What Can You Say in 3 Minutes About One of the World's Special Places? Del Puerto Canyon and the Proposed Dam

I was one of a large number of speakers at a forum this evening on the future of a proposed dam at the mouth of Del Puerto Canyon near Patterson, California. I counted roughly 200 people in attendance, and of the 30 or so of the attendees who spoke, none spoke in favor of building the dam (there may have been supporters, but they chose not to speak). What follows is approximately what I said tonight (I had my comments all written out, but I always go off-script!). I've added the pictures to this post (I couldn't use them at the session).

Statement on the proposed dam and reservoir in Lower Del Puerto Canyon

Thank you for the opportunity to speak to this important issue. My name is Garry Hayes, and I have been a professor of geology at Modesto Junior College for 32 years. I wish to speak to several issues about the canyon and proposed dam.

First, the national significance of Del Puerto Canyon

The Environmental Impact Report is a perfunctory report on the geological resources and hazards at the proposed dam site in Del Puerto Canyon, but fails to communicate the national significance of canyon. Describing Del Puerto as a “typical” canyon of the Coast Ranges is like describing Yosemite as just another glacial valley. If the Diablo Range were under federal ownership, I am convinced that Del Puerto would have warranted consideration as a national monument or national park. Why? It is the only place in Central (and maybe all of) California where one can drive from the earth’s surface into the mantle and do it in an exceedingly scenic manner. One passes through 25,000 feet of marine sediments, through the underlying ocean crust, and into rocks that were once part of the earth’s mantle. At the same time, as others will note, the canyon offers a unique assemblage of endemic plants and animals. It has a rare riparian wetland habitat in an otherwise arid mountain range. I have taken hundreds of students into the canyon for field studies over the last 30 years. The National Association of Geoscience Teachers, the Geological Society of America and other national organizations have conducted tours in the canyon and many geologists have done research there.

It is not widely known, and the EIR fails to mention that the first dinosaur fossils ever found in California were found on a slope just above the inundation zone of the reservoir. I am disturbed that such a significant site would be simply ignored in the planning for this reservoir.

Saurolophus, the species of dinosaur found in Del Puerto Canyon

My other concern about this reservoir involves the instability of the slopes above the proposed reservoir. There are huge and clearly active landslides within the inundation zone. The EIR fails to address the possibilities of large-scale slope failures when the dam is filled. I am concerned about what happens when a slide that formed under arid conditions is subsequently inundated beneath a hundred feet or more of water. I find the statement in the EIR that “the rate of movement of landslides would likely be slow…” to be inadequate and worrisome.

Del Puerto Canyon is a region of national scientific significance, and I am concerned that the EIR does not acknowledge this fact. The backers of the dam have not adequately considered the importance of this important educational locality, and I strongly and sincerely request that the Del Puerto Water District reconsider the Ingram Canyon alternative, given that the Ingram project will not have the detrimental impact that would occur if a dam is built in Del Puerto. We should be increasing educational access to the canyon, not restricting it.

Thank you for your time and attention. (end of comments)

Del Puerto can use your help! To get involved, there are several things you can do:

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:

1/21 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

Damning Del Puerto Canyon, a Geological and Natural Treasure in our County

I awoke this morning mildly astonished to see my own words making up the headline of a Modesto Bee article about a proposed dam in Del Puerto Canyon, a deep gorge cutting into the heart of the Diablo Range in the western part of Stanislaus County. Del Puerto is one of the most unique landscapes of California's Coast Ranges, and as I noted, a geological and natural treasure. I was hugely dismayed to find that a proposal exists to build a large reservoir in the lower canyon, and my email to a colleague ended up being quoted in the Modesto Bee article linked above (hence my surprise at being quoted; I wasn't directly interviewed). The article accurately describes my concerns about the project. There are large landslides in the lower canyon that canyon that would almost surely be reactivated (or accelerated; they show evidence of recent motion) if the base is inundated by lake water. There are definite seismic concerns, as a probable active fault lies just east of the dam site. But my biggest concern is the effect the dam will have on the natural environment of the canyon.

