range, maybe 5,000-10,000 feet high, but it lies thousands of feet beneath sea level except in extraordinary places like Iceland. Iceland is part of the Mid-ocean Ridge, but probably also includes magmas rising from a hot spot, giving it the extra elevation to rise above the waves.

So mountaineering on the Mid-ocean Ridge is beyond most mortal folk, but you can explore the surface of the ridge in a few places around the world where oceanic crust has been scraped off the sea-floor and accreted to the edge of a continent. One such place is on the Marin Headlands just north of San Francisco (I noticed in the geoblogosphere that Chuck Bailey at W&M Blogs has recently described another such spot in the somewhat more exotic Wadi Jezzi in Oman - Check it out!).

Some of the most spectacular scenery along the most beautiful coastline in the world can be found on the Marin Headlands (this post is actually a continuation of a blog series I started before the Christmas holidays). When you cross the Golden Gate Bridge headed north, take the Alexander Avenue offramp and follow Conzelman Road onto the headlands. Immediately you are treated with a gorgeous view of the Golden Gate Bridge, but the real story is behind you in the roadcuts. There are ribbon cherts (a form of deep ocean sediment derived from myriads of one-celled creatures called diatoms) lying directly on ocean-floor basalt. Lay your hands on the contact between the two and you are touching the surface of what once was part of the greatest mountain range on planet Earth!

The Marin Headlands don't lack for inaccessibility themselves. The coastline through this region is exceptionally rugged, with steep cliffs that slope directly into the surf. San Francisco Bay lies hidden behind the peninsula, and the ruggedness of the region prevented its discovery for many years (nearby Monterey Bay was visited as early as 1542, but San Francisco Bay was not discovered until 1769).

The odd looking cement structures found around the Marin Headlands are kind of a jarring reminder that the history of this region is not that of long-protected wilderness sanctuary. When World War II was being fought, San Francisco Bay was one of the most important strategic strongholds for the entire Pacific Theater of operations. There was the harbor itself through which much of the fleet operated, factories and ammunition depots, and the inland agricultural areas. That and a huge civilian population. The hills above the Golden Gate were studded with huge cannons and guns that could wreak devastation on any enemy flotilla that tried to enter the harbor. Below is Battery Mendell, just north of the Point Bonita Lighthouse.

The Marin Headlands region is a stunningly beautiful place with a fascinating geological story. The area is protected as part of the Golden Gate National Recreation Area, which is a level of protection that is more or less equivalent to a national park. It is certainly a popular destination, but it's usually possible to find some quiet corners, especially if you are willing to hike a little. While in the area, don't pass up Muir Woods National Monument. It's one of the most-visited monuments in the country, but wandering through the Redwood forest, it's not hard to see why.

Here's a Totally Unfamiliar Sight: Some Call it the Golden Gate

We are continuing our exploration of the most beautiful coastline in the world, the shoreline that runs from Big Sur to Bodega Bay. Today we'll have a look at one of the best-kept secrets of the entire state of California.

One of these times, you might find yourself in the city of San Francisco. It's a beautiful city built on a number of hills on the San Francisco Peninsula with a gorgeous stretch of beach, and a complex mix of rocks and sand dunes. If you do visit the city, try and take the time to seek out one of the unknown wonders of the city. As early as 1917, talk began of building a bridge across the opening of San Francisco Bay to the Marin Headlands. Much to the surprise of many visitors to the city, the bridge was actually built in 1937, providing a quick shortcut to the Sonoma/Napa Valley region, another part of California that no one has heard much about.

Locals call the span the Golden Gate Bridge. They try and keep it a secret from the tourists, fearing that it might get crowded and clogged by traffic. But if tourists are persistent, they can navigate their way through the back streets and find the bridge. Be aware that there will be people at one end collecting money from gullible drivers, insisting that they are simply charging a "toll" to use the bridge.

The bridge as seen from Fort Baker on the north side.

Oh, heck, I don't know how long I can drag this joke out. The bridge is only one of the most famous spans in the world and one of the most iconic symbols of California. Still it is a wonder to see, and I can't resist stopping whenever it comes into view. The setting is spectacular with steep coastal cliffs, the beautiful skyline of the City by the Bay, and the dark forests of the Presidio. When built, it had the longest span of any suspension bridge in the world at 4,200 feet, and is still the second longest in the United States. It rises nearly 250 feet above the waters of the Golden Gate. The two suspension towers are 746 feet high. A small army of thirty eight painters works continuously to protect corrosion-prone surfaces. And then there is the fascinating geology...

