What's Wrong With This Picture (Part 2)? The Problem of a Dam in Del Puerto

Actually, nothing is wrong with this picture, or any of the others in this post. Not yet...

These are pictures of the lower portion of Del Puerto Canyon, the parts that will be inundated if plans to build a dam are successful. There are viable options to this dam that will not destroy a popular and scientifically significant canyon in the Diablo Range of California's Coast Ranges. It is the only publicly accessible canyon in the range within Stanislaus County.

The canyon is a treasure for many reasons. Geologically, it is the source of a huge amount of research into the geologic history of Central California. Driving up the canyon is the equivalent of driving from the Earth's surface through 25,000 feet of oceanic sediment, another five miles of oceanic crust, and into the mantle, the Earth's layer that reaches all the way to the outer core. California's first dinosaur discovery was in the lower canyon, and other paleontological discoveries have been made nearby. The canyon has a unique mining history: mercury, chromite, magnesite, and others have been found in the upper canyon.

The canyon is a wonderful outdoor laboratory for botanists and zoologists. More than 160 bird species have been found here, making Del Puerto the third most diverse bird habitat in the entire county, exceeded only by the San Joaquin River National Wildlife Refuge, and the Modesto Water Treatment Plant (!). Canyon explorers will see a huge variety of mammals, reptiles, amphibians, and insects. I've seen badgers, bobcats, coyotes, rattlesnakes, king snakes, and all manner of lizards.

The wildflower displays in the canyon can be stunning. The unique soils in different parts of the canyon allow hundreds of flower species to flourish, including some that are found nowhere else in the world. There were few flowers last weekend, but come spring the slopes will be awash in color.

There are archaeological sites in the canyon of the occupation by the Yokuts and earlier cultures. These have barely begun to be understood or explored.

People of the San Joaquin Valley have few places where they can go in the Coast Ranges for recreation, and the building of a dam will remove yet one more access point. Del Puerto Canyon is a treasure for so many reasons, but not the least of which is that the canyon is a nearby place for spiritual recharge, exercise, and personal exploration. These intangibles are hard to put a price tag on, and that's the problem.

There will be many reasons offered as to why this reservoir is SO necessary, and how there will be SO many benefits. But the question needs to be asked, what is this taking away from all of us?

If you are on Facebook, check out https://www.facebook.com/groups/463664377903706/

Read the Environmental Impact Report at this link. If Del Puerto Canyon has significance to you, please respond and be active in the opposition! If you have expertise in any of the areas that will affected, you need to be heard from.

There are several important meetings and deadlines coming up very soon:

1/15 3:30pm Protest. Corner of Ward and Sperry
1/15 4:00pm Public Meeting. Hammon Senior Center 1033 West Las Palmas, Patterson
1/21 6:30pm City Council Meeting. 1 Plaza Circle. - request they take a stand, voice concerns
1/27 5:00pm Public Comments DUE. ahansen@delpuertowd.org OR Anthea Hansen PO Box 1596 Patterson CA 95363
1/28 9:00am Board of Supervisors Meeting 1010 10th St Modesto CA - voice concerns, they have final decision

What's Wrong With This Picture? The Problem of a Dam in Del Puerto

What is wrong with this picture?

This is a map of all the historical sightings of Red-tailed Hawks on eBird in a portion of the Diablo Range between the Bay Area and the Central Valley. The Red-tail is a common bird that lives in a wide variety of habitats, ranging from coastal areas to mountains to savannas. So why, except for a single east-west line across the center of the picture, have they never been sighted in the interior of the Diablo Range? Is there a no-hawk zone in the range interior? It turns out that you could do a similar map for any common (or uncommon) bird species and the pattern would be similar. Are there no birds at all in the Diablo Range?

We raptors want to know...

The answer is that of course there are birds all over the range. But there is no public access in the majority of the mountain range. It is largely under private ownership, and the most common sign to be seen anywhere is "No Trespassing". Except for that narrow stripe in the center of map, at Del Puerto Canyon. There is a public route, Del Puerto Canyon Road, that crosses the range and provides a (very slow) connection between Patterson and the Santa Clara Valley.

