Through The Sandglass

The "Dieu Protège"

In the previous post I cited the joys of serendipity, and they continue. Travelling down the west coast of Brittany, we were attracted to the old fishing harbour of Douanenez and Le Port-Musée there, a boat museum. This is a museum with a difference - not only are there indoor exhibits of everything from coracles to twentieth-century fishing vessels, but outside, in the harbour, is moored an extraordinary collection of boats that have been painstakingly collected and restored, and around which you can wander at will, clambering up and down ladders, trying out the wheelhouses. Serendipity arose when I discovered that, among these boats is Le Dieu Protège, a gabare sablière - a sand transport.

To the north of Douanenez is Brest, a major port for centuries and the focus of extreme conflict during the Second World War. Used as one of the extensive U-Boat bases and a harbour for German battleships, including the Scharnhorst and the Gneisenau, Brest was the target of constant allied bombing raids. In 1944 the battle for its liberation was vicious, and by the end of the war literally only a handful of buildings remained standing in the city, including significant sections of the mediaeval harbour fortifications. The images below show Brest harbour before and after the war.

The post-war reconstruction of Brest took years and required huge volumes of concrete that, in turn, demanded huge volumes of sand. Le Dieu Protège ("God Protects") was built in 1951 specifically to excavate, transport, and deliver sand for the rebuilding of the city - the sand trade was thriving. It seems that the first owner had proposed various names to the authorities, all of them refused because of other boats already claiming them. He came up with the name in memory of a small boat sunk in the treacherous coastal waters after delivering the church bell to the island of Ouissant (presumably the protection extended only to the bell....). The boat is 23 meters long, has a capacity of 150 tons of sand, and has double wooden hulls to protect against the abrasive effects of its cargo and the stresses of large volumes of sand being dropped into its hold. It was equipped with sails (below, right), but was motorised and was fitted with a substantial mechanical bucket grab (named in French, le crapaud - the toad).

Until this innovation, sand extraction had taken place by beaching the boat and loading the cargo by hand and shovel, assisted, if on the beach, by horses, as shown in the delightful model at the museum (above, left). The bucket could hold 70 cubic meters of sand at a time, and around two hundred bucketsful of sand needed to be hauled up to fill the hold. [Although quoted from the museum documentation, the bucket capacity seems excessive and the numbers are inconsistent - see comment below]. The remaining requirement was a water pump to remove the 15 tons of water that drained out of every cargo. Four anchors would secure her to an offshore sandbank while she worked.

Le Dieu Protège's working life continued for thirty-five years - her last job was dredging the local harbours. She was purchased by the museum in1990 and restored - and, as the photo below shows, she is being further restored today.

 

Go down the steep ladder to the hold today (now empty of sand), and there are fascinating movies being shown of the sablière at work in her heyday. I have taken the liberty of extracting a few still images from these:

 

My thanks to a superb museum - http://www.port-musee.org. There's much, much more to see than Le Dieu Protège, so, if you're in Brittany, take time for a visit.

And I'll leave you with a picture of le sablier at the helm of a gabare sablière  - it's a great place for kids.

 

Sand contributes to the economic stimulus

As a young kid, I was blessed with six great-aunts, who lived together in two groups of three, one group in Nottingham and the other in the East End of London. Visiting either somewhat eccentric group was always a treat for me, their rambling old houses filled with strange (but still memorable) aromas, exotic Victorian decoration, and sundry mysteries (including the outside toilets). The London group consisted of two spinsters and a widow plus cats and, generally, a lodger priest whom I tended to avoid. I loved visiting, particularly if my great-aunt Grace (my favourite) had made bread pudding, still the best I have ever tasted. But a culinary mystery, and a genuine piece of kitchen magic, was when she preserved eggs. These were still the post-war days when eggs were precious, and, in any case, old habits die hard. The eggs were immersed in this strange substance, which began as a viscous fluid, diluted for the purpose. "It's waterglass, dear," my great-aunt would explain; this struck me as being very descriptive, since it looked like liquid glass, but little did I realise how accurate the name was.