The environmental impact report was published recently (read it here.). Comments on the Environmental Impact Report can be made at a public meeting from 4 p.m. to 6 p.m. Jan. 15 at the Hammon Senior Center, 1033 W. Las Palmas Ave., in Patterson. Written comments will be accepted until Jan. 27 at Del Puerto Water District, 17840 Ward Ave., Patterson 95363. If you appreciate the intrinsic value of our precious local canyon, I hope you will comment and make your voice heard.

I've written often about Del Puerto Canyon over the years (see many of the articles here), and to give you a feel for the unique nature of the canyon, I'm adapting an article from last May.

The strange and alien landscape in upper Del Puerto Canyon.

California has some really strange landscapes. A state that has beaches, mountains, volcanoes, forests, and deserts is going to offer many perspectives of the complex geological influences on the state. But for alien and otherworldly, few places in the state can compare to the journey you take when you follow Del Puerto Canyon from its mouth in the Great Valley to the headwaters in the Diablo Range. It's a journey into the middle of the world.

"Del Puerto" refers to "The Gate", the constriction of hard sandstone at the mouth of the canyon. This will be the site of the proposed dam. It will be more than 200 feet high.

I guess I should be a bit more specific. We journey to rocks that had once been part of the Earth's mantle, the 1,800 mile thick layer that lies just beneath the thin crust (3-50 miles thick). We can't reach the core of the planet, because no one can (despite sci-fi movies that say otherwise). Since mantle rock is very hot and is subject to convection, it is at least conceivable that the rocks we are exploring have once been close to the Earth's core.

This is an active landslide that will be partially inundated by the proposed reservoir. I am concerned about the effect of adding water to the slip plane. California's first discovery of dinosaur bones was at the top of this slope.

So how does one explore the Earth's mantle? Well, first one has to get through the crust, and the thinnest crust is that which makes up the ocean floors. It's nominally composed of basalt, but the details are more complex.

In Del Puerto Canyon, the ocean floor is covered by...a bit of sediment. About 25,000 feet of it! The sediments poured off the mountainous edge of the continent during the later part of the dinosaur era, the Cretaceous Period. There was a huge subduction zone that formed as oceanic crust plunged into the mantle beneath the edge of the North American continent. This so-called Cascadia Subduction Zone caused volcanoes to form where the Sierra Nevada is today, but the area offshore of the volcanic arc, the forearc basin, collected sediments. As the sediments accumulated, they pressed the crust downward and even more sediment piled on top. Eventually the layers reached a thickness of five miles.

The basin collected fossils as well. There were the usual shells of clams, snails and ammonites, a variety of shark teeth, and three groups of seagoing reptiles, the plesiosaurs (think Loch Ness), ichthyosaurs (think reptilian version of a dolphin), and 35-foot-long mosasaurs (think "swim for your life!"). Even dinosaur fossils have been found. The first dinosaur ever found in California, a Saurolophus, was discovered in the lower reaches of Del Puerto Canyon in 1935.

Eventually, one will reach the base of the oldest sediments, and encounter the ocean crust itself. Faulting obscures some of the relationships, and so in the picture below we see some of the oldest sediment on the right (somewhat brownish shale) and basaltic/andesitic volcanic rock on the left (greenish gray), separated by a fault. The volcanic rocks are harder, and the canyon takes on a more rugged aspect as we climb higher into the mountains.

The Coast Ranges of California are one of the youngest mountain systems in the world, having been uplifted mostly in the last 3 million years or so. The streams in this dry environment have not been able to downcut as fast as the mountains are rising, so they flow much of the way over bedrock. There are few floodplains in these mountains.

The water flows almost year-round and thus the canyon is a critical habitat for all kinds of wildlife. Dozens of mammals and reptile species are known, and nearly 200 bird species have been observed here.

