A Short Mining History and Geology of the California Gold Rush

Note: I've maintained a website at MJC for quite a few years for the use of my students, and the website is being erased this year for online security reasons. I wanted to keep some of the information, so I am presenting it here, hoping some of you might find it interesting. This particular blog is a review of one of California's seminal events (for better or worse), the Gold Rush of 1848. I borrowed from a number of sources, listed in the references at the end.

Let's start at the beginning and have a look at the basics of gold:

Gold Nugget at the California State Mineral Exhibit in Mariposa. This incredible sample stolen and melted down for a few drug dollars several years ago, a horrible loss.

Chemical Symbol: Au

Atomic Number: 79

Atomic weight: 196.967

Specific Gravity: 19.3 (19.3 times as heavy as an equivalent volume of water; twice as heavy as pure lead; 1 cubic foot weighs over 1/2 ton)

Hardness: 2.5-3 on the Moh's hardness scale (teeth are harder, miners are sometimes pictured biting nuggets to test the authenticity - fool's gold is harder)

Melting point:1,945 degrees F

Boiling point: 5,378 degrees F

Crystal System: Cubic

Gold crystal at the California State Mineral Exhibit in Mariposa. Such crystals are quite rare. 

Gold is the most malleable and ductile of all metals.

One ounce of gold can be stretched into a wire more than 40 miles long.

Gold can be worked into a layer 1 millionth of an inch thick (it has been used on the face masks of astronaut's space suits as a shield).

Gold is Inert, therefore it does not corrode.

Gold is an excellent conductor of electricity

All of the gold ever mined in all of human history would fill a cube only 60 feet on a side!

Fricot Nugget at California State Mineral Exhibit in Mariposa is an extraordinary mass of crystalline gold. At about 13 pounds (201 troy ounces), it is thought to be the largest remaining nugget from the Gold Rush days in California. Other larger specimens were simply melted down.

How Did the Gold Get There?

Gold is present in very small amounts in literally all rocks and even in ocean water; but to be mined economically, it must be concentrated. Even so, the richest gold deposits may contain only a fraction of an ounce per ton.

Around 400 million years ago, California was a different place. It didn't even exist as land, and instead, lay at the bottom of the sea. The Pacific shoreline lay to the east, in present day Utah and Arizona. To the west, large volcanic islands erupted ash and lavas onto the sea floor. Hot springs on the ocean floor built up huge deposits of sulfide mineral deposits.

At various times between 400 and 200 million years ago, titanic crustal forces caused the offshore islands to collide with the American continent, crushing and folding the rocks derived from the sea floor and volcanoes (Keep in mind that this was still a slow process, with movements of only a few inches per year). The rocks, scraped off the sea floor and collected from innumerable volcanic eruptions, became the metamorphic rocks that make up the bedrock of the Mother Lode region.

Typical Mother Lode scenery in the vicinity of Bagby Grade north of Mariposa. Lake McClure on the Merced River in the middle of the picture.

Beginning about 200 million years ago, massive shifts of the tectonic plates that encircle the earth caused the sea floor crust to be pushed beneath the American continent, where it heated up and melted into huge molten masses of magma. These so-called subduction zones are in modern times responsible for the volcanoes and sometimes violent earthquakes of the Cascade- and Andes mountain ranges. The molten rock forced its way upward through the crust and slowly cooled to become the granitic rock that makes up most of the Sierra Nevada today.

Half Dome and Clouds Rest in Yosemite Valley, as seen from Highway 49 near Bagby Grade. The High Sierra has vast outcrops of granitic rock.

Water, derived from rain and snow, percolated into the ground in the Mother Lode region. Following fractures and cracks left by millions of years of geologic mayhem, the water came closer and closer to the hot molten magmas. At these elevated temperatures, water dissolved otherwise stable materials including quartz, gold, silver, copper and zinc.

