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!

The Karst Topography...of California? Getting Filthy Rich from Karst in Columbia

The Fricot Nugget at the State Mineral Museum in Mariposa, California

As I noted in the last post, California has some unique landscapes that are unfamiliar to most visitors, specifically karst topography. Karst results when a region is underlain by limestone or marble. The formation of caves is sometimes revealed at the surface when the roof of a cavern collapses, forming sinkholes and related features. At Chaw'se in the last post, we saw how karst topography was utilized by the Miwok people. Today we look at how another culture used karst, and in the process nearly brought about the end of the Miwok and other native peoples in California. It was all about gold.

The southern Mother Lode in the vicinity of Columbia and Sonora.

The western part of the Sierra Nevada is composed not of granite, but of metamorphic rock like slate, marble and metavolcanic greenstone. The metamorphic rocks contained widely disseminated bits of gold, but hydrothermal activity related to the intrusion of granite in Mesozoic time concentrated the gold into veins of quartz and other minerals. The veins came to be known as the Mother Lode, and they were the source of riches and misery. In most cases the miners were after gold-bearing river gravels along rivers that flowed through the Mother Lode, or they developed mines that cut directly into the quartz veins. When the easy gold was gone from the rivers, they expanded their search into unexpected areas. One of those places was the ancient riverbeds of another era, long before the present day Sierra Nevada existed.

Fifty million years ago, the Sierra Nevada didn't not exist as a mountain range. There were the eroded remnants of an ancestral Sierra Nevada that developed during the late Paleozoic and early Mesozoic eras when masses of land slowly mashed into the western edge of North America, and when subduction zones produced magma that eventually became the granite of the high Sierra. Those mountains were gone, and instead there were slow-flowing rivers that had their origin in what is today central Nevada, with hints of headwaters much farther away, perhaps even Canada. The rivers flowed into a coastal complex that was characterized by swamps, jungles, and sandy beaches. These ancient rivers were ultimately diverted here and there by lava flows, which forced the water into new channels that became today's rivers, like the Stanislaus and Tuolumne. The ancient river gravels that remained were perched on high flat ground between today's river canyons. The gravels often contained gold, but something very odd happened at the site of Columbia State Historical Park. It involved karst topography.

The rocks near Columbia form a landscape unlike those found anywhere else in the Mother Lode, or anywhere in California for that matter. The limestone that formed in Paleozoic oceans 300 million years ago was recrystallized by heat and pressure into marble. Groundwater moved through cracks and fissures in the marble, widening the fissures, and in some cases forming caverns. A channel of the ancient rivers flowed over the roughened surface, and the fissures captured stunning amounts of gold. Eventually the rocks lay covered by two or three meters of soil and gravel. Until the miners found the gold...

The gold deposits at Columbia were unbelievably rich. According to some estimates by geologists of the California Geological Survey, some 4 million ounces of gold, worth about four billion dollars at today's prices was mined here. The entire production of gold in California over the last 150 years amounted to about 110 million ounces, so this one mining area produced a significant percentage of all the gold mined in California.

This gives us an explanation for the strange landscape around Columbia. The miners had very little water from local sources, so flumes and penstocks were constructed to bring water from higher parts of the Sierra Nevada. Instead of using sluice boxes or pans, the miners shot water cannons at the soils, moving vast amounts of sediment and quickly exposing the gold (this was called hydraulic mining). Parts of the town were built on gold-bearing soils, and there was the occasional suspicious fire that destroyed buildings, allowing the miners to blast away a bit more ground. What they left behind was a kid's wonderland of boulders, fissures, and small caves that is every bit as fun as Tom Sawyer Island at Disneyland (I'm disappointed that I didn't grow up as a kid around here. I had to settle for Tom Sawyer Island at Disneyland...).

It's probably no surprise to realize that the Gold Rush was a disaster for the native cultures in California. The indigenous people already been decimated by European diseases and unfortunately some of them lived in gold country. They never had a chance against the influx of tens of thousands of miners from all over the world. Some tribes disappeared entirely, and others barely hung on to their lives and culture.

Columbia hung on as a town after the gold deposits gave out, although the downtown area fell into disuse. In the 1930s, efforts began to make the area a state park, and in 1945 the town center was established as Columbia State Historical Park. The stores and restaurants were refurbished and visitors are treated to park employees and business owners in period dress. And a dandy candy store that is often the only business still open when our students arrive in town at the end of our field trip.