Oceanic crust is basaltic in composition, but there are differences at depth. On the ocean floor, basalt flows form "pillows", globular masses of the volcanic rock. Beneath the pillow basalts, basaltic dikes fed the eruptions. Dikes occur when volcanic rock fills cracks and fissures in the surrounding rock. Since the surrounding rock is also dike material, the entire layer, a mile or two thick, is made of vertical sheet dikes. Feeding these dikes were magma chambers composed of...basalt! But some of the basalt was left at the base of the oceanic crust where it then cooled slowly to form a sparkling crystalline rock called gabbro. The entire suite of rocks is called an ophiolite sequence. The Coast Range Ophiolite sequence in Del Puerto Canyon is considered to be the second best exposed in the state, behind the Point Sal Ophiolite in southern California.

There is a spot in one of the most rugged parts of the canyon to investigate the gabbro where it was pierced by a vein of quartz (below). People have looked for gold here, but I doubt they found any.

Just a few more miles up the canyon we penetrate the uppermost part of the mantle. The rock originally consisted of ultramafic minerals like olivine and pyroxene, but here the rock has been metamorphosed into serpentine, California's state rock. The rock was sheared and faulted on its way to the surface, leaving shiny green and black polished surfaces (below).

And then we are there. In the uppermost part of the canyon, we reach the netherworld of mantle rock that was far less altered, so it retained some of its original appearance. In places we can see olivine and pyroxene crystals, as well as grains of chromite. These ultramafic rocks contain few nutrients needed by plant life, so only a few species can tolerate living on these slopes. Gray pines are among them, grasses generally are not. There are a number of wildflower species endemic to California that can be found here.

Looking at these shattered broken rocks from very deep in the Earth, one imagines hell freezing over. The forges of the demons and devils lie frozen in place, to be slowly removed by earthly weathering. They try to invade the surface realm, but they are defeated by the forces of the heavens, the water and ice falling from the sky.

It may have been a metaphorical battlefield, but in the end there is great beauty in the rarity of the flowers, plants and animals that thrive, or at least tolerate the conditions in the upper canyon.

Del Puerto Canyon is traversed (slowly) by Highway 130, originating in Patterson on the floor of the Great Valley. It can also be reached by way of Mines Road out of Livermore, a winding road out of the San Jose area over Mt. Hamilton and the Lick Observatory complex. It is not a fast way to go!

I know that there is a need for water in the Central Valley. But no matter how many dams get built, there will never be enough to meet the expressed needs and desire of agribusiness. But I feel that we need to keep some of the wild places, and Del Puerto is one of those especially unique places to learn about our planet. I hope you will make your voice heard about this project.

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

A Netherworld Incompatible with the Existence of Life: The Mantle Exposed at Del Puerto

Our journey into the interior of the Earth has reached a remarkable boundary, the base of the oceanic crust. Beyond the crust, underneath the Mohorovičić discontinuity, lies the mantle, a layer that extends halfway to the center of the Earth. We have been driving up Del Puerto Canyon, a scenic route that crosses the Diablo Range in the central part of California's Coast Ranges. The rocks at the headwaters of the canyon have formed a landscape unlike any other. It's a landscape of the underworld, the netherworld.

Serpentine in the upper canyon. Serpentine is highly altered mantle material.

The netherworld is a place in the mythology and religion many cultures, a dark and hot place deep within the Earth that is the habitation of death. It's no place that a human would ever want to be. And yet, here we were, on a sunny December afternoon. It's not necessarily a place of death, but at the same time the rocks were never really able to support life. They formed in an environment where life was impossible, and life at the surface is ill-adapted to survive in such rocks.

The Earth's mantle is composed of a variety of ultramafic rocks, rocks rich in iron and magnesium and a suite of elements not commonly found in the overlying crust: mercury, chromite, nickel, platinum and cobalt. The rocks found in the upper canyon include varieties of dunite and peridotite, which are olivine-rich rocks. In most places such rocks are quickly altered into serpentine, California's state rock, but in upper Del Puerto Canyon, you can find largely unaltered ultramafic rock.

The plant cover is strange, and quite unlike any found on "normal" crustal rocks. Grass is almost nonexistent, and the ubiquitous California oak trees are nowhere to be found. There are too many toxic substances in the soil, and not enough of needed nutrients. The few scattered plants that can survive on such soils include the California endemic Gray Pine and some species of manzanita. A great many wildflowers have adapted to these soils, and the slopes can be quite colorful in wet years.

