Where the Sierra Nevada Rises From the Sea: Tomales Bay and Bolinas Lagoon

Let's cross back onto North America for a short bit. In this off and on series about the most beautiful coastline in the world we have been jumping back and forth between the North American and Pacific plates as we followed the transform boundary called the San Andreas fault. In the last post on the Point Reyes Headland we were standing on the granitic rocks of the Sierra Nevada which have been displaced northward at least 200 miles along California's iconic fault line.

Source: http://pubs.usgs.gov/of/2005/1127/

We pick up our journey northward along the coast on Highway 1 as it winds its way along the cliffs beyond Muir Beach (and don't miss Muir Woods National Monument if you are ever in the region). We are on the east side of the San Andreas fault, meaning we are on the North American Plate (the fault is just offshore).

The rocks east of the fault are a melange of many different kinds of rock that accumulated in the trench deposits (the accretionary wedge) of the vast subduction zone that controlled the tectonic development of California for upwards of 200 million years. There are sequences of graywacke sandstone, dark shale, limestone, chert, and volcanic rocks that were jumbled and churned as the Pacific Plate sank into the mantle beneath the edge of the North American Plate. The earliest geologists who attempted to map these rocks, which today are called the Franciscan Complex, were generally stymied in their efforts because the rocks refused to follow the familiar "rules" of stratigraphy, like superposition or lateral continuity.The mess of rocks only makes sense when you realize they're supposed to be a mess (below).

The highway descends the cliffs and Bolinas Lagoon comes into view. Rocks within the San Andreas and related fault zones (the San Gregorio and Golden Gate faults in this instance) are highly sheared and ground up so they are easily eroded. Linear valleys will often form along fault zones, and such was the case here near Point Reyes. The valley extended from Bolinas Lagoon to the northern end of Tomales Bay, but when the last ice age ended around 12,000 years ago, sea level rose and flooded much of the stream floor. This is the origin of Bolinas Lagoon and Tomales Bay.

Waves in this region come mostly out of northwest, causing the majority of beaches in the area to transport sand southward. The waves coming into Bolinas Bay tend to refract as they encounter Duxbury Point and swing around so they carry sand northwest. This has caused the formation of a sand spit that closes off Bolinas Lagoon, forming one of the nicer tourist beaches in the San Francisco Bay Area, Stinson Beach.

The lagoon is shallow and has developed numerous mudflats that are exposed at low tide (and which make for a nice sheltered refuge for the seals and sea lions in the region). Three major faults cross the lagoon (as noted above, they are the San Andreas, San Gregorio, and Golden Gate), which must cause at least some consternation for the dozens of homeowners who live on the sandy spit.

A drive a few miles north through Olema Valley brings the traveler to Tomales Bay, the other drowned river valley that has developed along the San Andreas fault. The bay might have been a major harbor and port along the California coast except that the 12 mile long estuary is mostly very shallow, and the shoals and sneaker waves at the mouth of the bay are extremely hazardous to boaters (according to the state, in one year alone 13 boaters lost their lives there).

The rest of the bay is generally calm and is popular with small boaters, kayakers and fishing enthusiasts. Only a few small villages are present, including Olema, Inverness, Point Reyes Station and Marshall. Mostly the bay is undeveloped, and is a unique environment compared to the rest of the California coast. The calm water in the interior bay is the exception rather than the rule, and is an important natural habitat for a large number of species. A fair portion of the bay is protected as Tomales Bay State Park and much of the west shore is part of Point Reyes National Seashore. Several parcels along the eastern shore are part of the Golden Gate National Recreation Area.

We've nearly completed our off and on again web series on the "granite coast" of California, the place where the Sierra Nevada rises from the sea. The final post will be a visit to Bodega Bay...and..."The Birds"...

Where the Sierra Nevada Rises from the Sea: Point Reyes National Seashore

Drake's Beach, with outcrops of Purisima Formation that reminded early sailors of Dover.

Around 30 million years ago major parts of Central California were in Southern California. It's about that "simple". Around 29 million years ago a major restructuring of plate boundaries was taking place along the Pacific Coast. A subduction zone (convergent boundary) that had influenced the California margin for nearly 200 million years, forming the granitic rocks of the Sierra, was changing over to something new. It was transforming, so to speak.

For many millions of years, oceanic crust and exotic terranes were carried eastward towards the continent, only to be forced beneath the edge of North America (subducted). Eventually the source of oceanic crust, the East Pacific spreading center, made contact with the convergent margin and subduction ended. Because of the relative motion of the two plate edges, a new fault system developed, the San Andreas, and a portion of Southern California began to move northwestward along the new fault. Such zones that divide crustal plates from one another are called transform boundaries. Granitic rock that had formed as part of the Sierra Nevada Batholith was sliced off from the south end of the province and was carried north several hundred miles (see below). The displaced granitic rocks are referred to as the Salinian Block.

The Point Reyes Peninsula is one of the more intriguing corners of California. It's connected to the California Coast Ranges after a fashion but it is a world apart. Tomales Bay, Olema Valley, and Bolinas Lagoon mark the location of the San Andreas fault. The epicenter of the infamous 1906 San Francisco Earthquake was close to Point Reyes, and offsets totaled close to 20 feet during the magnitude 7.8 seismic event.

