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"...

A Picture Tells the Story. We're in Big Trouble Out West.

Source: http://drought.unl.edu/MonitoringTools/NASAGRACEDataAssimilation.aspx

It's a terrifying picture. We have been through 12 of the driest months ever recorded in California, and there is no relief in sight. One can tell we are in trouble when the only part of state not suffering a crippling drought is Death Valley and the Mojave Desert. The rain totals thus far in the Central Valley are at Death Valley levels.

A persistent high pressure belt has been diverting the winter storms north. They are then barreling through the central and eastern United States, causing moronic senators and representatives to bellow about how global warming/climate change is somehow an elaborate hoax designed to hurt their benefactors in the fossil-fuel energy business. If a snowstorm disproves a century of careful research by thousands of climatologists, what are we to make of the worst drought ever in California, and horrific heat waves in Australia and Argentina? It's been in the middle to high sixties here at home these last few weeks. If that makes my friends who are shivering back east a bit resentful, just consider what it's going to be like out here in a few months. Yosemite and my precious Sierra Nevada have already suffered through the worst wildfire ever, and more of the same is in store.

These kinds of climate changes that we are experiencing have been predicted and are happening now. It's not always easy to see the changes on a day to day basis, and sometimes the changes might seem positive. For one, we have not had a single Tule Fog day this entire winter. I hate the foggy sieges that used to last for weeks on end, but if it would mean rain and snow for us in California, I would welcome them today.

If we are moving towards a mega-drought, geologic history gives us a clue about what to expect. Two century-long droughts have already taken place in the last 1,200 years. I've discussed the effects previously in this post:  http://geotripper.blogspot.com/2010/05/fridays-fun-fotos-fluvial-forest.html

These dead trees in the middle of the Walker River in the eastern Sierra Nevada were growing to maturity while the river was largely nonexistent during one of California's horrific megadroughts.

Magnitude 4.4 Earthquake in Southern California

There was a magnitude 4.4 earthquake in southern California this morning at 1:35 AM, centered about three miles north of Fontana along the southeastern edge of the San Gabriel Mountains in a complex zone where three major fault systems intersect: the San Andreas, the San Jacinto, and the Cucamonga. Very minor damage was reported.

The picture above was taken relatively close to the epicenter area and shows the incredibly steep and rugged topography produced by the faults that have been active in the region. Cucamonga Peak on the left rises to an elevation of nearly 9,000 feet. The mountains have been pushed up in a very short period of time geologically speaking, only a few million years.

Source: http://www.cisn.org/shakemap/sc/shake/11413954/intensity.html

The focal mechanism suggests right lateral motion, consistent with the expected stresses on the San Jacinto fault.

Source: http://earthquake.usgs.gov/earthquakes/eventpage/ci11413954#scientific

There doesn't seem anything particularly unusual about the quake. As always, this region is traversed by several active faults that are capable of generating large damaging quakes. The San Jacinto has produced nearly a dozen magnitude 6+ quakes in the last 125 years, although most of the epicenters were well to the south of today's quake. The San Andreas fault is the more famous fault with quakes that are considerably larger, but also rarer. A magnitude 7.8 earthquake in 1857 was centered near Fort Tejon along Interstate 5, and around 1680 a very large earthquake ruptured the fault between Salton Sea and the Cajon Pass area. There may have been a serious earthquake here in 1812 as well. The recurrence interval for big quakes has been around 150 years so sufficient stress is currently built up to cause a serious event, so if nothing else, today's minor shaker is a reminder to be prepared for the BIG ONE (we always have to write it in big letters or something). Always keep an extra supply of water and food on hand, carry an emergency kit in your car, and have a family plan in the event that you are separated.

For the official information on the earthquake, check out the USGS page:  http://earthquake.usgs.gov/earthquakes/eventpage/ci11413954#summary

Birds of My Neighborhood: Geotripper Explores a Bit of the "Original" Great Valley

In the last few weeks I've been documenting the bird species in my local neighborhood, primarily around the cow pasture that sits a few blocks from my house. Much to my surprise, I was able to identify at least 25 different birds in a few days of walking, and I blogged some of my favorite pictures so far (here and here).