The rocks have yielded economical amounts of mercury, chromite and mercury. Our stop at the head of the canyon was at an old chromite mine. Despite the obvious use to make cars sparkle and shine, chrome has more important uses in making stainless steel and armor. During peaceful times, chrome is cheaper from overseas sources, but in wartime, the ores in the canyon become valuable. They were last mined during World War II.

Mercury was sought for use during the Gold Rush as a way to separate gold from the ore. There are extensive deposits in the upper canyon and just beyond the pass at the head of the canyon. Mercury mining was lucrative, but deadly. Miners who absorbed mercury into their nervous system quickly developed symptoms and often died. Mercury continues to poison water and sediments in the Central Valley and San Francisco Bay.

Altered ultramafic rocks from the upper canyon

The upper canyon has numerous kinds of rocks to observe. The two pictures above show ultramafic rocks that were in the process of altering to serpentine. The core sections are composed of enstatite pyroxene (which I believe is also known as bronzite when slightly altered like this).

Chromite ore from the upper part of Del Puerto Canyon

It's not too difficult to find small metallic grains of chromite. It takes a bit more searching to find some relatively unaltered peridotite or dunite. It's a beautiful rock, with the rich green of the olivine, which is also known as the gemstone peridot. That's right, there are outcrops of gemstones in the upper canyon! If you are ever up that way, take a close look at the rocks near mile marker 18.

Peridotite from the upper reaches of Del Puerto Canyon

So, our journey into the interior of the Earth is pretty much done. The rocks of the core are just too deep to visit, even with unobtainium (movie reference! The Core, and Avatar). It's a pleasant way to spend an afternoon, exploring the netherworld in the bright light of day!

If you want to pay a visit to Del Puerto Canyon, it can best be accessed from the town of Patterson in the Central Valley southwest of Modesto. A road (Mines Road) reaches the area from Livermore, and a slow winding road travels over Mt. Hamilton and Lick Observatory from San Jose in the Bay Area. There is a campground and day use area in the upper canyon (Frank Raines Park and Minniear Day Use), but most of the rest of the canyon is private land. Stay on the road!

What to do on a Saturday? Let's Go to the Middle of the Earth (via Del Puerto Canyon)!

The strange and alien landscape in upper Del Puerto Canyon.

California has some really strange landscapes. A state that has beaches, mountains, volcanoes, forests, and deserts is going to offer many perspectives of the complex geological influences on the state. But for alien and otherworldly, few places in the state can compare to the journey you take when you follow Del Puerto Canyon from its mouth in the Great Valley to the headwaters in the Diablo Range. It's a journey into the middle of the world. Our Geology Club made the trip a few weeks ago as an ending of the semester celebration. Odd way to celebrate? Did you simply party? We traveled half-way to the center of the planet!

"Del Puerto" refers to "The Gate", the constriction of hard sandstone at the mouth of the canyon.

I guess I should be a bit more specific. We journeyed to rocks that had once been part of the Earth's mantle, the 1,800 mile thick layer that lies just beneath the thin crust (3-50 miles thick). We couldn't reach the core of the planet, because no one can (despite sci-fi movies that say otherwise). Since mantle rock is very hot and is subject to convection, it is at least conceivable that the rocks we explored had once been close to the Earth's core.

So how does one explore the Earth's mantle? Well, first one has to get through the crust, and the thinnest crust is that which makes up the ocean floors. It's nominally composed of basalt, but the details are more complex.

In Del Puerto Canyon, the ocean floor is covered by...a bit of sediment. About 25,000 feet of it! The sediments poured off the mountainous edge of the continent during the later part of the dinosaur era, the Cretaceous Period. There was a huge subduction zone that formed as oceanic crust plunged into the mantle beneath the edge of the North American continent. This so-called Cascadia Subduction Zone caused volcanoes to form where the Sierra Nevada is today, but the area offshore of the volcanic arc, the forearc basin, collected sediments. As the sediments accumulated, they pressed the crust downward and even more sediment piled on top. Eventually the layers reached a thickness of five miles.