The metal and sulfide laden stew of hot water then rose along fractures adjacent to the Melones Fault Zone in the Mother Lode. As it cooled, it began to precipitate the mineral riches that it carried as large quartz veins with varying proportions of gold and silver, along with iron, copper and zinc sulfides. Some may have even emerged at the surface as hot springs, like those that exist today near Reno and Carson City. This process is called hydrothermal mineralization.

At least twice in the last hundred million years, the Sierra Nevada rose into a mighty mountain range only to be attacked by the forces of erosion: water, ice and wind. Many thousands of feet of rock were stripped away, and the gold veins were exposed to the elements. Rivers carried fragments of gold downstream and on into the Great Valley. During the most recent mountain-building episode, which may have begun within the last 10 million years or so, many ancient streambeds (which had their headwaters in Nevada) were abandoned, and the thick gravels they contained were left as isolated patches on high mountainsides, and on the plateau-like topography between deep canyons. In some cases, lava flows covered and protected the gold-bearing gravels. All that remained was for the gold to be discovered by human beings who valued the strange metal.

Ancient stream gravels near North Bloomfield in the Mother Lode

How Did They Get The Gold Out?

Numerous methods of mining the gold of the Mother Lode were utilized. Some of these methods were pioneered here.

Placering:

When the Gold Rush began, few of the people in California knew anything of the methods used to procure gold from the quartz veins and river gravels. Because of the richness of the river gravels in the earliest days, panning was an early method of choice, but it was inefficient, back-breaking labor. Panning soon gave way to cradles, rockers, and long-toms. Hardworking miners could process several cubic yards a day.

Modern-day 49ers at North Bloomfield

It is difficult to assess the total production of these placering methods, but in the period of the Gold Rush itself (roughly 1848-1853), perhaps 10-12 million ounces of gold were produced (worth many billions of dollars at today's prices). This was the era when the individual could hope to strike it rich, and quite a few lucky miners did. Most barely made a living at the placers, and as the boom waned, many drifted away, or found work with some of the industrial mines that were starting to develop. Other miners began looking at the canyon walls and terraces, and some of them noticed the terrace gravels. Gold must be there, but how could they get it out? . . .

Hydraulic Mining:

In 1853, the first successful hydraulic nozzle was brought to bear on the terrace gravels. In this method, a high pressure hose was used to direct a violent stream of water at the slopes and cliffs containing the terrace gravels and their load of gold. The loosened sediment would be washed over a set of riffles constructed out of bedrock. At intervals, the hoses were turned off, and the gold collected from the bottom of the huge sluices. By 1884, some 11 million ounces of gold had been produced by this method. Hydraulic mining was particularly successful due to the relatively low labor costs coupled with the fact that many hundreds of cubic yards of gravel could be processed in a matter of hours.

One of the hydraulic nozzles utilized at North Bloomfield

Water was brought onto the mine property by a series of flumes and ditches that connected to reservoirs that had been constructed in the high country in and around the Tahoe Crest (today's Emigrant Wilderness). By 1865, some 5,000 miles of waterways had been constructed across the west slope of the Sierra. When hydraulic mining was abandoned, many of these canals and reservoirs became the infrastructure for the generation of hydroelectric power.

Debris-filled channel in the northern Mother Lode caused by hydraulic mining. The tree "trunk" is actually the 60-foot level of the original tree.

Unfortunately, hydraulic mining had a devastating effect on the local environment. Entire hillsides and ridge tops were removed by the hoses. Vast amounts of sediment filled Sierra river channels, and spread into the Central Valley and San Francisco Bay. In response to legal challenges by farmers in the Central Valley (who were suffering serious flooding as a direct result of the hydraulic mining), the method was severely restricted as of 1884, and largely abandoned after that time. Meanwhile, attention shifted to the sediments of the Central Valley. Gold had been carried for millennia into the fine grained sediments of the valley floor, but so far, no efficient method had been found to profitably mine the resource . . .