And that's the problem with the picture. Del Puerto is the only public access to the mountain range in Stanislaus County, and as such is one of very few places where anyone can study the unique geology, biology, botany and archaeology of this intriguing place. And it is the only place that offers year-round recreational opportunities (there are several county parks and a campground in the upper canyon). But now there are plans afoot to eliminate much of that access, and indeed to severely impact the natural environment. There are plans to build a large dam that will inundate five miles of the extraordinarily scenic canyon.

There will be many reasons offered as to why this reservoir is SO necessary, and how there will be SO many benefits. But the question needs to be asked, what is this taking away from all of us? Look to this blog for some answers in coming days. Also, check this blog from several days ago. If you are on Facebook, check out https://www.facebook.com/groups/463664377903706/

Read the Environmental Impact Report at this link. If Del Puerto Canyon has significance to you, please respond and be active in the opposition! If you have expertise in any of the areas that will affected, you need to be heard from.

There are several important meetings and deadlines coming up very soon:

1/14 9:00am Board of Supervisors Meeting 1010 10th St Modesto CA
1/15 3:30pm Protest. Corner of Ward and Sperry
1/15 4:00pm Public Meeting. Hammon Senior Center 1033 West Las Palmas, Patterson
1/21 6:30pm City Council Meeting. 1 Plaza Circle. - request they take a stand, voice concerns
1/27 5:00pm Public Comments DUE. ahansen@delpuertowd.org OR Anthea Hansen PO Box 1596 Patterson CA 95363
1/28 9:00am Board of Supervisors Meeting 1010 10th St Modesto CA - voice concerns, they have final decision

A Look Back at Ten Years of Geotripping: The Story Told by a Single Rock

I'm commemorating a decade of blogging by dredging the archives for some of my favorite posts. 2010 was a turbulent year. We were still in the depths of the Great Recession (at least in our area), my summer trip was cancelled for the only time in the last 29 years, we broke ground on our new Community Science Center (affordable only because of the recession and the drop in construction bids), and I posted 298 blog entries, my most productive year on Geotripper. A great many posts continued the Other California series, and a really strange political controversy over the California state rock serpentine accounted for many more(more on this in the next post). It was hard to choose, but my favorite post of the year was the story of a single rock...
From November 20, 2010:

Meet my new deskcrop! I was out on the last field trip of the year, a journey through the Diablo Range to the San Andreas fault and Pinnacles National Monument. We had a wonderful day, which we didn't exactly deserve or expect. It poured all night, and somehow the rain stopped at daybreak. We had a few showers during the day, but a big storm was brewing in northern California (four feet of snow expected in Lake Tahoe, for instance). Happily, the deluge didn't start until we arrived back in town, and it has been pouring ever since.

In between we were treated to a beautiful display of clouds and rainbows, and we had a full day learning about California's most famous fault, and a few others as well. I picked up one rock, though, which captured my imagination, and reminded me of why I fell in love with the science of geology so long ago. It's a porphyritic andesite that I picked up on the shoreline of San Luis Reservoir near Pacheco Pass in the Diablo Range. The mountains form the western boundary of the Great Valley of California for a distance of about 125 miles.

It's not a remarkable rock in and of itself. Andesite is one of the most common of volcanic rocks, found pretty much all over the Pacific Rim in places like the Cascades, the Japanese islands, Indonesia, and by incredible coincidence, the Andes! The story of this rock could be very straightforward; it could be as simple as: it erupted...the rock weathered a bit...and I picked it up.

But that isn't the whole story. Context is so important; I picked the rock up from a layer, seen below, that was exposed on the eastern flank of the Coast Ranges. It is a sedimentary conglomerate. Again the story could be fairly simple; conglomerate is a fairly common rock too. We could hypothesize that the rock journeyed a bit further from the volcano, down a river and the river flowed from the mountains onto an alluvial fan. Along the way, it was rounded off by bouncing against other rocks...then I picked it up.

It turns out that story doesn't quite work as a hypothesis either. This conglomerate has some odd features that tell us that it didn't form in a river. It is topped by a gray sandstone that is in turn covered by siltstone and shale. Above it is another layer of conglomerate that is covered by sandstone, siltstone and shale. This pattern is repeated over and over, until we have a sedimentary formation more than 20,000 feet thick (four miles)! The layers contain marine fossils; some are shells of snails and clams, but sometimes the rocks include shark teeth and the remains of giant marine reptiles like ichthyosaurs, plesiosaurs, and mosasaurs.