Waterglass is sodium silicate, or, more strictly, sodium metasilicate, Na₂SiO₃, a chemical that easily absorbs water, starting off in my Great-Aunt's kitchen as a powder, and hydrating into a viscous fluid and then a true liquid. It seems that it penetrated the shell of an egg, sealing off the porosity and thereby preventing access by bacteria; eggs preserved this way could be kept for months. Waterglass showed up later in my life in various guises, including as an  sealant for various leaking bits of my early cars - it was particularly effective for a hole in the exhaust pipe, the heat of which would solidify a patch soaked in sodium silicate into a hard seal. The irony today is that sodium silicate is hitting the headlines not as a means of mending your car, but of completely crippling it. As a recent USGS news item reported:

The Nitty Gritty of Cash for Clunkers

The government-run “Cash for Clunkers” did much more than just stimulate the economy and raise awareness of carbon emissions. It also caused demand for a little known and little used mineral compound called sodium silicate. The program required that buyers of clunkers immediately kill the engine of the car — a task most efficiently done by running the engine with sodium silicate. When a vehicle runs for a few minutes with this compound in place of engine oil, the engine seizes, and it cannot be reused. The other parts of the vehicles, however, can be recycled.

The "Cash for Clunkers" project has caused a massive demand for an obscure mineral. As the Wall Street Journal reported a few days ago, under the headline "The Killer App for Clunkers Breathes Fresh Life into 'Liquid Glass'," the "designated agent of death for cars" suddenly became a highly sought-after commodity. One chemical supplier

ordered enormous supplies and purchased prime space on Google, so that his company popped up in searches for sodium silicate. Last week, he sold 4,600 gallons of it, and the rush is continuing. "We're working 16 hour days, and we've got friends and family helping out filling orders."

So where does all this magic, lethal, mineral come from? As the USGS reports, "Sodium silicate, the only soluble silica compound, is made by fusing two industrial minerals, high-purity silica sand with soda ash." Sand to the rescue of the global economy!

"Soda ash" is sodium carbonate, the naturally occurring form of which is the mineral natron. And so we come back to glass - "the stone that flows." The satisfying story that Pliny the Elder tells of the origin of glass is, sadly, apocryphal, but it's entertaining and illustrative of the process. He describes, in his Natural History, how Phoenician traders with a cargo of natron, perhaps from the desert lakes of the Sahara, had put in for the night on the eastern Mediterranean coast. Unable to find adequate rocks to support their cooking pots over the fire, they resorted to using some blocks of their cargo. Whatever their dinner recipe was, they had unwittingly assembled the ingredients for glass: the soda lowered the melting point of the beach sand, and out of the fire flowed streams of translucent liquid. It's a nice fairy-tale, but with an important point: to melt sand on its own requires temperatures in excess of 1,600°C (2,900°F), far beyond the capability of traditional wood-burning furnaces. The melting point has to be lowered to a practicable temperature, and this is where the natron plays its critical part in the story. Soda has a much lower melting point than sand and acts as a flux, an additive that makes silica sand meltable at realistically achievable temperatures.

But of course what is being made here is sodium silicate - waterglass, a product soluble in water, which is hardly desirable in making a wine glass or a window. To stabilize the glass, calcium in the form of powdered limestone has to be added, and it is entirely possible that this was first discovered by accident through using silica sand that also contained fragments of seashells. The resulting concoction is soda-lime glass, the everyday kind of glass that has been used for the last three thousand years.

Sodium silicate, while it is a somewhat obscure chemical, does, nevertheless, continue to be used in a number of different ways. It's still used as an adhesive and sealant, in treatments for timber, concrete, and water, and as a fire protection. Perhaps its most important use today is in the manufacture of synthetic zeolites, clever minerals that have a whole host of uses. It's also used in Magic Rocks. Although that name hadn't been yet coined when I was a kid, this was the other incarnation of sodium silicate that I encountered. In the days when chemistry sets really were chemistry sets, with ingredients capable of causing real domestic havoc and incurring real maternal wrath, one of the more benign but common chemicals provided was waterglass. With this, and some compounds of different metals, a "chemical garden" could be created. Put a layer of sand at the bottom of a glass container, add waterglass dissolved in water, and then chunks (the magic rocks) of different metal compounds, generally nitrates or chlorides of manganese, cobalt, copper, iron, and watch your garden grow. The metals combine with the waterglass to form insoluble metal silicates that precipitate and grow upwards to form exotic plant-like structures. The colour depends on the metal, just as the colour of glass depends on the metal added. The phenomenon was discovered in the 17th century, when waterglass was known as "oil of sand" and played a common role in alchemy. It would seem that Sir Isaac Newton, in his experiments in the transition between alchemy and "chymistry," created magic gardens in oil of sand, the organic appearance of the structures confirming the old alchemical idea that metals have a kind of life and can "vegetate".

