by John McPhee
Moores cast a final glance over the man-made valley by Gold Run, and said, “It’s not all that bad. Some places like this do not look bad. They are spaced out. They are not the English industrial Midlands. I like to drive cars. I like to move rapidly from place to place. There is a price we pay. If people wish to eschew all that, let them walk. When they get rid of their cars and their hi-fi sets, their credibility will rise.”
He lingered long enough for a change of mood. His voice resumed at a lower and softer register. “In a couple of hundred years we are doing a good job of extracting minerals deposited over billions of years. High-grade gold deposits are just gone. Ditto copper. The U.S. has had it. There just won’t be any more until we go through a few million more years of erosion, allowing the geologic processes of secondary enrichment to take place. Meanwhile, technology must extract lower- and lower-grade resources. We don’t realize what we’re doing.”
I said I thought that we knew what we were doing and didn’t give a damn.
He said, “Americans look upon water as an inexhaustible resource. It’s not, if you’re mining it. Arizona is mining groundwater.”
Soon we were dropping toward two thousand feet, among deeply weathered walls of phyllite, in color cherry and claret—the preserved soils of the subtropics when the unrisen mountains were a coastal plain. Geologists call it lateritic soil, in homage to the Latin word for brick. All around the Sierra, between two and three thousand feet of altitude, is a band of red soil, its color deepened by rainfall that leaches out competing colors and intensifies the iron oxide. Not only phyllites but also mica schists, shales, tuffs, and sandstones in the roadcuts were red. When the road dipped far below the rooflike plane of the western Sierra Nevada, the dissected inclines around us had the appearance of red mountains covered with manzanita.
At Weimar, a little off the highway and close to the twothousand-foot contour, was a narrow band of serpentine, the California state rock. Moores said, “Worldwide, there is an association between serpentine and gold-bearing quartz, as there is here, in the belt of the Mother Lode. Gold-quartz deposits and serpentines just go together. Where there’s a hard-rock mine, serpentine will not be far away. The relationship between serpentine and quartz-vein gold is not well understood, but the miners talked about it. It was a fact of their life.” On the geologic map, the serpentines showed up as strings and pods in a rich wisteria blue, like some sort of paisley print, trending north-south, signing the Mother Lode.
Also accompanying the Mother Lode was a family of major faults, confined to a zone that was scarcely fifteen miles wide but extended, both to the north and to the south of Interstate 80, more than a hundred miles. Three of the faults crossed the highway in and close to Auburn, about twenty miles below Gold Run and thirty-five above Sacramento. Auburn, once known as Rich Dry Diggings, is now the seat of Placer County. Gold was found there in a living stream less than four months after Marshall’s discovery at Sutter’s Mill, and mined hard-rock ore was still being stamped to powder at Auburn well into the twentieth century. The placer discovery was in Auburn Ravine, which the interstate touches as it passes through the town and under the Southern Pacific. In 1849, Auburn was as far uphill as you could haul things conveniently from Sacramento in a wagon. For people and pack mules, it was the trailhead to the burgeoning mines. Within a few years, Auburn had become known as the Crossroads of the Mother Lode. In masonry walls of block schist, in windows arched with sawed soapstone, something is left in Auburn of the Roaring Fifties.
In Auburn Ravine, a couple of hundred yards below the railroad overpass and exactly twelve hundred feet above sea level, the interstate had been cut through charcoal-gray rock that had very evidently been damaged by a great deal more than human engineering. We pulled over as soon as the shoulder was wide enough, and walked back to have a look. We walked past talc schists and sheared serpentines and integral blocks of volcanic rock separated by shear zones. The cut that had caught Moores’ attention was ten feet high and nondescript, below gray pines and trees of heaven. It was tight to the interstate, and tandem trailers were screaming past us. A billboard across the road said, “Placer Savings, It’s the Extras That Count.” Picking and prying at the Sierra Nevada a roadcut at a time, Moores had crossed the mountains showing all levels of absorption and excitement. In the presence of unusual rock, he variously fizzes and clicks. Now, as he leaned into this outcrop with his lens, he began to do both.
It was fine-grained diabase, in magnification asparkle with crystals—free-form, asymmetrical, improvisational plagioclase crystals bestrewn against a field of dark pyroxene. It was a much finer diabase than you would find in, say, the Palisades Sill, across the Hudson River from Manhattan. It had cooled and frozen more rapidly, but it derived from a chemically identical magma—that is to say, essentially identical, there being no exact copy in geology except a Xerox of your last mistake. Had this magma been extruded into air or water it would have become basalt, but—like granite, diorite, gabbro—it had chilled and formed its crystals in the absence of both. There was a signal difference, however—far beyond cooling rates or chemical composition—between this diabase and the rock of the Palisades Sill or any magma that intrudes and then hardens as a single body. To see the difference, you did not need to make a thin section—a tiny slice of rock for a microscope slide. You did not even need the hand lens. This rock had been assembled in vertical laminations like successive layers of wallboard. It had frozen not all in one piece but in continual fashion, layer after layer—a history that could be read from one lamination to the next, like bar codes indefinitely extended. Moores, ebullient, said, “We’re in Fat City.” Lens to eye and leaning into the outcrop, this professed and practicing agnostic said, “God, it’s fantastic! God Almighty! This is a jackpot, a tremendous bit of serendipity. We’ve struck gold.”