OK, but what in the world could be moving boulders and cobbles around on the sea floor? Isn't that one of those places where the water is calm all the time? Sometimes that is the case, but not here. These rocks accumulated in large river deltas extending into a shallow sea that lay off the coast of California in late Cretaceous time, the final period of the dinosaurs. Because of constant large earthquakes, the edges of the river deltas were unstable and were often shaken loose. The mass of rocks turned into turbulent chaotic masses flowing along the sea floor at 30 or 40 miles per hour. These violent events are called turbidity currents. The turbidity currents can carry rocks for many miles. As the flow slows down, the larger particles settle first, followed by the finer particles, and so on. The sedimentary beds become graded by grain size (graded beds).

But why earthquakes? And where did the lavas and magma come from? It turns out these things are related too. This story seems to grow more complicated by the moment! In Cretaceous time, there was no San Andreas fault, and there was a lot less of California, at least a California that could be seen from above sea level. A vast subduction zone lay offshore, a place where the Pacific Ocean crust was being pushed under the North American Continent. As the ocean crust was pushed deeper and deeper into the underlying mantle, it heated up, and assisted by the presence of water, melted into magmas that rose upwards through the crust. Some of the magma exploded and flowed out the surface forming andesitic volcanoes, but other magma chambers cooled slowly three or four miles down in the crust, forming crystalline igneous rocks like diorite, granodiorite, and granite. Those are the characteristic rocks of the Sierra Nevada batholith, exposed in places like Yosemite Valley, Tuolumne Meadows, and Sequoia/Kings Canyon National Park.

I've often stood on the edge of Yosemite Valley and looked at the vast exposures of granitic rock, and imagined the volcanoes that once stood above, miles higher. I even wrote an extensive web series about being under the volcano. In my mind I could stand on the flanks of the Mesozoic volcanoes, watching out for predatory dinosaurs and gazing at pterodactyls flying overhead. It took a lot of imagination, but today the feeling was a little more tangible. I was holding a piece of the volcano in my hand.

I love where my geologic journeys lead; there are lots of pretty spectacular places to see in our present world, but so many more incredible landscapes of the past. It was a good day in the Coast Ranges!

Driving to the Center of the Earth in Del Puerto Canyon...Piercing the Ocean Crust

Rugged terrain in the upper Del Puerto Canyon just beyond the Tesla-Ortigalita fault (on the right near the people)

As in the last post, I'm exaggerating a little bit. We're not going to the center of the Earth, we are instead using California's unique geology to explore the mantle, the layer that extends from 20 miles to about 1,800 miles depth, about half way to the center. These rocks have been on a long journey to reach the Earth's surface, and they are not often seen by casual explorers.

Volcanic rocks in Del Puerto Canyon. These are either pillow basalts or highly jointed rocks.

In our first post, we had driven through the five mile (8 km) thick sequence of sedimentary rocks laid down on the floor of a relatively shallow ocean (the Great Valley Group). We reached a major fault near the base of the sedimentary rocks and the canyon changed in a major way. The smooth gentle slopes of grass gave way to rocky slopes covered with brush, scrub oak and the occasional cypress tree. The rocks had changed. We had reached the ancient oceanic crust, known here as the Coast Range Ophiolite.

An ophiolite sequence is a unique series of rocks that are usually understood to represent a cross-section of oceanic crust. The top of an ophiolite is composed of pillow lavas, lumpy chunks of basalt that form as molten rock encounters cold ocean water (see some forming in this short video). Those might be some pillow basalts in the picture above, but I've never been able to get close enough to confirm it. It could also be highly jointed volcanic rocks.

Beneath the pillow basalts, one might expect to find sheet dikes, fractures that have been filled with volcanic rock that had been on the way upwards to the ocean floor. The sheet dikes are not obvious in Del Puerto, although they can be picked out in a couple of places (we didn't stop in the right places on this trip).