It's sort of fun where a simple story of disabling clunker engines can get to - my great-aunt Grace would be pleased.

"Sand to Chips" - what's the real story?

One of the thorny issues that I grappled with in writing the book was how to review the staggering number of ways in which sand is important in our lives - in the end, the diversity was so overwhelming that I resorted (as readers of the book will know) to an alphabetic medley approach; interestingly, it would seem to be a chapter that is often highlighted in reviews and comments as either highly entertaining or awkward. In my ruminations about how to weave all these disparate topics in to  a narrative, one idea had occurred to me: given that an immediate and recurring (if subconscious) encounter we all have with quartz sand is every occasion that we use an electronic device, why not tell the story, the journey, of a sand grain from its geological origins to the silicon chip in the computer on which I was writing?

"Sand to chips - a biography" - seemingly a simple task, this proved anything but. To begin with, given the need for various but rigorous levels of extreme purity in silicon-based manufacturing, just any old sand won't do - the raw material has to be as pure as possible to begin with. As I wrote:

First of all, the public is understandably more interested in the technology of making the silicon chip than in its provenance in a sandbank by a river somewhere. Second, the absolute volume of semiconductor-grade silicon is only a small fraction of all silicon production. But more important, a small number of companies around the world dominate the technology and the market, and while their literature and websites go into considerable and helpful detail on their products, the location and nature of the raw materials seem to be of “strategic value,” and thus an industrial secret. I sought the help of the U.S. Geological Survey, which produces comprehensive annual reports on silica and silicon (as well as all other industrial minerals), noting that statistics pertaining to semiconductor-grade silicon metal were often excluded or “withheld to avoid disclosing company proprietary data.” The survey staff were, as always, extremely helpful, but were themselves perplexed that such an apparently simple question was not simple to answer. They put me on the trail of a number of sources, but telephone and e-mail inquiries did not shed a great deal of light. What I could deduce is this: the common source of silica for manufacturing the high-purity grades of silicon used for, among other things, silicon chips is not loose, unconsolidated sand from a beach or river sandbank, but sand that has already been ultrapurified by nature: quartzite.

Quartzite is a rock that was originally a silica sandstone; it has been so deeply buried in the Earth’s crust, cooked by such high temperatures and pressures, that many of the impurities have been distilled out and the sand grains completely annealed and welded together. Hit quartzite with a hammer and it rings like a bell because of its hardness, purity, and uniformity. Hit it hard enough and it breaks across the ghosts of the grains, not around them. Quartzite can be well over 99 percent pure silica. Grind it up to a powder of a consistent grain size and it’s a good starting point for making silicon.

The geological kitchens of the Appalachians were good at making quartzites, and my research (including phone calls to sand and gravel suppliers) led me to one possible source:

The Coosa River, for example, originates close to the border of Georgia and Tennessee and, together with its tributaries, drains a large and geologically diverse area of Appalachian rocks, including quartzites. It crosses from Georgia into Alabama, where it joins the Tallapoosa to become the Alabama River. As it does so, it enters the broad coastal plain, slows down, and dumps its sediment load. Large volumes of aggregates for all kinds of industrial purposes are extracted in this part of the country from the sediments of the Coosa and other rivers, but among those everyday aggregates are pebbles of great value—pure quartzites from the kitchens of the Appalachians. And those, I believe, are one source of raw material for high-grade silicon. So it’s true that computer chips are made from sand—but sand that was first deposited several hundred million years ago.

But the confidence level that I (and the USGS) had in this being a definitive example led me to abandon the sand grain biography idea. Then, earlier this week, the whole issue arose again: BBC Radio in the UK had a programme (it is the BBC, hence the extra "me") called "Chips with Everything" and an associated article titled "Silicon Valley's Secret Recipe." The thirty-minute broadcast is well worth listening to, for the "Silicon Valley" referred to is in the Blue Ridge Mountains of North Carolina, around the small community of Spruce Pine which calls itself the Mineral City - for good reason. The programme is culturally fascinating but also highly informative with respect to silicon chip technology - and I learnt something that I had missed in the opacity of my earlier research.