Such a Peaceful Scene...and the Birth of Horrific Destruction

A beautiful serene stretch of river on a sunny spring evening. How could anything like this be associated with the extermination of not just a culture, but of many cultures? How could this be linked to one of the most environmentally destructive periods in our state's history? Yet it is...

In late January of 1848, James Marshall had a contract with John Sutter to construct a sawmill on this stretch of the American River near Coloma in the Sierra Nevada foothills of California, and on January 24th, he was realigning the millrace to get the sawmill working properly. They let the river do some of the work by letting water flow through the culvert all night, and in the morning he and his workers found yellow flakes of metal in the bottom of the millrace ...

On February 2, 1848, far to the south in Mexico, the Treaty of Guadalupe Hidalgo was signed, ceding California to the United States in the aftermath of the Mexican-American War. This strikes me as one of the stranger chronological coincidences ever.

Word spread quickly about the gold in the Sierra foothills, and the rush was on. Sutter's Mill didn't operate for very long. It was essentially abandoned by 1853, and all surface evidence was swept away during a flood in 1862. Many years later, the foundation timbers were excavated, allowing archaeologists to calculate the dimensions of the mill. A replica was constructed on the site in 1967, and the original timbers have been put on display (below).

The village at Coloma is now a state park that commemorates the discovery of gold and the rush that followed. There is a fundamental fascination with the yellow metal that caused hundreds of thousands of people to abandon their homes to seek a fortune in the gold fields of California. Legends abound of miners who found giant nuggets and rich pockets that made them instantly rich. Other legends tell the story of bandits and desperadoes. Few stories are ever told of the people whose culture disappeared into the mists of history without ever being recorded. The Native Americans of California, who may once have numbered over a million people, had already been decimated by European diseases, but for some cultures the Gold Rush was the last nail in the coffin.

The 1967 reconstruction of Sutter's Mill in Coloma

The Gold Rush did not last for very long. By 1853, the rivers had been overturned in the quest for the yellow metal, and the miners were beginning to disperse in hopes of finding ores in eastern California and across Nevada. The attention of the mine owners and investors shifted to methods of gold extraction that were more efficient and even more destructive.

Gold-bearing gravels could be found on ridge lines where ancestral rivers once flowed prior to the main uplift of the Sierra Nevada. The miners couldn't efficiently work the deposits because of the lack of water. Mining companies put together a system of flumes and canals in the high country to bring water to the gravels, and one of the most destructive kinds of mining ensued: hydraulic mining. A water cannon, called a monitor, was pointed at the cliffs with the gold bearing gravels, and the explosive spray washed away the gravel into a large tunnel. Large riffles filled with mercury trapped the particles of gold, and the waste material was dumped into the nearby rivers.

A monitor (water cannon) used in hydraulic mining at North Bloomfield.

Millions upon millions of cubic yards were washed away. In the scene below near Interstate 80, the landscape was once level with the white cliff in the far distance. The valleys downstream were choked with debris, and silt filled the floor of the Central Valley. Floods became commonplace as river channels filled with silt and overflowed. Mud filled parts of San Francisco Bay to the extent that it is only about 70% of its original extent. Mercury that escaped from the mines contaminated the sediments, and continues to be a problem today. Hydraulic mining was so destructive to the environment that it was essentially outlawed in 1884.

And then there were the dredges. It took the mining companies a long time to figure out a way to find the very fine gold particles that filled sediments in the Central Valley. The first successful dredge went into action in the late 1890s, and profitable production continued into the 1970s. The dredges were large factory barges with a system of shovels on one end, a waste conveyor belt on the other, and a series of sieves and mercury coated copper plates in the middle to trap the microscopic particles of gold. The dredges were floated in a pond, where they dug away at one end, and filled in the other. In this way, they "sailed" across parts of the Central Valley, producing huge rock piles where fertile soils once existed. Huge swaths of land were ruined in the chase for the elusive metal.