Dredging:

The gold contained in the flood plains of the rivers that flowed out of the Mother Lode was very fine, widely disseminated throughout the sediment, and very hard to concentrate with the methods available during the early part of the Gold Rush. The first attempt at using a dredge to mine gold in 1853 was a failure (it immediately sank). It was not until 1898 that a dredge was used profitably to mine gold on the Feather River. The method was a great success, and dredges were active through the 1960's. More than 20 million ounces were mined this way.

Dredges were profitable because they could move and process thousands of cubic yards per day with minimal labor costs. Many dredges could operate at a profit when the sediment they processed contained only 10-15 cents of gold per cubic yard.

In essence, dredges were floating factories with a huge set of buckets at one end, sediment sorters, sieves and screens in the center for processing sediment, and a crane at the other end for distributing the waste material (tailings). The buckets dug sediment out of one end of the pond, sometimes to a depth of 100 feet or more, and the tailings were dumped at the other end. Working this way a dredge could navigate across the landscape, carrying its pond along with it!

"Widowmaker" drill in the Sutter Creek Mine

The Hardrock Mines and the "Mother Lode"

Most of the methods described thus far collected gold that had been concentrated by the work of rivers and erosion. The forty-niners were keenly aware that the gold had to be coming from somewhere in the bedrock, and they found quickly that the gold was associated with quartz veins that ran the length of the Mother Lode District. The first attempts at hardrock mining began in 1849 at Mariposa, and eventually the mines would become the biggest producers of gold in the Mother Lode. They also introduced a certain degree of economic security that was lacking in the boom camps that could be ghosted almost overnight. Hardrock mines in the Mother Lode region operated for almost 100 years, until a presidential order during the height of World War II shut most of them down, in 1942. At least eight of the mines boasted total production in excess of 1,000,000 ounces. Overall, hardrock mining accounted for about 60% of the gold produced in the Mother Lode and Sierra Foothills.

Checking for traces of gold in narrow veins of quartz and pyrite in the Sutter Creek Mine

The most serious problem of mining the gold veins themselves was that quartz, the host ore, was an extremely hard mineral. It was difficult to tunnel through it, and the quartz ore had to crushed to a powder before the gold could be separated out. The mining, done with hand-powered tools, and using relatively weak black powder for explosives, was time-consuming and labor intensive. By the 1860's most of the mines were barely hanging on, but two inventions changed the fortunes of the mining companies: the steam-powered drill, and dynamite. With these two new technologies, the mines were able to process ores quickly and economically, and they expanded quickly. The Kennedy Mine, in Jackson, by 1870 had only reached a depth of 600 feet, but by the 1920's, it had reached nearly ten times that depth (5,912 feet at the time it closed). Mines that had formerly measured tunnel and shaft lengths in the hundreds of feet could now boast of miles of tunnels (the Kennedy Mine had 150 miles; the nearby Argonaut Mine had 62).

Headframe of the Kennedy Mine in Jackson

Once the ore was brought to the surface, it was processed through a stamp mill. It was here that huge 1,000 pound hammers set on crankshafts were set to crush the ores. The stamp mills ran 24 hours a day, and many of the largest mines would have dozens of these noisemakers. The Mother Lode was not a quiet work environment!

Renovated stamp mill at the California State Mineral Exhibit in Mariposa

After crushing, the ore was chemically treated to tease the gold out;  mercury was most commonly used for this purpose. One of the most enduring problems of the gold rush mines is the contamination of the soil and water by the mercury, as well as by the acids and arsenic released during the milling process. Some of the worst toxic waste sites in the United States are those surrounding the old mines. Despite their historic nature, the sites will need to be cleaned up to prevent further damage to soil and water resources.

Open Pit Mining and Cyanide Heap Leaching:

Following the shutdown of most mining operations due to World War II, gold production reached a historic low in California. When the war ended, attempts were made to reopen several of the mines, but higher prices and flooding in the mines themselves doomed the efforts to failure. The price of gold, which was set by the U.S. government at $35 per ounce, was not high enough to justify most efforts at mining. The last hardrock mine shut down in 1965, and the last dredges ceased operation in 1968.