The reason the Spruce Pine calls itself the Mineral City is that in the surrounding hills is a treasure trove of precious and highly valuable industrial minerals - including some of the purest quartz in the world. These minerals, exploited since long before the ancestors of today's residents arrived, are contained in igneous rocks, solidified from molten material deep below the earth's surface; but these are special, rocks that are made up of unusually large and unusually pure, crystals - rocks called pegmatites. Below is a photograph of one of these (thanks to Callan at NOVA Geoblog), with large feldspar crystals embedded in quartz. It was long thought that these unusually obese crystals formed because the cooling rate of the molten rock was extremely slow, but this apparently logical model has been replaced by recent research that suggests that the cooling rate was unusually rapid, faster than the rate of crystallisation could keep up with.

 

The hills around Spruce Pine have been described as the most valuable strategic acreage on the planet. Sixty-eight-year-old Ira Thomas, a ninth-generation Spruce Pine miner who used to dig up aquamarines and prospect for mica as a child justifies this description: “Because the world runs on computers, we all know that now. And if we locked the gates to Mitchell County they could not make any more computers.” As a consequence, the quartz pegmatite mines are protected by security akin to the Pentagon or the Bank of England. Spruce Pine is billed as the world's only source of this uniquely pure quartz - this is geologically probably not quite true, but it is by far the world's major supplier of a raw material that sells for $50,000 a ton.

But so what was my revelation? Simply that the Spruce Pine quartz is valuable not as the source of silicon for chips, but as the material for making the crucibles and other bits of kit needed to process electronic-grade silicon. Silicon can be extracted from any quartz relatively easily (you can do it, albeit somewhat dangerously since silicon is an excitable element, in your kitchen from beach sand); but to process it to the level of purity needed, then ultra-pure equipment is essential - and it's made from Spruce Pine quartz. The level of silicon purity for a chip is extraordinary: electronics-grade silicon has to be at least 99.99999 percent pure - referred to in the trade as the “seven nines” - and often it’s more nines than that. In general, we are talking of one lonely atom of something that is not silicon among billions of silicon companions. Or, as a Dow Corning scientist, put it:

Imagine stringing tennis balls from the Earth to the moon, and wanting them all to be yellow. He said this would take about 5.91 billion tennis balls. For the color coding to be of semiconductor quality, "you could only tolerate two, maybe three that were orange," Lane said. For solar cells, which are slightly less demanding, Lane said, "You could tolerate five or six orange balls."

Following up on the BBC material, I came across a useful series of articles on Geek.com, which, in turn, led me to a press kit put out by Intel, titled "From Sand to Silicon: the Making of a Chip." And there, to my delight, at the head of the document is an image of a pile of sand. One of the Intel images is at the head of this post, and in the document is accompanied by the description:

Sand

With about 25% (mass) Silicon is –after Oxygen –the second most frequent chemical element in the earth’s crust. Sand –especially Quartz -has high percentages of Silicon in the form of Silicon dioxide (SiO2) and is the base ingredient for semiconductor manufacturing. [my highlighting]

The silicon is purified in a series of technologically sophisticated (and highly secret) steps; to the right of the pile of sand is melted silicon, and then a "Mono-crystal Silicon Ingot .... produced from Electronic Grade Silicon. One ingot weights about 100 kilograms (=220 pounds) and has a Silicon purity of 99.9999%." (Why six, not seven, nines?)

So there we have it - sand, or potentially sand in its quartzite incarnation, is the source of raw silicon for electronic chips. But I am no further forward on identifying geologically which sands - or sandstones, or quartzites - are the sources. The quest continues - if anyone can shed light on this, I would be hugely grateful: I can provide an update for the paperback edition of Sand!  

 

[My profound thanks to Pete Modreski at the USGS for his continuing help throughout this quest. And there are other sources of high purity quartz in the world, among them from Norway: the NGU, the Norwegian Geological Survey, has an excellent web page, in English, on high purity quartz -  http://www.ngu.no/no/hm/Georessurser/industrimineraler/Kvarts-og-kvartsitt/Hoy-ren-kvarts/]

 

All that glitters ...