The hard rock mines of the Mother Lode had their own problems. The first mines were dug in 1849, and they were active until 1942, when presidential orders shut down the mines for strategic reasons. A few tried to start up again after the war, but were ultimately unsuccessful. The miners brought huge amounts of ore to the surface, where the rock was crushed to a powder in stamp mills and treated with mercury  to tease out the gold (cyanide is used today). The mine dumps are toxic, not only with the mercury and arsenic, but also the acids produced by exposure of the sulfide minerals in the gold veins to oxygen in the atmosphere. Clean up efforts are still ongoing today, 70 years after the mines closed.

The mines weren't just damaging to the environment. The miners themselves were subject to constant dangers from explosions, cave-ins, falls down shafts, and the gold miner's equivalent to black lung disease: silicosis, a deadly lung disorder caused by breathing in fine quartz dust.

Source: Kennedy Mine Foundation

Today the Mother Lode is a tourist destination, and the miners have achieved a sort of legendary status. The heritage of the Gold Rush is more equivocal: the destruction of cultures, and the environmental devastation of a wide swath of the Sierra Nevada foothills, the Central Valley, and San Francisco Bay. And yet, every time I take a field trip through the Mother Lode, I can't help but look through the mine dumps for the bright yellow gleam of the strange metal.

The Fricot Nugget, the largest surviving nugget from the Gold Rush era. Thieves attempted to steal the nugget last year, but failed. They still made off with numerous other beautiful specimens.

Getting hosed in the Mother Lode: Columbia

A recent post included a description of the hydraulic mining that took place in the Sierra Nevada Mother Lode here in California. We visited some of these sites on our recent field studies exploration of the gold mining region. The last stop of the day was at Columbia Historical State Park, where in the last rays of the setting sun we took in one of the more unusual "ghost" towns of the Mother Lode.

First off, it is not really a ghost town, as several thousand people live in the area, and the region supports a community college that is part of our district. It is, on the other hand, one of the best preserved towns of the Gold Rush era. After it burned down a few times early on, the inhabitants caught on that brick might be a better construction material than wood, and so a rather large selection of 1800's vintage buildings survive. Museums, stores, and restaurants, even an old-time theatre are available for tourists.

For we geologists, and for children (is there a difference?), the town has a special treat: big rocks! What's the story here?

In Paleogene time (I still like saying "Tertiary time"), the Sierra Nevada was a vastly different place. The mountains were lower, and large rivers traversed the landscape in shallow valleys. Flowing across the Mother Lode quartz veins, the rivers picked large amounts of gold, and carried it downstream. A particular channel, the Tertiary Calaveras River, flowed across an exposure of marble, part of a metamorphic exotic terrane called the Calaveras Complex.

Marble, like limestone, is composed of calcium carbonate and is easily dissolved by slightly acidic water. Over the millennia, caves developed underground, and the surface was punctuated by sinkholes and blind valleys, features typical of karst topography. The surface rocks were extremely uneven with deep crevasses and cracks. The rough surfaces acted like riffles where the river passed over, trapping vast amounts of placer gold.

Much of the surface around Columbia and nearby Rawhide Flat is composed of these unusual looking rocks, pictured above. But Columbia itself sits atop a flat shelf around 15 to 20 feet higher than the bouldery surface. Why?

It turns out that the town sits on a section of the original soil surface that existed before the hydraulic miners attacked the rocks in the area. Before the miners arrived, the boulder outcrops were not visible, as they were buried in clays, soil and river deposits. Once the miners figured out that huge amounts of placer gold occupied the cracks and fissures, they pursued the riches with wild abandon, spraying away the soil and sediment, ultimately recovering between 3 and 4 million ounces of gold (think $3 to $4 billion at today's gold prices!). It has been called one of the richest single placer deposits in the entire world.

The miners worked their way right up to the very edge of town, and I have heard that occasionally buildings would burn down under suspicious circumstances. Instead of rebuilding, the site was mined instead.

The town is a pleasant place to visit, and the Candy Shop comes highly recommended. Be aware that because of the California state budget mess, parts of the park are shut down to save some piddling amount of money for the state. For instance, a number of the park bathrooms are locked up. I don't know the present status of the museum.

Hosing Down the Mother Lode: Hydraulic Mining

Continuing our journey through the Sierra Nevada Mother Lode on our field studies trip last weekend, we arrived at Chili Gulch south of the town of Mokelumne Hill to talk about another way of wresting gold from the earth. We were gathering at a rather poor roadcut when a local rancher drove up and offered to let us onto his property where the walls of an old hydraulic mine could be seen. I was thrilled (ranchers don't always like geologists, as many of you know). The picture above shows some of the Tertiary "Auriferous" Gravels exposed in the old quarry wall. These gravels were obviously carried and deposited by a vigorous river, but at this point we were several thousand feet above the present-day Mokelumne River. How did river gravels end up near the top of a mountain??