In the early 1970's, the United State removed controls on the price of gold, and its value rose sharply. By 1980, the price per ounce reached more than $600 per ounce, and the attention of the mining industry once again turned to the Mother Lode. Surveys were undertaken during the 1970's and by the middle 1980's, several mines were once again in operation. Efforts were now directed towards low-grade deposits that had been largely ignored in the past. New technology allowed mines to operate profitably when ore grades amounted to as little as 0.025 ounces per ton of ore.

The flooded pit at the Harvard Mine near Jamestown in the Mother Lode

Instead of following tunnels and adits as had been done in the past, the new mines were designed as huge open pits in which all the ore (not just the highest grades) could be mined and processed. The ore would be crushed to a powder, and placed in huge dumps where it would be sprinkled with a solution containing cyanide. The cyanide solution would dissolve any gold or copper present, and percolate to the base of the heap, where the "pregnant" solution would be collected and processed to remove the gold. The process requires state of the art technology, and is hugely expensive, and yet can be profitable as long as the price of gold remains stable at high levels.

Unfortunately for the newly opened mines in the Mother Lode, the price of gold did not remain high enough to justify continued operation of the mines. The Carson Hill Mine closed in 1989, and the Harvard Mine near Sonora closed in 1994. The former produced about 100,000 ounces of gold in three years of operation, while the latter was produced about 660,000 ounces during its operation from 1986 to 1995.

Through the 1990s the price of gold continued to stagnate and even drop further. In November of 1997, the price of gold dropped below $300/oz for the first time in 10 years. The profitable operation of major mines in California, Nevada, and elsewhere were threatened by this continued stagnation. The greatest value of gold had been as a hedge against inflation, but inflation was low throughout the 1990's. The steep rise in the value of gold since 2002 has probably raised interest in renewed mining, but regulatory obstacles and societal opposition will probably limit production for the time being.

What were the effects of the Gold Rush on California?

Environmental Effects:

Dredge tailings along the Yuba River (from Google Earth)

Wide expanses of prime farmland destroyed by dredging.

Worsened flooding in the Great Valley.

Destruction of extensive old-growth forest by hydraulic mining and logging.

Destabilization of slopes and hillsides from hydraulic mining activities.

Contamination of soil, groundwater, rivers and lakes by arsenic, mercury, cyanide and acid mine drainage.

Sociological Effects:

Genocide committed against Native American people. Source: https://nativephilanthropy.candid.org/events/california-indian-genocide-campaign-during-gold-rush/

The destruction and elimination of native cultures in the California Region. Tens of thousands of people dead from starvation, disease and murder.

Hastening of the exploration and colonization of the American west.

Diverse and cosmopolitan nature of California’s population began with the Gold Rush.

Economic Effects:

Support for the Union war effort in the 1860's.

Increase in the national money supply.

Some people became fabulously rich.

Historic mining districts are now important tourist destinations and resorts.

Technology:

Expansion of the agricultural frontier by the need for a food supply in the mining areas.

Numerous improvements and innovations in mining technology.

Construction of the infra-structure for hydroelectric power development in the Sierra Nevada.

Resources:

U.S. Geologic Survey circular on gold

Barabas, A. H. ed., 1991, Geology, gold deposits, and mining history of the southern Mother Lode: National Association of Geology Teachers - Far West Section, Fall Meeting Guidebook, October 11-13, 1991, 123 pages.

Bowen O.E., and Crippen, R.A., Jr., 1948, Geologic Maps and Notes along Highway 49, in Jenkins, O.P., ed., The Mother Lode Country, Geologic Guidebook along Highway 49 - Sierran Gold Belt, California Division of Mines and Geology Bulletin 141, pages 35-86.

Clark, W.B., 1970, Gold Districts of California, California Division of Mines and Geology, Bulletin 193, 186 pages.

Landefeld, L.A., and Snow, G.G., eds., 1990, Guidebook to Yosemite and the Mother Lode gold belt: Geology, tectonics, and the evolution of hydrothermal fluids in the Sierra Nevada of California, with articles on operating mines in the Mother Lode, land use and permitting, history and natural history of the Sierra Nevada: Pacific Section, American Association of Petroleum Geologists, Guidebook 68, 200 pages.