 

At the end of my last post on granular segregation and placer mineral deposits, I included a photo of a sand sample from Yosemite that is reputed to contain flakes of gold, with a slightly enigmatic candidate grain. Lockwood at Outside the Interzone and the Lost Geologist kindly commented with testing advice - press the grain with a needle and, because gold is relatively soft and malleable, it will deform, the needle leaving an imprint; mica, being flaky and brittle, will split apart. I couldn't find the same grain, but found a similar one, and gave it a poke with a needle. The point left a dent - aha, I thought, my hopes rising. But, on applying further pressure, the grain fragmented, flying apart into tinier flakes. It was muscovite, a mica, coloured in this case yellow, and slightly rotten from the processes of weathering - hence the dent, and its deceptively lustrous appearance. I should have known - these are really young sand grains, the recent sawdust of the crumbling Sierran granites, and the mica minerals (including an almost perfectly hexagonal biotite grain, left) form a significant membership of this granular tribe - they'll rot out of existence as time goes by.

So, all that glitters is not gold. Of course, pyrite, the shiny yellow iron sulphide mineral, is the traditional "fool's gold" (left in the photo above) but I'm not that foolish: if I found something that looked like the placer gold shown on the left, below, I like to think that I might have leapt to the correct conclusion. But the little muscovite grains (above, right) do sparkle in a most alluring way - I came across the second photo, below right, which purports to show placer gold nuggets and, while I'm no mineralogist, they look a bit suspicious. But then most prospectors know a lot more about what they're doing than I do.

 

My wife would agree. When we got married, she found the old picture below and felt that, with a geologist for a husband, this would be illustrative of the way her life would evolve (note the contents of the woman's bag). We still, ever-hopefully, have this picture on the wall, but alas, so far ......  

Granular segregation and the Gold Rush

Anyone walking on the beach or through the dunes will often notice areas of dark gray smeared across the surface, often outlining ripples, as if an artist had highlighted the sand's topography with charcoal. I encountered many examples of such designs recently at the dunes of Kelso (above) and Oceano, and on California's beaches. Look closely, and you'll see that these patterns are characterised by concentrations of dark grains, gathered together in a much higher proportion than elsewhere in the sand. Look even more closely, down a microscope, and there they are, little black nuggets nestling among the glittering quartz grains (the example below is from Kelso).

 

As my messing around with kitchen physics showed, sand, like all granular materials, dislikes being mixed and will find endless ways of sorting itself out into its components. Pouring and shaking cause it to segregate by size and shape, transporting it by wind and water differentiates grains by weight (as well as by size and shape), and that's what's going on in the dunes and the beaches. The black nuggets are grains of a mineral much denser than the quartz grains of the same size, and they are therefore slightly less easily moved. With time, the lighter grains are winnowed out and the heavier ones left, concentrated. In the case of the Kelso sand, the heavier mineral is magnetite, an iron oxide; I suspected this, and repaired, again, to the kitchen, to get a fridge magnet which I applied to the sand with the result that the fine black grains flew onto the magnet, piling themselves up into minute towers (see the photo at left; warning - removing the grains from the magnet is far more difficult and can result in scratched white goods).

The natural concentration of minerals by wind, waves and currents is economically important - large deposits of iron were formed this way, today and in the geological past. Such a natural concentration was the cause of one of Thomas Edison’s many business failures. On a fishing trip with friends off the coast of Long Island, Edison put into shore for lunch and found the beach covered with a layer of black sand. He took some home (perhaps to his kitchen) and discovered that the black grains were magnetite. Edison’s enthusiasm ran, as it often did, ahead of his business sense, and he immediately arranged for the purchase of the beach and the manufacture of separating machinery. Unfortunately, by the time he and his colleagues returned to Long Island, a winter storm had reworked the beach and completely removed the black sand.

These kinds of mineral deposits are called placers - in addition to iron, platinum, tungsten, titanium, tin, niobium, zirconium, and other vital elements are all sourced from placers - plus other treasures. The composition of sands betrays their origins, and if they came from the crumbling of precious mineral deposits, then they will contain precious minerals, often more easily accessible than by mining the hard rocks of their origins. Diamonds, rubies, sapphires, garnets, and gold are mined from placer sands in many parts of the world; in places like Namibia, beaches have been stripped to bedrock in the search for precious gemstones. Gold, being heavy and un-reactive, makes for an ideal placer mineral, and such deposits have been exploited for as long as we have been obsessed with gold. An ingenious early method (probably used in ancient Egypt) employed a fleece bag, the woollen side facing inward: water and sediment were passed through the bag, and the heavy gold flakes became embedded in the wool, remaining behind when the bag was emptied of lighter sand and gravel. A similar method was still being used in the mountains of the Caucasus in the 1930s; it also explains Jason and his Golden Fleece.