The trick is to realize how much the topography of the Sierra Nevada has changed in 50 million years. If one were able to travel back to that era, a flight over the Mother Lode would have revealed a shoreline where the Central Valley is today. The shoreline would merge into swampy estuaries and lagoons, and every so few miles along the coast a large river would be flowing into the sea. Looking eastward would reveal a surprise: where the present Sierra Nevada Crest pierces the skyline there would be...no mountains! They had not yet risen (I hereby acknowledge that there is some fierce debate about the uplift history of the Sierra). Instead, the rivers would be flowing from sourcelands in central Nevada, or maybe even farther north and east (one has to wonder where the diamonds came from). The path of the rivers through the Sierra is fairly well established, and becomes more speculative as one travels farther east (see the map below).

Ash eruptions and lava flows covered parts of the Sierra Nevada while the mountains were rising, and rivers were diverted to their present-day channels. The rivers carved deep canyons as the mountains rose, isolating the patches of older stream gravels on the adjacent ridge tops.

It didn't take the forty-niners long to dig up and process all the gold-bearing gravels in the present-day rivers of the Mother Lode: they were pretty well played out by 1853. Some enterprising (i.e. hungry) miners started exploring the slopes and mountains above and soon found the Tertiary River Gravels, but there was a problem: they needed water to sift through the gold-bearing sediment, and water was not abundant on the mountain ridges.

It did not take the miners very long to come up with an ingenious and devastating solution to their problem. They built reservoirs in the high country near the Sierra Crest, and diverted the water before it flowed into the deep canyons. Using literally thousands of miles of flumes, canals and penstocks, they delivered water to the mines on the mountain tops, and using giant nozzles (called monitors) they blasted the cliff with stream of high pressure water (photo below). They had (re)invented hydraulic mining, one of the most destructive human activities to ever affect the Sierra Nevada and adjacent Central Valley.

To capture the gold, large riffles were carved in the rock where the water left the mine quarry. Hundreds of pounds of mercury sat in the riffles to amalgamate the gold, although the sheer quantity of water swept much of the mercury (and plenty of gold) downstream. Of course, ALL of the loosened debris was sent downstream as well, and there was a LOT of it. What was left behind was widespread devastation.

The hydraulic pits were cleared of all topsoil, and many square miles of forest were washed away, sometimes to a depth of several hundred feet (the original surface of the mine shown above coincided with the gray ledge on the far ridge). In a few places, like Malakoff Diggings State Park, the old quarries have taken on a vaguely Bryce Canyon-type beauty, but that is the exception.

Downstream, sediment choked river channels many tens of feet deep. In the picture below, the full thickness of the tailings can be seen in the terrace at the top of the photo. The stumps in the channel are actually the fifty foot level of mature trees that were buried in the gravel. The mud and silt continued downstream to fill the shallow channels of rivers in the Central Valley. Flooding became a yearly problem in towns like Yuba City and Marysville. About 40% of San Francisco Bay filled with mud from the mining operations. The total amount of sediment (1.5 billion cubic yards) was eight times what was moved to dig the Panama Canal.

The devastation of the mining finally drove the victims to court, and in 1884 the Sawyer Decision declared that the hydraulic mines had to confine the waste sediments to the mine property. The ruling effectively ended large-scale hydraulic operations in the Sierra, but the not pollution problems. Mercury continues to contaminate the mine properties, the sediments downstream, and San Francisco Bay. At some of the worst mine sites, an ounce of mercury could be panned out of two pounds of sediment.

As an aside, a few years I was watching the movie "Pale Rider", the Clint Eastwood western. It was set in the Gold Rush era Sierra, and the good guys were the gentle placer miners, who with their gentle and beautiful wives and children gently pulled gold from the little streams near their village. The bad guys of course were hydraulic miners who wanted to destroy the bucolic little town. The pictures of the hydraulic mines were well done, and there was, as I recall, a mention of the Sawyer Decision that ended the mining. They solved all the problems at the end of the movie by shooting at each other, of course.