 Thanks to Forrest Hopson for helpful comments!

Red Hills Area of Critical Environmental Concern (Couldn't the Bureaucrats Just Call it a Preserve?): Spring Update

Welcome to one of the most unique ecosystems and habitats in California's wealth of incredible places. It was ignored initially by miners in the region because it had no gold (the Mother Lode is a scant two miles away).  It was more recently ignored because it had no grass for grazing and poor soils incapable of growing anything "useful". The Bureau of Land Management, tasked in its early days of giving away land by way of the Homestead Act couldn't give the free acreage away at all. All but ignored by federal government, the absentee owner, it became an unofficial garbage dump, unauthorized shooting range, and unsanctioned off-road vehicle route. 

Why were the soils so poor in nutrients and toxic to plant growth? It was the underlying rocks: the serpentine and other ultramafic rocks associated with the Melones Fault in the Sierra Nevada Mother Lode. The soils developed on these rocks were severely deficient in important macronutrients, and contaminated (so-to-speak) by toxic metals. Most plants, including most grasses, cannot survive in these soils. Some select species tolerate the soils, and a few rare ones thrive. It is literally a different world, and as you can see in the picture above, the change is abrupt, as the prairie grasses and oak woodland suddenly give way to ceanothus or buckbrush and gray pine.

By the late 1980s the Bureau of Land Management, the federal agency in charge of the Red Hills area, belatedly recognized (with the encouragement and assistance of community and environmental activists) the unique nature of the ecosystem here, and declared it an "Area of Critical Environmental Concern", the kind of a designation that only a bureaucrat could love. It turned out that this landscape contained a large variety of endemic species found almost nowhere else, including a unique fish species, the Red Hills Roach (Lavinia symmetricus). Clean-ups were organized, and minimal tourists facilities (trails, parking areas, vault toilets, and a nature trail) were constructed. 

Bitter root (Lewisia rediviva)

For much of the year, especially in the hot dry summer, there is little to recommend about the Red Hills. Most of the plants are dead and gone to seed. But for a few months during the cooler wetter spring season, the hills come alive. It turned out that many of the plants adapted to living in these soils produce a vivid display of color when they bloom.

California Goldfields (Lasthenia californica) and scattered poppies

We took a bit longer to make our way up to the Red Hills this year, in large part because there hasn't been more than a spatter of rain since December. We are in the midst of another year of lingering drought. But in the end we decided to have a look, and although it came nowhere near the pyrotechnics of color of past years, there was more than enough to make for a satisfying excursion.

Poppy, a flower that seems able to grow in just about any environment California can fling at it.

Enjoy a little bit of color!

Monkeyflower (Erythranthe sp.)

Blue Dicks (Brodiaea)

The parking area and trailhead at Red Hills ACEC

Poppy sp. and Blue Dick (Brodiaea)

Serpentine

The Red Hills got their name from the red soils that develop on the ultramafic rocks, including serpentine. The rocks are rich in iron, thus the red color, but fresh surfaces of the rocks are generally shades of green, as can be seen above.

Paintbrush sp.

Lupine sp.

Lupine sp.

The Red Hills Area of Critical Environmental Concern is in the Sierra Nevada foothills near Chinese Camp off of Highway 120. If you want to learn more, or pay a visit, information about the Red Hills can be found on this BLM website, and the trail and road map can be found here.
 

A Day on the California Prairie: Finding the Precious Places Near Home

Yosemite National Park is closed. The coastline is closed. The state parks are closed. Am I complaining? Not in the least. We are facing an unprecedented threat, so we must curtail our freedoms a bit until the scientists and the doctors (and all of their heroic support staff from nurses to foodworkers to custodians) can do their work protecting us. And we must continue to do our part, isolating ourselves from each other, and from the virus itself. I pray that you are able to stay healthy and safe, and if tragedy strikes, that you find peace in some way.

