Saturday, September 23, 2023

An Aerial Tour of the Stanislaus Table Mountain

This is a short blog series of informational articles from my college faculty website that is soon to be extinct (arcane unsafe software, they say). Way back in 2002, the parent of one of my students offered me a flight of my choosing, and I knew it had to be Stanislaus Table Mountain. It is one of the more famous geological features of our region, and it is best appreciated from above. Please buckle your seatbelts, and comply with the no smoking signs...

Our flight takes us from Oakdale, a small town at the foot of the Sierra Nevada, to the Sonora-Columbia area in the Sierra Mother Lode. Our objective was to get a bird's-eye view of the Stanislaus Table Mountain, regarded by many as one of the finest examples of an inverted stream in the world. The "mountain" formed around 9 million years ago, when a latite lava flow streamed westward from vents in the vicinity of the Dardanelles, near Sonora Pass at the crest of the Sierra Nevada. 

The latite is dark-colored with phenocrysts (crystals) of plagioclase feldspar scattered throughout. It superficially resembles basalt, but is more closely related to more silica-rich lavas like andesite and dacite. The lava flowed down a river channel carved out of the tuffs and mudflow deposits of the Valley Springs  and Relief Peak formations, ending somewhere just west of Knights Ferry, having traveled some 60 miles. The lava resisted erosion better than the softer surrounding rock, and the flow eventually was exhumed, forming a winding, sinuous ridge, especially in the vicinity of Jamestown and Columbia.

Soon after takeoff, we approach Knights Ferry. The modern Stanislaus River makes a prominent loop, with orchards and agricultural fields on the left-side flood plain. The higher terraces are drier, and are used primarily for grazing. The exposed rocks in the barren areas are mostly Mehrten formation, which consists of volcanic mudflow deposits around 4 to 9 million years old.

North of Knights Ferry, exposures of the Gopher Ridge volcanics are visible. These are metamorphic rocks dating from the Jurassic period. These rocks formed as island arcs (volcanic islands like Alaska's Aleutian Islands) on the oceanic crust of the Pacific Ocean, but were scraped off against the edge of the North American continent as the rocks were subducted. The rocks have been metamorphosed, and turned almost vertical by intense east-west pressure. They are more resistant than the surrounding slates, and so stand out as a prominent ridge. The town of Copperopolis is barely visible in the upper right corner of the photo.

Farther along, we pass the site of the Harvard Mine. The open-pit mine was active from 1986 to 1994, producing about 660,000 ounces of gold from about 17 million short tons of rock. The lake in the pit is about 300 feet deep. Just prior to closing down, the miners recovered a huge mass of crystallized gold, weighing more than 40 pounds. The gold is on display at the Ironstone Winery in Murphys. The body of water in the background is New Melones Lake.

Our turnaround point was just east of the town of Sonora. The town had its beginnings in 1848 when gold was discovered by Mexican nationals who had lost their citizenship as California was signed over to the United States. The Mexicans were soon displaced ("invited to leave") by American miners.  The original town is visible in the center-right part of the photo, while the newer urban development is visible in the center and left-hand part. Scars from the on-going construction (completed long ago) of a bypass can also be seen.

Turning west, we start to see the inverted stream of Table Mountain. Highway 108 passes along the lower left hand part of the photo. Very little soil has developed on the top of the old lava flow (note the lack of trees, and widely scattered grassy areas). The sinuous nature of the flow is becoming evident.

A look out the back of the plane offers the best view of the flow. The barren-looking surface of the flow is actually a unique ecosystem of native wildflowers that are largely free of the invasive European grasses that have overwhelmed the original grasses over much of the region. It is a fascinating hike, especially in the spring after a few good rainstorms. A relatively recent hike to the top of the lava flow can be seen here:

Looking west, with Knights Ferry in the far distance. The flow is wider, especially where some of the lava backed up into some ancient tributary streams. Tulloch Lake on the Stanislaus River is visible to the right.

The rest of the flight was a bit more mundane, as we buzzed my house and returned to Oakdale.

My thanks to Ken Iwahashi, the pilot on our journey. 

Addendum: If you are wondering what the latite rock looks like, I went out today and got a few shots of it.

Friday, September 15, 2023

A California Love Letter: It's the Best Geology to be Found Anywhere!

This is the second of some resource materials I have on our college website that are being removed, so I wanted to preserve them. The following is some info I give to my students of my "Geology of California" course. For the majority of these students, it is their first introduction to geology, and their first introduction to the extraordinary state that is their home. For a more complete explanation of each superlative, click on the orange links (down the rabbit hole!). Enjoy!

Highest point in the lower 48 states: Mt. Whitney, 14,505 feet 

Lowest point in the western hemisphere: near Badwater, Death Valley, -282 feet

The deepest canyon in North America (maybe): Kings Canyon, Giant Sequoia National Monument. Hells Canyon on the Oregon/Idaho border may be 19 feet deeper. Maybe...

Largest living things in the world: Sequoia Trees

Tallest living things in the world: Coast Redwoods

Oldest living things in the world: Bristlecone Pines (5,000 years), White Mtns, or Creosote Bushes in Colorado Desert (11,000 years)

My Scottish BBC Interview at a relatively balmy 110 degrees

Hottest Place on the Planet and Driest Place in North America: Death Valley: 134 degrees, precipitation 1.4"/year

One of the Snowiest Places in the U.S.: Tamarack, Sierra Nevada, 76 feet in one year, 32 feet in one month, 37 feet on ground at one time

Highest Waterfall in the North America (no. 7 in world): Yosemite Falls, 2,425 feet

Second tallest active volcano in the U.S.: Mt. Shasta, 14,162 feet

Second most recently active volcano in lower 48 states: Mt. Lassen (1914-21)

Most voluminous volcano in the lower 48 states: Medicine Lake Highland (around 130 cubic miles) in northeastern California

Scarp from the 1872 Lone Pine earthquake

Some of the largest earthquakes in the lower 48 states: 1906 (San Francisco 7.8), 1872 Lone Pine (7.8), 1857 (Ft. Tejon 7.8)

Old oil derrick near the Santa Clarita Valley

One of the more prolific oil and natural gas producing regions in the world: Los Angeles Basin, Bakersfield, and Santa Barbara-Ventura Channel

One of the biggest explosions ever: Long Valley Caldera, 750,000 years ago, 125 cubic miles of ash spread all over the western states as far east as Nebraska and Kansas

McWay Falls at Julia Pfeiffer-Burns State Park on the Big Sur Coast

No other state has the combination of landscapes: Coastlines, deserts, mountains, river valleys and plateaus, due in no small part to the fact that California is influenced by all three different kinds of plate margins: Divergent, convergent and transform. Few places in the world have this kind of diversity.

The San Andreas fault on the San Francisco Peninsula. San Andreas reservoir, from which the fault took its name, is in the foreground.

What would you add to this list???

Tuesday, September 12, 2023

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.


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 . . .


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:

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.


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.


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!