The bridge from the Marin Headlands.

San Francisco Bay is one of only two natural harbors along the California coast, and for two centuries has been a critical transportation and trade corridor for the state. Rivers that drain 40% of the state of California flow into the bay. It originated only 8,000 years ago as the last major ice age ended and sea level rose to flood the lower Sacramento River Valley (There were no doubt previous incarnations of the bay during earlier interglacial periods). During the ice advances sea level would drop three hundred feet or more, and the Sacramento River would flow to a delta near the Farallon Islands more than twenty miles offshore. Despite the urbanization, the bay is one of the most important estuary-wetland environments on the entire west coast of North America.

Several important fault systems cut across the Bay Area, including the well-known San Andreas, but also the less familiar Hayward, San Gregorio, Calaveras and many others. The proximity of these faults to the bridge is cause for some concern to the engineers who desire that the bridge would survive a major seismic event (retrofits have recently been completed). Part of the problem is the that the southern abutment is situated in serpentinite, a notoriously unstable rock (the cliff just visible on the right of the picture above is Baker Beach, which is made up of numerous landslide blocks).

The Golden Gate as viewed from the summit of Mt. Diablo in the east bay region.

The geology is incredibly complicated. The hills are composed of exotic terranes in the Franciscan Complex, bits and pieces of continental and oceanic crust that were carried hundreds or thousands of miles from their point of origin and accreted to the western edge of North America along a vast subduction zone. The subduction zone changed to a transform plate boundary (lateral movement along the San Andreas fault) only in the last few tens of millions of years.

Source: National Park Service

The rugged cliffs of the Marin Headlands (seen in the picture below from Fort Point) will be the subject of a follow-up post soon, but in short they are made of a complex mix of ocean floor basalt and reddish chert, a quartz-rich sedimentary rock that formed originally on the sea floor as diatomite. The diatomite was composed of trillions of microscope shells of one-celled creatures called (you guessed it) diatoms. The rock forms a solid foundation for the northern bridge abutment.

As mentioned already, the foundation of the southern abutment is in serpentinite, a far less stable rock (seen below from Baker Beach). You can see the landslides on the right side of the picture. I probably had a chance of being arrested the day I took the picture. It was September 15, 2001, just a few days after the tragedy of the Twin Towers and Flight 93. We were on a field trip with the National Association of Geoscience Teachers that had almost been cancelled. Airlines were not yet flying, and security around iconic symbols around the country was very tight. I imagine that if we had wandered closer to the base of the bridge there would have been trouble, or at least some very pointed questions from the California Highway Patrol.

Why Did the Military Get Such Nice Digs? Fort Baker on San Francisco Bay

The reasons are partly geological. But before we mention the geology, there must be pictures of cute and interesting animals...like this young sea lion that was eying the crowd of fisherman on the wharf. We were in the Bay Area today celebrating a birthday, but everywhere we went was crowded, as if it were a holiday weekend or something. No parking on the beach at Half Moon Bay. Nothing at either end of the Golden Gate Bridge. The Marin Headlands didn't look particularly crowded from a distance, but the offramp on 101 was backed up for a quarter mile. There was a crowded art festival at Sausalito. We despaired of finding any place to hang out, but a short distance from Sausalito we found a quiet uncrowded park that had somehow missed my notice in the past: Fort Baker.

The military got all the best spots on San Francisco Bay. The Presidio is a huge complex on the northernmost part of the San Francisco peninsula, and the Marin Headlands were covered with forts and outposts, as well as an imposing group of gun batteries. It isn't hard to figure out why. Before the days when wars could be fought with buttons and missiles and drones, you had to assault other countries with some kind of invading force, and if the countries were bounded by shorelines, you had to invade with ships. A brief look at a map of the California coastline reveals that the state has a grand total of two natural harbors: San Francisco Bay, and Coronado Bay down south at San Diego. Two harbors that controlled essentially all major ship traffic south of the Columbia River in Oregon and Washington. Strategically, this is a vulnerability.