In the meantime, there are a few legal ways to maintain sanity. Exercising in isolation from others near home is one way. If the city parks are crowded, find a deserted road on the edge of town to walk along. It is a way that we can find the precious and undiscovered treasures that have always been just outside our towns. As it turns out, some treasures are ephemeral, lasting only a few weeks.

My town lies on the edge on one of the few remaining largely untouched prairies left in California. These prairies used to extend across the entire Great Valley, but agricultural development has displaced 95% of the grasslands. There are a series of wildlife refuges up and down the valley that protect some of the remaining wildlands or rehabilitated farmlands. The rest of the prairie tends to be found on the margins, in the foothills of the surrounding mountains where the soils are too thin to support agricultural development.

The rainfall this year has been perilously low, with not a drop in the entire month of February. But March and April saw a resurgence in precipitation and the drying sprouts reawakened, and flowers have appeared in abundance throughout the Sierra Nevada foothills.

We took a drive through the prairie that remains between the Tuolumne and Stanislaus Rivers, and were treated with an explosion of color. I'd love to say I was an expert botanist and tell you all the species, but beyond lupines and poppies, my knowledge of flowers is sadly lacking. What I enjoyed today was the profusion of color.

We are living in one of the most challenging times many of us will ever experience short of all-out war. Natural disasters horribly affect particular regions, but there is always help from elsewhere, and there are places of retreat and refuge if an earthquake or hurricane strikes. But this one is a worldwide viral attack, and all we can do is shelter in place until it passes. It's scary, it's dangerous, and many are suffering. I hope there are a few moments of peace and serenity to be found here in these little treasures close to home.

The Magic of a Pristine Cave: Wanderings in the Black Chasm

The miners were not kind to the caverns of the Sierra Nevada.

The search for gold in the foothills led fortune-seekers all over the hills and valleys of the Mother Lode in the Sierra Nevada, and practically every slope and hollow was prospected for the elusive metal. They usually came up empty of the riches they sought so eagerly, but many times the miners would encounter an unexpected opening in the outcrops of marble that occurred here and there in the Calaveras Complex. They discovered dozens of caverns.

Helictites at Black Chasm cave

For light they used torches and candles, and they didn't hesitate to break off cave decorations (speleothems) as souvenirs. A few enterprising miners realized they wouldn't get rich with gold so they charged admission to their caverns and made money from tourism instead. But they didn't control the behavior of their guests, and more speleothems disappeared. The once beautiful and intricate caverns were reduced to mere holes in the ground, and what decorations remained were grimy with soot and muddy fingerprints from visitors using them as handholds.

The "dragon" at Black Chasm

To be sure, times haven't changed that much. Spelunkers learned long ago that publicizing a newly discovered cave was often a death knell, so locations are kept a highly-guarded secret, and when possible vulnerable caves are fitted with locking gates. But there are two exceptions. California Caverns and Black Chasm cave have rooms that were inaccessible and undiscovered during the Gold Rush, and these have been developed carefully in such a way that the original appearance of the cave has been preserved. We visited Black Chasm today as part of my field class on the caverns and karst topography of the Mother Lode

Soda straws (straight) and helictites (twisted) at Black Chasm cave

Folks are sometimes surprised to find out the true color of stalactites and other speleothems; they're pure white. And they are so incredibly delicate. So-called soda straws and helictites are so fragile that a single touch will break them off, destroying thousands of years of exceedingly slow growth.

Soda straws (straight) and helictites (twisted) at Black Chasm cave

The helictites of Black Chasm are particularly memorable. There are thousands and thousands of them, possibly the greatest accumulation of any cave in the country. The room where the visitors can view them is just one of several. The others are off-limits, and appropriately so. The origin of the helictites is obscure and is debated by cave researchers (speleologists). They obviously ignore gravity.