California was founded on placer sands: forty-niners during the Gold Rush sought the metal not only in subsurface mines, but in the streams and rivers that drained the gold-bearing ores. Panning the stream sediments was backbreaking work, and so a technological breakthrough was called for. It happened in the form of hydraulic mining: miners used high-powered water hoses to erode the valley sides (photo below, USGS). The gold, being heavier than the rest of the dirt, collected in sluices, and everything else drained away; mercury was commonly used to further concentrate the gold. It has been estimated that over a twenty-year period 750 million dump truck loads of sand, mud, and gravel, together with mercury, were flushed into the Central Valley—with dire environmental consequences. The Yuba Valley is just one of the watersheds that remains physically and chemically scarred today, and it can be argued that the environmental movement in the US has its roots there. A few years ago, the New Scientist had a fascinating piece on this - it's only available to subscribers, so I have reproduced it in full at the end of this post. And for further details, go, as usual, to the wonderful USGS search box and put in "hydraulic mining." And where did all this sedimentary debris from hydraulic mining end up? Well, a lot of it is in San Francisco Bay, moving out into the Pacific and forming some remarkable sedimentary features on the way - but more of that in a later post.

And, while I'm at it, I was given by a friend a sand sample from a Yosemite lake, a sample that was reported as containing flakes of gold. I've had a quick look; it's full of the hardly surprising debris of the weathering of the Sierran granites, including many flakes of yellowish mica, red herrings in the search for gold. But, in the middle-right of the photo below is a single grain that looks different - it's still a flake, but more chunky than the typical micas and with a different surface lustre. I'm no mineralogist, and certainly not a precious mineralogist, so if anyone has a view as to whether or not this might be gold, then I'd like to hear from you - but can't yet afford to offer a reward!

  

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From the New Scientist, 12 November, 2005, by Richard Lovett

Histories: The day the gold rush stopped

Although American environmentalism did not begin to flourish until the 1960s, the movement's roots date back to 1878 and a town meeting in Yuba City, California. Not that the assembled townsfolk saw themselves as doing anything so altruistic as protecting nature. Their concern was to save their land and livelihoods from torrents of mud unleashed on them by gold mines upstream. Angry and desperate, they formed the Anti-Debris Association, hired lawyers and went on the offensive. The fight that followed was so bitter it's amazing no blood was spilled. At one point, the association armed an anti-debris militia of 70 men - but law prevailed, and in a decision nearly a century ahead of its time a federal judge decreed that an estimated 45 tonnes of gold must remain in the ground.

IN FEBRUARY 1852, a Connecticut Yankee called Edward Matteson was working his gold claim at a place called American Hill in the Sierra Nevada foothills. There was plenty of gold in the compacted gravel of his claim, but it was widely dispersed and would take months to dig out with a pick and shovel. Perhaps, Matteson thought, there was a better way.

Matteson, like other miners in the early years of the California gold rush, had been using techniques that had hardly changed in millennia. The concept was simple: if you shovelled gold-bearing soil into a sluice box and scoured it with fast-moving water, the lighter sand, gravel and mud would be washed away, leaving the precious metal behind.

Unfortunately, shovelling dirt into a sluice box is hard, time-consuming work - and dangerous if you are digging into the base of a steep cliff. Matteson's brainwave was to attach a canvas hose to a tank of water high above him. Then he could stand safely back and direct the hose so that the water did the excavating and then channelled the debris along a system of ditches into his sluice box.

The Romans had much the same idea: they built dams and ditches to direct water where they wanted it, breaking the dams to create artificial flash floods along the channels. Matteson's innovation was to send the water through a nozzle, magnifying the force of the flow and allowing him to direct it precisely where he wanted.

Thanks to his high-pressure hoses, Matteson's operation became so efficient he could do several weeks' worth of work in a single day. But the new technique, called hydraulic mining, also sent weeks' worth of sediment downriver each day.

Word spread and soon hydraulic mining was booming. Most of the early mines were small and their environmental effects predominantly local. And in the early 1860s severe drought forced many to close. But when the rains returned, so did the miners, this time backed by wealthy investors from as far away as England.

"Hydraulicking" worked best along an 80-kilometre belt of the Sierra Nevada at elevations between 1200 and 1500 metres, where huge deposits of gold-bearing sediments had accumulated in ancient streambeds. There were mines everywhere, but by far the largest was Malakoff Diggins, which was so efficient that it could profitably mine sediments containing only a few pennies' worth of gold per cubic metre.