The eastern and southern coasts of the United States are geologically stable and are replete with fine harbors as a result of the steep rise in sea level that accompanied the melting of the glaciers at the end of the last Ice Age. River mouths were drowned, providing extensive bays, and deep anchorages.

California is different. It sits on an active plate boundary, the San Andreas transform, and many parts of the coast rise steeply from the depths of the sea. The tectonic uplift has been geologically rapid, and erosion has not kept up in most places. The one exception in central California is San Francisco where the Sacramento/San Joaquin river system was able to carve a canyon through the rapidly rising Coast Ranges. When the ice ages ended, the rise of sea level filled the lower canyon of the Sacramento and San Francisco Bay was the result.

Fort Baker dates from around 1905 when the United States military was beginning to realize the vulnerability of the west coast to attacks by sea. By the time World War II rolled around, the hills above the Golden Gate bristled with cannons. Any armada that tried to enter San Francisco Bay would have been carved to pieces by the crossfire.

The nature of warfare has changed and the gun emplacements and military forts at San Francisco became obsolete. The Presidio was given over to the National Park Service in 1994, and the last soldiers left Fort Baker in 2000. The areas are now managed as part of the Golden Gate National Recreation Area.

On this holiday weekend we discovered Fort Baker was uncrowded and beautiful. There was a simply spectacular view of San Francisco and a unique perspective of the Golden Gate Bridge. There was plenty of animal life to keep us entertained, and from the look of things, the fishing was good. It was a fine day....

Now THIS is why I love California beyond words (but I will use words anyway!)

California gets a lot of crap from around the country for whatever reasons (San Francisco liberals, Berzerkely, L.A. riots and floods, earthquakes, the whole nine yards), but you know what? I think people are just jealous, because my fair state is one of the most beautiful and interesting corners of planet Earth. Today provides a fine example.

I've been blogging a lot about Yosemite National Park of late, and frankly, lots more is coming, but I had an errand in the San Francisco Bay area this afternoon. Yosemite is a two hour drive from where I live. The Golden Gate Bridge is a two hour drive from where I live. My home town routinely comes up 49th or 50th out of 50 on those "best places to live in America" lists, and it has its problems, but I bet a lot of my geology friends would give up a lot just to be able to play in the snow for a few hours before heading to the beach to work on that sunburn. Or to put it more, eh, sophisticated-like, to go from the granite intrusions of a Mesozoic magmatic arc to the deep heart of a fossil subduction zone in the space of a few hours.

So, a few pictures of what we did after we finished our errands. We headed out onto the Marin Headlands, part of the Golden Gate National Recreational Area. The headlands are those dramatic hills at the north end of the Golden Gate Bridge. There's a winding narrow road that traverses the cliffs above the bridge, providing incredible views of the sea cliffs and San Francisco, which just has to be one of the prettiest cities in the world. The road ends at Rodeo Beach (picture above), which is just another typical northern California beach, meaning utterly breathtaking.

The rocks are strange; deep red brown and black, occasionally gray and green. The deep red layers in the picture above are ribbon cherts from the ocean floor, while the black bulbous looking rocks in the picture below are pillow basalts. They are the remains of deep ocean sediment and oceanic crust that were swept into the subduction zone that lay off the coast of central California from around 200 million years to as little as 10-15 million years ago (it's still there in the northernmost part of the state). The destruction of the subduction zone was the origin of the San Andreas fault, which lies just offshore of here.

If you are looking for the white sand beaches of the Bahamas or Florida, you won't find them in this part of California. Those beaches are made of quartz grains or coral fragments. In Northern California, the sand is composed of the same grand selection of minerals that are found in the rocks of the nearby cliffs, and almost any color can be picked out of a handful (for my sed-pet friends, yes, this is a gravel, not a sand sample; but it shows the colors better!).

An exploration of the roads around the tops and sides of the seacliffs reveals a bunch of strange looking concrete structures that are clearly old, and clearly abandoned. It turns out that if the Japanese Navy had ever decided to enter San Francisco Bay to cause havoc during World War II, they would have faced a devastating barrage of artillery fire coming from these concrete bunkers.

Today, these cliffs and rocky islands are home to hundreds of thousands of birds. The Marin Headland, clear-cut of lumber in the 19th century, covered with structures of war in the 20th century, is now a place of tranquility and contemplation. Sometimes we do a few things right...

I love California...