Black Chasm cave is privately owned and offers tours (for a fee of course) all year long. They are located adjacent to the village of Volcano a few miles up the hill from Jackson on Highway 88. There are other "attractions" for kids at the site (panning for gems, breaking open geodes and that kind of thing), but they also offer some surprising sophisticated cave research publications for sale. They have always been very cooperative about arranging tours for our college groups. If you are ever in the Mother Lode, don't pass up this opportunity to see some truly pristine caverns.

A Look Back at Ten Years of Geotripping: Driving Through the Most Dangerous Plate Boundary in the World

Driving through the most dangerous (kind of) plate boundary in the world is actually not very easy to do. Subduction zones, with the exception of the volcanoes, are mostly deep under the sea. But Central California is a unique case, being an ancient subduction zone that has been uplifted and exposed by erosion, so that interested parties can literally drive through what once was miles underground or at the bottom of the deepest oceans. I got the idea for this blog series when I spent an afternoon driving the winding road that travels over the Coast Ranges at Lick Observatory, and down through Del Puerto Canyon into the Great Valley. It's the equivalent of driving twenty or thirty miles into the Earth's crust. Looking back over the titles, I'm worried that I am using up my lifetime supply of bad jokes...

I've been reviewing the archives this week to find some of my favorite posts from ten years of geoblogging. This compilation appeared on July 4, 2015.

Without a doubt, subduction zones are the most dangerous plate boundaries on the planet. Divergent plate boundaries produce earthquakes and occasional volcanoes, but nothing on the fearsome scale of the calderas and stratovolcanoes and magnitude 9 earthquakes experienced at convergent boundaries. Transform boundaries produce earthquakes, but they are magnitudes smaller than those produced at convergent boundaries (despite what certain Hollywood movies have asserted recently). Hot spots, while not a plate boundary, can produce huge caldera complexes like Yellowstone, but such monsters have not had much of an effect on human history of the last few thousand years. It is the subduction zones of our planet that have caused the most human misery, in the form of massive earthquakes, tsunamis, and violent volcanic eruptions.

We have been driving through an example of one of the most dangerous plate boundaries in the world, but our particular example has been inactive for a very long time. Central California was a subduction zone complex for more than 150 million years, primarily during the Mesozoic era, but it changed into a transform boundary only a few tens of millions of years ago. The San Andreas fault is the resulting feature, and it is capable producing damaging earthquakes, but even the most destructive quakes, like 1906 in San Francisco (death toll 3,000), is but 1/30 of the energy of a magnitude 9 quake like that of Indonesia in 2004 (where the resulting tsunami killed 200,000 people).

It's taken a couple of months to work through our journey, so I've compiled all of the posts here so one can catch the continuity of the story. Here goes...

A New Blog Series

The introduction to the new series, a geological transect from the California Coast to Yosemite Valley, crossing an ancestral subduction zone that once caused geological havoc in a zone from Mexico to Canada (and still is in a few places).

Reconnaissance

An overview (in the most literal sense) of the lands we will traverse on our journey. We have a look at central California from above.

These Rocks are All Wrong!

Granite is exposed in the rocks of the Point Reyes Peninsula. But the arrangement of rock and sediment in subduction zones suggests that granite shouldn't be anywhere near here. It's the San Andreas fault. In California, it's always the San Andreas' fault.

Looking for the Big One

The peace and serenity of Tomales Bay belies a violent past. The San Andreas fault slices right through the bay, and produces large earthquakes with disturbing irregularity. The epicenter of the San Francisco quake in 1906 was not far from here.

Welcome to Geology's Junk Drawer

The Marin Headlands began as a giant collector of geological flotsam and jetsam from the crust of the Pacific Ocean. The jumble of rocks accumulated in an accretionary wedge, and were later lifted up into the mountains of the Marin Peninsula.

Geology's Junk Drawer on the Marin Headlands

Exploring the hidden corners of the Marin Headlands, we find Redwood forests, beautiful views of San Francisco, and disturbing reminders of World War II.