The mine was run by the well-financed North Bloomfield Gravel Mining Company, which ploughed $3.5 million into the operation. The Diggins eventually produced more than 6 tonnes of gold.

Today all that's left at the site is a 250-hectare pit 200 metres deep. But at the peak of operations, crews laboured day and night in the red-tinted gravels with hoses whose "Little Giant" nozzles could shoot 20-centimetre-thick jets at speeds of up to 50 metres per second - enough force to kill any miner who got in the way. In 1879 a reporter from the San Francisco Bulletin spoke of huge rocks being washed away "like chaff", with "a cloud of red foam" hanging above the points where the water hit the cliff. When water wasn't enough to do the job, entire hillsides were loosened with gunpowder - sometimes 15 to 20 tonnes of rock in a single blast.

When a US federal judge called Lorenzo Sawyer visited the site in the 1880s, he too was impressed. "The excavating power of such a body of water, discharged with such velocity, is enormous," he wrote. He was particularly amazed by the mine's night-time operations, conducted under "brilliant" lights that ran on hydroelectricity. "A night scene of the kind, at the North Bloomfield mine, is in the highest degree weird and startling, and it cannot fail to strike strangers with wonder and admiration."

By 1880, 'hydraulicking' had buried 6000 square kilometres of farmland

But Sawyer wasn't there to admire the scene. He was there because of where all that water, mud and silt was going. The first part of its journey was along a 3-kilometre tunnel through a mountain. The tunnel was effectively a vast sluice box that caught as much as 90 per cent of the gold. The debris, however, carried on into the Yuba river and towards the farmlands below.

In 1880, California's state engineer had estimated that 6000 square kilometres of farmland had been buried by mining debris. Sand bars had sprung up in rivers all the way to the coast. Hardest hit were the towns of Marysville and Yuba City, directly below the mines. As debris built up in the riverbed, the water rose so fast that the levee builders could hardly keep pace. But by now the people of Yuba City were fighting back. In 1878 they had banded together to form the Anti-Debris Association and sued the owners of the mine. Six years later, a panel of judges headed by Sawyer decided the case.

Judge Sawyer's 56-page ruling is a litany of environmental horrors, but the worst were his projections for the future. So far, the North Bloomfield mine had dumped 90 million cubic metres of debris into the Yuba river. About a quarter of that had reached the lowlands. The rest was still perched upstream, clogging some canyons to depths of up to 50 metres and creeping downward with each spring flood. If there was a really big flood, there was a chance the whole lot would come sliding downriver.

Worse, Sawyer estimated that there were 500 million cubic metres or more of gravel still to excavate. There was no choice but to shut down the mine under the ancient law of nuisance, which prohibits using your property in ways that damage someone else's.

Sawyer didn't ban hydraulic mining outright, though. He ruled that if the miners were to continue, they must find a way to keep the debris on their own property - and prove in advance that it worked. The miners appealed for leave to experiment with remediation techniques while continuing their operations. But Matthew Deady, one of Sawyer's fellow judges, gave that idea short shrift. Asking the people in the valley below to put up with that, he said, "may be likened, at least, to living in the direct pathway of an impending avalanche".

Sawyer's judgement was remarkable because it pre-dated America's best-known environmental rulings by more than 80 years. The stakes were enormous: the judgement would determine California's future as either an agricultural state or a mining state. But Sawyer didn't seem to see it as a trailblazing decision. He was simply applying established law, albeit on an unprecedentedly large scale.

It took years to force out miners who flouted the ruling, but the court's decision effectively killed off hydraulic mining in the California goldfields. Hydraulicking itself has never died: it has been used elsewhere to mine everything from coal to rubies and even aluminium. The difference is that today's mines exploit higher-grade ores, which make it economical to trap the tailings and recirculate the water in closed loops.

The hydraulic miners' legacy still surfaces from time to time in California's courts. In 1986 a flood breached levees built nearly 100 years earlier out of debris washed down from the North Bloomfield mine, resulting in lawsuits over whether such materials were adequate for flood protection. And in 1995 the California supreme court took on the tricky question of who owns those sandbars created by the shifting sediments from the old mines, some of which are now prime sites for urban development.

But nobody ever found an environmentally sound way to reopen the Malakoff Diggins. Today it's a little visited state historical park, even though, if Sawyer's back-of-the-envelope estimate is correct, gold worth $650 million remains in the ground.