What are the Most Important Geologic Roadcuts in California? A Candidate in Coulterville

In a recent post, I explored the geology of the Charlie Brown outcrop. It included excellent examples of normal faults, welded tuff, and airfall tuff, features that served as a microcosm of the geology of the entire province. I was on a field trip this weekend and saw another famous roadcut that encapsulates the geology of a different region. It was the Coulterville exposure of the Mother Lode quartz veins. It can be seen a few hundred feet west of the main junction in the village on Highway 132.

Despite extending for more than a hundred miles through the western Sierra Nevada foothills, the quartz veins of the Mother Lode are surprisingly hard to see in detail. In many places the rock has simply been removed by mining. In other instances the veins are on private property and are not accessible. But in Coulterville, the full extent of the minerals and faults of the Mother Lode are wonderfully exposed.

During the Jurassic Period, a vast subduction zone formed the margin of the western edge of the North American continent. Oceanic crust was being driven downward into the mantle, where parts of it melted to form granitic intrusions. In the Cretaceous, the focus of subduction moved west to where the Coast Ranges are today. The old subduction zone provided an avenue for mineralized solutions to rise towards the surface, where they precipitated quartz and metal deposits including gold and copper. The Melones fault was part of this system of fractures, and runs from Mariposa to Sierra City. Much of the Gold Rush gold was found in the vicinity of this fault zone. Highway 49 follows closely along the trace of the fault as well.

Mariposite at the Coulterville roadcut

The outcrop at Coulterville is the boundary between two terranes, bits of crust that were carried across the Pacific and added to the edge of the continent. The Calaveras Complex lies east of the fault, and the Foothills Terrane is found to the west. The fault zone exposes quartz veins, pyrite, and a unique mineral assemblage of quartz, ankerite, and mariposite. The ankerite is an iron-magnesium carbonate mineral similar to dolomite. The mariposite is an exceedingly rare mineral elsewhere in the world, but is found at many sites along the Melones fault. It is a chrome mica mineral formed when serpentine rock was altered by hot fluids moving up through the fault system. It was a good indicator of possible gold mineralization.

So, I'm wondering what your candidates are for the most significant geological roadcuts in California? I've reviewed the Charlie Brown outcrop, and the Big Pumice Cut that was critical to understanding the age relationships between the glacial tills and volcanic deposits in the eastern Sierra Nevada. I imagine the spectacular folded beds adjacent to the San Andreas fault along Highway 14 in Palmdale might be included. What others would you suggest?

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 or arsenic 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.

Ghosts of the Gold Miners - The Kennedy Mine

As a geologist, I understand the attraction that historians have for history. Geologists are historians in their own way, although our "history" covers, oh, just a few billion years more than that of students of human history. On the other hand there is a certain distance between ourselves and our subjects. I can't exactly feel an emotional kinship with a trilobite or tyrannosaur, but I can look at pictures like these, and see something in the eyes and lives of the people who stood together in 1914 in front of the Kennedy Gold Mine in Jackson, California. The picture hangs in the museum at the mine. If this had been a picture of a Pennsylvania shipyard at almost the same time, one of the young men could have been my grandfather.

These were the miners, millers, muckers, and mechanics who ran the gold mining operation. Even though it ceased operations in 1942, much of the mining complex still remains, and is open for tours. We were there for our field trip to the Mother Lode last weekend.

Looking at pictures like this, and looking at the artifacts from that time, one quickly realizes that there was nothing romantic about being a gold miner. If such "romantic" lives ever existed, it was during the first heady years of the Gold Rush in 1848, but it didn't last long, and few people actually became rich. Most of the miners were the farm laborers of their day. Pay was low, and conditions were appalling. A diagram at the museum shows that with the exception of the Depression years, the miner's pay was much lower than other related positions in the workforce.

Imagine working in a place where the management required that all workers had to change their clothes and shower in front of observers, to make sure they weren't trying to walk out with gold samples. Safety regulations didn't exist. There were no hard hats. Mining accidents were a fact of life; exhibits at the mine note that accidents took 36 lives between 1887 and 1942. In 1922, one horrible tragedy (an underground fire) took 47 lives in the adjacent Argonaut Mine. Statistics don't record how many miners led shortened lives from breathing the quartz dust ("miner's phthisis", or silicosis).

The miners and muckers worked in the dark, with a single candle for illumination in the early years (carbide lamps came later). The work was backbreaking. Early on, holes for explosives were drilled with hand tools. Steam operated drills came along later, but they produced far more dangerous silica dust.

Still, the mine was an impressive operation. From 1860 to 1942, the mine produced around 1.4 million ounces of gold. The underground workings exceeded 130 miles in length, and reached a depth of 5,912 feet, the deepest mine of its day. The site had a 100-stamp mill for crushing the gold-bearing rock (about 1/4 ounce per ton) that ran 24 hours a day. The noise had to be deafening.

The present day metal headframe is 125 feet high. The earlier wood headframe (visible in the historical photo above) burned in 1928. The fire actually burned all of the above-ground buildings except for the stampmill and the mine/milling office (below).

The most jarring building on the property is the mine office. It looks totally out of place given the harsh industrial nature of the other surviving buildings at the mine. Despite the Victorian appearance, there was a foundry on the left side of the building where the gold ingots were forged. The mine offices and walk-in safe were on the right bottom floor. Offices took up the second floor, and the top floor included accommodations for investors and VIP visitors.

The mine produced a lot of gold and yet it also produced mountains of toxic waste (they were polluting the town water supply and were eventually forced to put the acid-rich tailings in another valley entirely, using a unique set of waterwheels). It was a critical part of the town's economy, and yet broke lives. It is an interesting conflict of priorities. The last owner of the mine site desired that mine be preserved, and yet she also hoped that the property could preserve the natural environment as well (there are far more trees on the mine property today. The Kennedy Mine Foundation attempts to fill that role, and they have done a good job at presenting the story and the history of the operation.