Terra Fatale on the Marin Headlands

Although the subduction zone that formed the rocks of the Marin has been extinct for a long time, there are still plenty of hazards remaining in the region, both geological and nautical. The Point Bonita Lighthouse has been present in one form or another for 160 years. It hasn't always worked, as there are upwards of 300 shipwrecks in the area. There are other hazards too.

The Alien Bursts Forth in the Diablo Range!

We head across San Francisco Bay and start an arduous journey through the Diablo Range. The range formed as an exotic (read "alien") terrane pushed upwards through the sediments of the Great Valley Group, piercing the surface and rising into the sky. John Hurt would no doubt approve...

Exploring the Belly of the Beast in the Diablo Range

We make the first part of a long drive through a rugged portion of the Diablo Range, one of the largest sub-ranges in the Coast Ranges of California. Along the way we cross through two exotic terranes of rocks that had been carried miles deep into the crust in the accretionary wedge of the subduction zone.

At the Portal of Hell in Diablo Range

Making our way down Del Puerto Canyon in the Diablo Range, we do the equivalent of traveling through the crust of the Earth all the way into the mantle, finding outcrops of peridotite and dunite, rocks containing the mineral olivine among others. For the mercury miners, this truly was a portal to Hell.

Exploring the Ocean Crust without Unobtanium

Del Puerto Canyon cuts a swath through the Coast Range Ophiolite, the remnants of the ocean crust that once lay at the bottom of the Pacific Ocean. Though mostly in private ownership, the canyon is one of the most scenic in the Coast Ranges, and a pair of county parks in the upper reaches invite exploration.

Into the Realm of the Drowning Dinosaurs

Sediments accumulated in a deep trough along the west coast of North America called a forearc basin for upwards of 100 million years. The layers reached a depth of 5 miles! In those waters swam mosasaurs (yes, like in the recent Jurassic World movie, but they forty feet long, not a hundred), plesiosaurs, ammonites, and occasionally a drowning dinosaur. The first discovery of a dinosaur in California happened here in 1936.

The Sea Floor that became the Greatest Agricultural Region on Earth

The American Serengeti is a vast plain 400 miles long and 50 miles or so wide that once was the sea floor. The grasslands of the Great Valley once supported millions of migratory birds and grazing animals. It still supports millions of organisms, but these days, those organisms are humans. 95% of the original prairie has been developed for agriculture and the region produces a quarter of the nation's produce. And all of the almonds and walnuts.

In the Pleistocene, a Different Kind of Danger

The Great Valley would have been a most dangerous region for a different reason in the Pleistocene. Among the great herds of grazing animals there were predators, and they were adapted to bringing down giant prey, not the small game we find today. Gigantic Short-faced Bears, Saber-tooth cats, American Lions, Jaguars, and Dire Wolves.

The Dr. Who of Mountain Ranges

At least three Sierra Nevada ranges have existed throughout time. They might have even once been higher than today. The distorted deformed metamorphic rocks tell the story of the earlier ranges.

A Gentle Landscape Belies a Fiery Past

The Valley Springs formation, exposed throughout much of the Sierra Nevada foothills region, forms gentle grass and oak covered slopes, but the rock is made of volcanic ash that originated in monumental explosions millions of years ago. The ash came from gigantic calderas, some of which were hundreds of miles away in central Nevada.

A Landscape Buried in Hot Mud, and a 6-foot Long Saber-tooth Salmon

A lot of volcanoes in the world are made of mud. Lots and lots of mud. But this mud formed in violence. The Mehrten formation of the Sierra Nevada has other surprises too: gigantic tortoises, and six-foot long salmon...with fangs...

A Tale of Two Subduction Zones

There have actually been at least two subduction zones in California. Remains of the older one still make up the rocks of the Sierra Nevada Mother Lode, and those rocks were the source of gold in the Gold Rush.

Exploring the Underside of the Volcano

We wrap up the series in the heart of the ancient magmatic arc: Yosemite Valley. Walking among the towering cliffs, we are reminded that the rocks were actually formed within the magma chambers of volcanic systems, perhaps similar to Lassen Peak, Mt. Shasta, or even at times, Yellowstone.