There is a prominent dike in the canyon, but it is not actually part of the ophiolite. The rugged ridge is composed almost entirely of quartz. It probably formed millions of years after the others as hot hydrothermal fluids flowed through cracks and fractures. It's been investigated for gold mineralization, but I don't think anyone has found any ores worth mining (not that they wouldn't try; it's still under claim).

Quartz vein and gabbro outcrops in Del Puerto Canyon

Beneath the sheet dikes, one would expect to find the plutons that once fed the eruptions of basalt and other lavas on the seafloor. The magma that remained cooled slowly over thousands of years, forming a coarse-grained rock composed of crystals of amphibole, pyroxene, plagioclase feldspar, and maybe some olivine. The plutons are usually composed of a dark rock called gabbro, but some of the rocks are lighter-colored, a variety called diorite.

Highly jointed gabbro and diorite in Del Puerto, rocks of the lowest part of the oceanic crust

The presence of diorite and some silica-rich volcanic rocks in the upper parts of the sequence throws  a wrench in the normal interpretation of ophiolite, especially those that occur in California. Ophiolites that form at oceanic ridges (divergent boundaries) are usually poor in silica (composed almost entirely of basalt and gabbro). The ophiolite in the Coast Ranges of California may have formed in a more complex tectonic setting, in and near an island arc (a chain of volcanic islands like the Aleutian Islands today) associated with an oceanic trench.

Diorite in Del Puerto Canyon

In any case, we've penetrated the oceanic crust, a thickness of around three or four miles (6-7 km). These rocks don't often see the light of day, because when you think about it, what does it take to bring the ocean floor and crust to the slopes of a mountain range on land? Geologists have been trying for years to drill a hole through the oceanic crust, unsuccessfully so far, but a new effort has begun this year. Del Puerto Canyon is a place where we can literally walk from the base of the oceanic crust to the underlying mantle.

And that's what we'll do in the next post!

Gabbro near the quartz vein in Del Puerto Canyon

Driving Through the Most Dangerous Plate Boundary in the World: Into the Realm of the Drowning Dinosaurs

The sediments of the Great Valley Group form the parallel ridges trending diagonally across the photograph.

California has a lot of potential for geological mayhem, with the San Andreas and many other faults, mountain-building, and volcanoes of many kinds. But it once was worse. During the Mesozoic era and the early part of the Cenozoic, the entire California coast was rimmed by a massive subduction zone. The huge earthquakes produced as the plate sank beneath the western edge of the continent would have produced horrific tsunamis. The sinking plate eventually produced bodies of molten rock that found their way to the surface, producing huge volcanoes, much like those found in the Andes Mountains today.

We've been making our way on a blog journey through this most dangerous plate boundary. The remains of the subduction zone have been lifted and exposed by erosion in the Coast Ranges. So far we have been observing the rocks of the accretionary wedge, the intensely deformed material that has been churned up within the trench of the subduction zone (see this post for an example). In our last post, we crossed through the oceanic crust of the Coast Range Ophiolite in Del Puerto Canyon in the Diablo Range. At the end of that post we crossed the Tesla-Ortigalita fault and entered into the exposures of the sediments of the Great Valley Group. We are now in the rain shadow of the Coast Ranges, and the slopes are barren of trees.

This large slump in the lower part of Del Puerto Canyon is the location of the discovery of the first dinosaur fossil ever found in California, in 1937.

The rocks of the Great Valley Group were deposited in a sea that lay between the trench and the western shoreline of the North American continent. The shallow ocean environment is called a forearc basin. The sediments consist of primarily of sandstone, siltstone, and shale that cascaded off the submerged edge of river deltas along the shoreline. The underwater landslides were called turbidity currents. The sediments pushed on the crust, and subsidence allowed vast thicknesses of sediment to accumulate. In the region around Del Puerto Canyon east of the Bay area, the layers total as much 25,000 feet. At the south end of the valley near Bakersfield, the rocks are around twice that.

All in all it doesn't sound like a good place to search for dinosaur fossils. The rocks are the right age, Cretaceous, but the dinosaurs were terrestrial creatures. They no doubt roamed the slopes of the the volcanoes and coastal plains of the continent, but few are known to have spent much time in the oceans. Finding a dinosaur fossil here seems about as likely as finding a cow or coyote skeleton at the bottom of the sea in the modern day. 