A cow pasture and an urban neighborhood feel a bit out of place, like an environment where the species make do, and opportunistic species like crows, pigeons and starlings dominate. We decided to go looking for birds in a more "natural" setting and headed south to the San Luis National Wildlife Refuge. We were greeted by the hawk on the entrance sign.

I say "natural" in quotes, because the refuge is not exactly a section of of the valley that retains its primeval wilderness nature. There are very few of those areas still in existence. It is instead an effort to return some reclaimed agricultural lands to wetlands status, and the location and pattern of the swamps is carefully regulated to encourage natural plant growth to allow the migratory and native birds to flourish. In essence the refuge is farming birds. That sounds kind of negative, but it's not meant that way. The wetlands stops on the Pacific Flyway have been decimated by agricultural and urban development, and this is a great effort to undo some of the damage.

We headed out a short looping auto tour off of Landers Road and slowly made our way down the dirt road. The swamps had recently been flooded, and birds were all over, although they tended to be rather shy. A great many of our shots are at extreme zoom.

The American Coots (Fulica americana) were present in abundance. They seemed the least disturbed by our intrusion.

There was a Great Blue Heron (Ardea herodias) in the reeds, but I only got a few shots before it headed deeper into the marsh.

I think the bird below is a Say's Phoebe (Sayornis saya) that lurked in the low weeds and grass at the edge of the capabilities of my camera. With the distinctive orange belly I thought it was a robin at first.

 The bird below may be a sandpiper or willet (any assistance appreciated!).

And finally I got a few shots of the most common bird that we could see that  day, the Red-winged Blackbird. This pair was kind of meek about showing off their red plumage...

 But the one below had no problem!

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

Science on Screen at the State! Whale Rider, Sunday January 12 at 3:00 PM

For my Modesto area readers: I've been involved for the last 18 months with an innovative program at the State Theatre called Science on Screen. The program offers movies with science themes and speakers who can offer additional perspective (and sometimes "constructive" criticism) of Hollywood's version of science. I want to invite you to attend tomorrow's event, a screening of the movie Whale Rider, and a presentation by Kimberly Stevenot.

Read below for more information (from the State Theatre Facebook page):

Topic: Does Saving a Culture Sometimes Rely on Challenging its Traditions?
 
Science On Screen begins the New Year with Whale Rider, a magical film about a 12-year-old Maori girl who dares to defy tradition and challenge her family, only to bravely fulfill her destiny as the leader of her village. This family-friendly film is rich in its imagery, its depictions of the Maori tribe, and in its own bravery in seeing the need for cultures to change, possibly in order to save them. The speaker is Kimberly Stevenot, a noted Miwuk traditionalist, basket weaver, artist and teacher who will discuss tribal customs and cultures, and will also have native regalia, jewelry and baskets on display in the lobby. The Miwuk are native Americans who have long lived in the foothills of the Sierra Nevada, the Sacramento and San Joaquin valleys, and the Sacramento-San Joaquin Delta.

Science On Screen at The State Theatre is made possible through a grant by the Coolidge Corner Theatre, with support from the Alfred P. Sloan Foundation. This pioneering program creatively pairs Hollywood films with notable experts from the world of science, technology, mathematics and medicine. Interactive activities and displays in the lobby from 2 to 3 p.m.; Ms. Stevenot, who will speak at 3, will return for a Q&A following the screening of the film.

Doors and interactive activities at 2; presentation and film at 3 p.m.; Q&A to follow film
 
Admission: $8 or $6 for student with ID
 
Group discounts are available for classes and organizations (comprised of 10 or more) wishing to attend films in the Science On Screen series. For more information please call State GM Sue Richardson at 209-527-4697.

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.