The basin collected fossils as well. There were the usual shells of clams, snails and ammonites, a variety of shark teeth, and three groups of seagoing reptiles, the plesiosaurs (think Loch Ness), ichthyosaurs (think reptilian version of a dolphin), and 35-foot-long mosasaurs (think "swim for your life!"). Even dinosaur fossils have been found. The first dinosaur ever found in California, a Saurolophus, was discovered in the lower reaches of Del Puerto Canyon in 1935.

Eventually, one will reach the base of the oldest sediments, and encounter the ocean crust itself. Faulting obscures some of the relationship, and so in the picture below we see some of the oldest sediment on the right (somewhat brownish shale) and basaltic/andesitic volcanic rock on the left (greenish gray), separated by a fault. The volcanic rocks are harder, and the canyon takes on a more rugged aspect as we climb higher into the mountains.

The Coast Ranges of California are one of the youngest mountain systems in the world, having been uplifted mostly in the last 3 million years or so. The streams in this dry environment have not been able to downcut as fast as the mountains are rising, so they flow much of the way over bedrock. There are few floodplains in these mountains.

The water flows almost year-round and thus the canyon is a critical habitat for all kinds of wildlife. Dozens of mammals and reptile species are known, and nearly 200 bird species have been observed here.

Oceanic crust is basaltic in composition, but there are differences at depth. On the ocean floor, basalt flows form "pillows", globular masses of the volcanic rock. Beneath the pillow basalts, basaltic dikes fed the eruptions. Dikes occur when volcanic rock fills cracks and fissures in the surrounding rock. Since the surrounding rock is also dike material, the entire layer, a mile or two thick, is made of vertical sheet dikes. Feeding these dikes were magma chambers composed of...basalt! But some of the basalt was left at the base of the oceanic crust where it then cooled slowly to form a sparkling crystalline rock called gabbro. The entire suite of rocks is called an ophiolite sequence. The Coast Range Ophiolite sequence in Del Puerto Canyon is considered to be the second best exposed in the state, behind the Point Sal Ophiolite in southern California.

We stopped in one of the most rugged parts of the canyon to investigate the gabbro where it was pierced by a vein of quartz (below). People have looked for gold here, but I doubt they found any.

Just a few more miles up the canyon and we penetrated the uppermost part of the mantle. The rock originally consisted of ultramafic minerals like olivine and pyroxene, but here the rock has been metamorphosed into serpentine, California's state rock. The rock was sheared and faulted on its way to the surface, leaving shiny green and black polished surfaces (below).

And then we were there. In the uppermost part of the canyon, we reached the netherworld of mantle rock that was far less altered, so that it retained some of its original appearance. In places we could see olivine and pyroxene crystals, as well as grains of chromite. These ultramafic rocks contain few nutrients needed by plant life, so only a few species can tolerate living on these slopes. Gray pines are among them, grasses generally are not. There are a number of wildflower species endemic to California that can be found here.

Looking at these shattered broken rocks from very deep in the Earth, one imagines hell freezing over. The forges of the demons and devils lie frozen in place, to be slowly removed by earthly weathering. They tried to invade the surface realm, but they were defeated by the forces of the heavens, the water and ice falling from the sky.

It may have been a metaphorical battlefield, but in the end there is great beauty in the rarity of the flowers, plants and animals that thrive, or at least tolerate the conditions in the upper canyon.

We turned around and headed back to more familiar habitats.

Del Puerto Canyon is traversed (slowly) by Highway 130, originating in Patterson on the floor of the
Great Valley. It can also be reached by way of Mines Road out of Livermore, or  a winding road out of the San Jose area over Mt. Hamilton and the Lick Observatory complex. It is not a fast way to go!

Driving Through the Most Dangerous Plate Boundary in the World: Exploring the Oceanic Crust Without Unobtainium

One of the movies I most love to hate is "The Core". There are plenty of ridiculous parts, there are several earnest geologist characters, there are the many gory deaths while the heroes save planet Earth from being severely microwaved or something like that, and there is of course unobtainium, the magical element that makes everything else possible, including "Avatar". But mostly I scream at the end when the heroes make it though the oceanic crust at the "plate boundary" near Hawaii. Just like physicists yelling "you can't hear explosions in space", I yell "there's no plate boundary at Hawaii". But there is oceanic crust, and it is very hard to explore. In fact most oceanic crust is pretty much inaccessible except by the use of expensive drilling rigs at sea, and they haven't penetrated the deepest parts of the crust yet. Oh, and unobtainium doesn't really exist...