So, a hypothetical question: what if you did find a cow or a coyote skeleton on the sea floor? Could you explain it? It might take a moment, but one could imagine an intense flash flood along one of the rivers that flow off the Sierra Nevada and through the Great Valley, trapping and drowning a few cows or other creatures along the way. Their carcasses would have floated downstream, and eventually the bones could have sunk to the sea floor. I bring up this point because the sediments of the Great Valley Group have in fact yielded a few dinosaur fossils, and they probably did originate in a river flood.

Source: http://upload.wikimedia.org/wikipedia/commons/4/4b/Saurolophus_scalation.png

The first dinosaur ever found in the state was found in 1936 in our own county, Stanislaus, by a 17 year old boy named Al Bennison. He was searching for shell fossils in Del Puerto Canyon near the landslide seen in the photo above when he found some bone fragments on the hillside. They proved to be the remains of a duck-billed dinosaur (or hadrosaur), possibly a creature called a Saurolophus.

These were big creatures, as much as 30 or 35 feet long. They were plant-eaters, and were among the last of the dinosaurs, along with Triceratops and the tyrannosaurs. Bennison's discovery made news at the time, but few people in our county are aware of the awesome paleontological heritage of our region today. I'm hoping that will be changing soon as we prepare a display for the new Great Valley Museum of Natural History at Modesto Junior College.

Source: http://en.wikipedia.org/wiki/Saurolophus

But Bennison wasn't done yet. A year later in another canyon nearby he found the remains of a fearsome marine reptile called a mosasaur. Mosasaurs were probably the top predators of the Late Cretaceous seas, possibly even consuming sharks (yeah, that's a mosasaur you are seeing in the Jurassic World movie trailers, acting like a Sea World Orca). They are descended from ancient terrestrial lizards similar to the Komodo Dragon.

Bennison's mosasaur was a new species and now bears his name: Plotosaurus bennisoni. Like the hadrosaur, few people are aware that they once lived in this region. We have a four-foot long skull of a mosasaur on display in the museum, and a more extensive display is in the planning stages.

Another species of mosasaur, Plotosaurus Tuckeri, was found a few years later a bit further to the south. As can be seen in the diagram below, these creatures approached the size of whales, and would be a terror if they lived in today's oceans.

There have been many other discoveries of Mesozoic dinosaurs and other reptiles like plesiosaurs and ichthyosaurs. The best resource for learning about these fascinating creatures is the book Dinosaurs and Other Mesozoic Reptiles of California by Richard Hilton of Sierra College. Check it out!

From Dinosaurs and other Mesozoic Reptiles of California, by Richard Hilton

I reached the mouth of Del Puerto Canyon and was met with a disturbing sight that was emblematic of the conflict between humans and the natural landscape that we inhabit. More on that in the next post.

Driving Through the Most Dangerous Plate Boundary in the World: At the Portal of Hell in the Diablo Range

At the portal of hell? For the miners who once worked these tunnels, it was...

I miss the tabloids sometimes. Sure, there's the Onion, which gets mistaken for real news sometimes. But there was nothing like standing in the grocery store lines perusing headline after headline of "real" news, like Elvis sightings, UFO reports, and "Loch Ness Monster Meets Yeti" stories. On extended field trips when group morale was slipping a bit, I could pick up a copy of one of those tabloids and read stories around the campfire, and it worked like magic to bring a group out of the doldrums.

Source: http://www.snopes.com/religion/wellhell.asp

One story has stuck in my mind over the years, the one about the Russians (or Alaskans, or Norwegians) who were digging the world's deepest well on the Kola Peninsula in Russia (or on the North Slope, or in Scandinavia) and when they reached a depth of 7 miles (or 9 or 12), the drill started spinning wildly, and they measured temperatures of 2,000 degrees, but scariest of all, they lowered a microphone (into the 2,000 degrees...) and heard the unmistakeable sounds of screaming souls in Hell. In some versions, gases rose and took the form of the devil, and...well, you get the general picture. As can be seen from the link at the start of the paragraph, the story spread so widely that it merited a Snopes article. What does it tell you that some people (including one or two of my students) took this seriously?