The peninsula is protected from development as Point Reyes National Seashore, and is a haven for wildlife. During my visit several years back I captured my best shot ever of a California Quail. On our way to the beaches, we saw a herd of Tule Elk.

The Peninsula has several distinct geological sections. The mountainous terrain of Inverness Ridge is composed mostly of the granite and metamorphic rocks of the Salinian Block. Much of the western and southern parts expose sedimentary rocks of Miocene and Pliocene age. During the journey northwest from Southern California, the peninsula was submerged beneath the ocean waves. Active dune fields can be found along the extensive beaches of the peninsula.

Drake's Estero is an interesting feature. The branching arms of the bay suggest that it is a submerged river valley. The shallow water has proven to be a good spot for oyster farming, and the 80 year old business is embroiled in a battle with the park service over land-use issues.

Source: USGS (http://pubs.usgs.gov/of/2005/1127/)

From the headland of Point Reyes, Point Reyes beach stretches northward for around 10 miles, making it one of the longest stretches of sandy beach in Central or Northern California. As popular as it might have been, the fierce waves and very cold water make for uncomfortable water play.

The rocky headland where the lighthouse is located is composed of granitic rock and some of the overlying late Cenozoic sedimentary rocks. The lighthouse is visible from as far as 24 miles away, but fog obscures the view for around 140 days a year. Winds are nasty (highest recorded was 133 mph), and the temperature generally hovers between 50 and 60 degrees Fahrenheit all year.

Reaching the lighthouse requires descending 300 steps, but during the rare clear days, the spot is one of the best in California for whale-watching. Expect crowds on the nice days...

Steps leading to the Point Reyes Lighthouse

In the next post, we'll explore the more inland part of Point Reyes National Seashore, mainly Tomales Bay and Bolinas Lagoon.

Point Reyes Lighthouse, constructed in 1870

Not Seeing the San Andreas Fault in the Bay Area...

The San Andreas fault is a major transform boundary that runs through much of California, and it is justly famous for damaging earthquakes, including the 1906 San Francisco, the 1857 Fort Tejon, and the 1989 Loma Prieta events. As I've noted many times on this blog, it is not the only fault in the state, but it is the best known.

Sort of...

It's one thing to know the name of a fault, and quite another to know important facts about the fault, such as quake probability, potential quake magnitudes, and...where it actually is. The fault is not too hard to find south of San Francisco. The fault stays on land all the way to the Salton Sea region hundreds of miles to the south. North, though, the fault runs offshore, then briefly onshore, then offshore again. When it is onshore, the trace is often obscured at ground level because of thick vegetation, but the general location is usually easy to find on Google Earth and aerial photographs because of the straight linear valleys that tend to form along the fault system.

I was in the north bay over the weekend for purposes other than geology, believe it or not. But as I went through my photographs, I realized that I took a fair number of pictures of the San Andreas fault, except that it was invisible, being underwater and all.  In the picture at the top of the post, we are standing at Muir Beach Overlook on the Marin Peninsula. If you click on the photo for the larger version, you can pick out Lands End and the residential buildings of San Francisco on the upper right part of the picture. The fault goes offshore out there, along the coast at Pacifica, and runs parallel to the coast to the right of the shot.

Looking the other way (northwest) from Muir Beach Overlook, we can see Bolinas and Point Reyes Peninsula. Both localities are on the Pacific side of the San Andreas fault. We drove north on Highway 1 to have a closer look at where the fault comes inland again.

Wave action along the coast at Bolinas has produced a long baymouth bar that forms Bolinas Lagoon. One of those expensive homes, out on the left end, has a major plate boundary running through the living room. It will be an interesting day in that house when a large 1906-style quake hits.

Harbor seals use the mudflats inside Bolinas Lagoon to rest and to keep their pups safe. The San Andreas comes on land at the north end of the lagoon in the center of the picture above. It runs for thirteen miles to Point Reyes Station, and disappears underwater again at Tomales Bay. Offsets resulting from the 1906 earthquake reached the greatest length in this area, around 19-20 feet. The epicenter was close to here.

Tomales Bay is a unique feature of the California coast line. With a length of 15 miles and an average width of 1 mile, one might have thought that it would have been utilized as one of the rare natural harbors in the state. The bay is exceedingly shallow, however, with an average depth of only 15 feet, and it has a treacherous sand bar at the entrance. It is essentially a linear river valley that was submerged as sea level rose after the last ice age receded around 13,000 years ago. The San Andreas runs right down the middle.

It was a beautiful afternoon as we drove along the west side of the bay through Inverness and then south to Olema. If you really want to see the San Andreas fault for real, you need to stop in at the visitor center at Point Reyes National Seashore, where there is an "Earthquake Trail", and blue posts mark the precise trace of the 1906 break.

We found a delightful restaurant for dinner, the Farm House Restaurant. I was amused that they advertise their location "one half mile from the 1906 epicenter", and kind of wonder about that as a selling point. But the service was good, and the food delicious, especially the clam chowder.

For an overview of the region, check out the Google Earth images below, with the location of the San Andreas fault marked on the second. It is a beautiful region, even if the fault isn't always visible! Click on the photos for a larger version.