But many flowers do exist in Del Puerto, at the right time of year.

The thing is, we can explore the oceanic crust, and for us in Central California, it involves simply driving up Del Puerto Canyon in the portion of the Coast Ranges called the Diablo Range. We began our exploration in the last post of our series about driving through the most dangerous plate boundary in the world. We were journeying through the depths of hell as represented by the peridotite and serpentine of the Earth's mantle. That portion is in the uppermost parts of the canyon. As we descend towards the Great Valley, we make our way through a section of oceanic crust called the Coast Range Ophiolite.

We saw some elements of an ophiolite sequence while we explored the Marin Headlands in this post a few weeks ago. The big difference is that the rocks of the Marin formed a few thousand miles from the California coast as much as 200 million years ago. The Coast Range Ophiolite is thought to have originated much closer to the margin of the continent around 140 million years ago.

The upper canyon has some nice exposures of highly contorted deep-sea chert similar to those of the Marin Headlands (above), but pillow basalts are a bit harder to find in Del Puerto Canyon. Other parts of the oceanic crust are well-represented however. Pillow basalts form during eruptions onto the sea floor. The rocks of Del Puerto formed much deeper in the crust. The eruptions of basalt on the sea floor were fed by numerous sheet dikes in the mid-levels of the crust, which were in turn supplied by plutons of basaltic magma that later cooled slowly to form gabbro plutons. Gabbro looks like no other rock in the coast ranges (below).

The blocky exposures reveal a rock full of mafic (dark iron-rich) minerals. The "black granite" of many stone countertops is not granite at all, but gabbro. The gabbro of Del Puerto is somewhat finer grained than most rock used in countertops, but under a microscope it is very pretty. It often contains small crystals of green olivine (the gemstone peridot).

The gabbro exposed in the middle part of Del Puerto has been split by a quartz-rich dike that captures the attention of most canyon travelers. It is probably related to hydrothermal fluids (hot water solutions) developed during faulting and uplift of the range.

The quartz vein is very resistant to erosion and forms a wall of rock running up the cliff. It has caught the attention of gold seekers, although I am unaware of any economical ores in the region. This hasn't stopped the speculators. The vein was recently staked for mining, even though no one has found anything there in 90 years of easy access to the outcrop.

This stretch of the canyon gets a fair amount of precipitation and reasonably good soils develop on the slopes. Some water is usually present on the valley floor all year. The vegetation is some of the most diverse to be found anywhere in Coast Ranges, and several hundred bird species have been sighted in the canyon at one time or another.

The sediments of the Great Valley Group were deposited on the Coast Range Ophiolite. The boundary between the ophiolite and sedimentary rocks along Del Puerto Canyon road is a fault, the Tesla-Ortigalita. It's a little hard to see in the photo below, but note the color change in the rocks from lower right to upper left. Some years ago, about 1996, I stood here with Al Bennison, who as a child discovered the first dinosaur ever found in California. He went on to a career in paleontology, and mapped much of the region. He mentioned that the shale on the right side of the fault sometimes yielded up ammonite fossils. He walked over, looked at the rock a moment, then pulled out an ammonite specimen. I have gone back to that exposure a dozen times or more, staring for hours at the rocks, hammering away, and I have never found one.

A final note about exploring Del Puerto Canyon. It is a marvelous habitat for many wonderful creatures, but there are some that one may enjoy better from a distance. In 2008 we saw this rattlesnake.

And in 2012 a flipped rock revealed this small arachnid. Explore carefully!

This post is part of a continuing series about the ancient subduction zone complex exposed in Central California. It is no longer active as such, but once was a zone of earthquakes, tsunamis, volcanoes and many dinosaurs must have perished when it was active in Mesozoic time. For a preview of the series, check out this original post for the series.