In any case, we are back on our journey through the most dangerous plate boundary in the world, and when we finished our last episode we had passed through the Franciscan Complex, the rock unit that formed in the accretionary wedge of California's Mesozoic subduction zone. We are now making our way down the eastern margin of the Diablo Range through the gorge of Del Puerto Canyon. The association of "Diablo", and "Puerto" ("Devil" and "Portal") is what brought to mind the "screams of the damned" in the old tabloids.

The geologists were trying to dig a hole into the deep crust. They wanted perhaps to break through the continental crust and into the underlying mantle, a layer of peridotite and other ultramafic rocks that make up about 80% or so of the Earth's volume. They failed, but what an effort they made! The thing is, if one really wants to reach the mantle, there is an easier way: find the places where the mantle has risen to the Earth's surface. And that's what happened in Del Puerto Canyon.

Peridotite is composed largely of the mineral olivine (the gemstone peridot). The Earth's mantle is made of gemstones!

As we travel down this canyon we are going to do the literal equivalent of traveling from the Earth's mantle upwards through four or five miles of the oceanic crust, and then "climb" through 25,000 feet of oceanic sediments. All while coasting down a pleasant country avenue in one of the most scenic of the Coast Range's canyons. Along this journey we will also finally leave behind the accretionary wedge of the Franciscan subduction zone, and enter into the intriging forearc basin.

The ultramafic rocks of the upper canyon host a number of unusual minerals and ores. Among them are mercury and chromite. The mercury mined here was used in the process of separating gold from the tailings in the Mother Lode on the other side of the Great Valley. As I understand it, one could make very good wages mining the mercury, but you didn't last long at the job. The mines literally killed the miners. For them, the tunnels really were the portals of Hell. The mercury vapor in the air got into their nervous system, first destroying their minds, and then their bodies.

Chromite from upper Del Puerto Canyon

The chromite that was mined in the upper canyon was not nearly so dangerous. Chromite is of course the source of chrome, which most people associate with shiny car accessories. The most important use of the metal is much more pervasive: it is a major component of stainless steel. The United States usually imports cheap ores from overseas, but during periods of war domestic sources were exploited, and that's what happened in the upper part of Del Puerto Canyon. The road we are traveling was once a railway that serviced the mines.

The orange streaks in the ultramafic rocks of Del Puerto Canyon are slickensides, scratch marks left by faulting

There is a large quarry face in the upper canyon where the rocks can be readily observed. The unweathered rock is dark in color, but weathers readily into an iron oxide rich patina. The region came to be known as the Red Mountain Mining District. The ultramafic rocks are very poor in important in plant macronutrients, and also contain some toxic elements as well. The vegetation on the slopes reflects the difficulty of surviving in this poisonous chemical environment. A number of the species found here are endemic to California and the Coast Ranges. And few invasive species ever gain a roothold on these slopes.

Next, we'll see if we can find the oceanic crust, the rock sequence known as the Coast Range Ophiolite. Stay tuned...

Driving Through the Most Dangerous Plate Boundary in the World: Exploring the Belly of the Beast in the Diablo Range

A long time ago (a few weeks; that's years in internet time) I began this series about driving through the most dangerous plate boundary in the world. Convergent boundaries with their subduction zones or gigantic thrust faults are the most dangerous boundaries that exist on the planet. We need no more reminder than the horrible tragedy that unfolded this week in Nepal.

California was once dominated by such a boundary, from about 200 million years to just a few tens of millions years ago. The subduction zone was replaced by a transform boundary known to most as the San Andreas fault. The San Andreas is dangerous in its own way, but the biggest quakes it causes pack only a thirtieth of the punch of a really big subduction zone quake (magnitude 8 compared to a magnitude 9+). The subduction zone still exists in the northern part of the state; it's called the Cascadia Subduction Zone.

The subduction complex includes three main features, an accretionary wedge, a forearc basin, and a magmatic arc. We have been traveling through parts of the accretionary wedge as we made our way through the Marin Headlands and Golden Gate. We've now entered another section of the wedge, the core of the Diablo Range. It's the domain of the Franciscan Complex, the "belly of the beast".

In the last post I likened the Diablo Range as being the structural equivalent of the alien baby popping out of John Hurt's chest in the movie Alien. Bear with me on this one. Material scraped from the ocean crust along with sediments washed in from the nearby continent accumulate in the trench, and get carried deep into the lithosphere where the rocks were subjected to intense pressure and elevated temperatures. The appearance and composition of the rocks was changed, and some of the minerals serve as proxies for estimating the depth the rocks reached: as much as 20 miles (32 kilometers)! The rocks churned upwards again, reaching close to the surface, but in the aftermath of the end of subduction the rocks were covered by the sediments of the Great Valley Group.

Then, as the pressures rose as a result of regional compression along bends in the San Andreas fault, the rocks of the Franciscan Complex were thrust upwards, piercing through the cover of Great Valley sediments, somewhat like the beast in Alien. They were exposed to erosion for the first time. And it is those rocks we traverse as we drive from Mt. Hamilton to the headwaters of Del Puerto Canyon.

After the narrow deserted road up the flank of Mt. Hamilton, the Lick Observatory complex felt positively urban.

The rocks themselves are unremarkable to look at, being composed mostly of a clay-rich sandstone called graywacke (a German term for "gray barren rock"), and black mudstone and shale. A few interesting chunks of rock are exposed here and there, red or green chert sequences and pillow basalts like those of the Marin Headlands, and some other blocks called "blueschist knockers" (please don't ask). The blueschist is a form of metamorphic basalt or other volcanic rock that has been altered under high pressure into a bluish-gray rock containing the minerals glaucophane and lawsonite.

The observatory complex is really a small village with dormitories and tourist parking.

The earliest geologists to study the Franciscan Complex had a real challenge on their hands. The rocks didn't make much sense. They didn't "follow the rules" of normal stratigraphy. They were intensely faulted and folded, sometimes upside down, and thus were difficult to map. The geologists had no known process in their knowledge base that could explain such rocks. It wasn't until the formulation of the theory of plate tectonics in the late 1960s that these rocks could be explained as the product of deformation and metamorphism in the accretionary wedge of a subduction zone.

The views from the summit of Mt. Hamilton are expansive, to say the least.

Although the rocks are similar in appearance, detailed mapping and chemical analysis allowed the researchers to distinguish several unique zones, or terranes, within the Franciscan, rocks that had formed together at a particular time. There are two of these sequences in the core the Diablo Range, the Eylar and Burnt Hills terranes. The Eylar is the older of two. It contains some fragments of the volcanoes that once existed at the summit of the ancestral Sierra Nevada. The Burnt Hills terrane includes bits of granite, indicating that the ancestral Sierra had undergone significant erosion by late Cretaceous time.

Some of the Manzanita bushes near the summit were more like small trees

San Antonio is a peaceful valley owned by some ranch outfit called "No Trespassing". His signs were everywhere.

So, the thing is, I've been living here in the Great Valley for 25 years, and I've been up Del Puerto Canyon dozens of times, but until last month I had never traveled State Route 130 to Lick Observatory and San Antonio Valley. I finally did so, and I took the pictures gathered together in this post. There aren't all that many rocks in the pictures, because graywacke (the German "gray barren rock") is gray. And barren. The chaparral and oak woodlands were far more interesting, at least on a macro-scale. In a petrographic microscope, all kinds of interesting things have happened to these rocks due to their deep burial.

An American Wigeon in a stock pond in San Antonio Valley

After all the tight curves and winding roads on the ridge of Mt. Hamilton, San Antonio Valley was a refreshing change with straight stretches of pavement and wide open oak woodlands and meadows. It was late March and the spring wildflowers were blooming, though maybe not for long in this drought year.

And then an abrupt transformation. The gentle landscape of San Antonio Valley is rimmed in on the east by the ridge of Red Mountain, and the bleached tailings piles from the old mercury mines indicated a major change in the rock structure. We are crossing the boundary between the rocks of the Franciscan Complex and the base of the oceanic crust and upper mantle. Del Puerto Canyon, lying a short distance east over a low pass, cuts a deep swath through the Coast Range Ophiolite and the overlying Great Valley Sequence. We'll be talking about that region in the next post.

Mercury was mined from rocks of the Earth's mantle in the Red Mountain Mining District.