Given the fact that we were at twelve hundred feet in the western foothills of the Sierra Nevada and in close proximity to serpentine and quartz, I could be forgiven if at first I took him literally. Yet all that glistered in this outcrop was pyroxene. Gold is where you find it, though, and for Eldridge Moores this indeed was gold. Unlike all the other rock we had seen as we traversed the mountains, or were likely to see in most of the aerial world, this rock in its origin was not of any continent. It was not from slope, not from shelf, not from lake, stream, or land. It had no genetic relationship to continental rock. Like a blue-water fish on a farmhouse platter, it had been moved a great distance. Only a meteorite could have been more out of place.
Nineteenth-century geologists would have called this rock augite porphyrite; the miners would have called it blue diorite or slate. It was rock of the ocean crust. Formed at spreading centers, ocean crust gradually turns cold as it travels away from the hot rift of its beginnings toward the deep trenches where nearly a hundred per cent of it is consumed. Down the vertical column from salt water to mantle rock, ocean crust has varying components, of which these laminations are the clearest record of lateral movement. A layer at a time, the fluid rock is driven upward in the spreading center, solidifies, and takes its place in the long march. Most of this happens in the mid-oceans, in the world system of separating boundaries of plates. It also happens in the short, isolated, and slice-like spreading centers that develop near island arcs. In all geology where rock forms in successive layers, the layers are initially horizontal—with this one exception. The laminations of the ocean crust form vertically, and remain vertical as they move to become the floors of abyssal plains and until they disappear into trenches. In Moores’ words, “This is the only situation where age progression goes sideways.”
Although the rock in this outcrop had obviously been shattered by a very great tectonic force—and although it had to some extent been recrystallized as well in the attendant heat and pressure—neither its disfigurement nor its metamorphosis had masked its structure. The laminations—known in geology as sheeted dikes—were as narrow as ten centimetres and as wide as eighty. By looking close
ly at their edges, you could all but see the spreading center that the accumulating rock had slowly moved away from. Layer after layer was glassy along its right-hand edge. The magma had cooled quickly there after touching solidified rock. The spreading center, therefore, had been to the left. After a new lamination of magma touched hard rock and turned marginally to glass, the rest of the lamination froze more slowly, forming the fine crystals. Some layers had glassy margins on both sides. They had split the weak center of previous and still-cooling layers. In a minor and local way, they corrupted the chronology.
When seismology first revealed the dimensions of the ocean crust, it proved to be surprisingly thin—about fifteen thousand feet thin—with remarkable uniformity all over the world. The sediments upon it, generally speaking, are not much more than a veneer. Rock of the ocean crust—departing from spreading centers with bilateral symmetry, ultimately disappearing in the subduction zones—is everywhere younger than most rock of the continents. The oldest known continental rock was discovered east of Great Bear Lake, in the Canadian Northwest Territories, in 1989, and has a uraniumlead age of 3.96 billion years. The earth itself, according to radiometrics, is six hundred million years older than that. The oldest ocean-crustal rock that has yet been found in any seafloor in the world is early-middle Jurassic—a hundred and eighty-five million years old. That is less than one-twentieth the age of the oldest continental rock and one-twenty-fifth the age of the earth itself. From spreading to subduction, from creation to extinction, the ocean crust completely cleans house in fewer than two hundred million years. A lithospheric plate will typically include both continental rock and ocean crust, but trenches get rid of the ocean crust while the continents stay afloat. Since rock of the sort that Moores and I were looking at does not form on continents and will not be found under a Hudson Bay, a Sea of Okhotsk, or any epicontinental sea, what was it doing in Auburn, California, more than five hundred miles from the nearest abyssal floor?
Moores did not have to be asked, for if he had a tectonic and petrologic specialty this was it. He had travelled the earth to see this kind of rock. Where you found it up on dry land, it proclaimed an event in the making of new country, in the mobile history of plates. It was not a signature after a fact but a precursory signing in. In its transportation from the deep and its emplacement on a continent, it was not merely a clue but an absolute statement that scenery had been shifted in an operatic manner.
Toward the end of the middle Jurassic—in the high noon of dinosaurs, about a hundred and sixty-five million years ago—an island arc like the Aleutians or Japan had moved in from the western ocean and docked here. This was the third terrane at this latitude: the one that followed Sonomia and smashed into it with crumpling, mountain-building effects that propagated eastward turning soils into phyllites, sandstones into quartzites, siltstones into slates—the metamorphics we had seen up the road. In aggregate, the three terranes extended the continent by at least four hundred miles. The third one, suturing here, had doubled the width of what is now California.
The sheeted diabase that we found in Auburn—shattered so grossly in the collision—was a part of the ocean crust at the leading edge of the third terrane. As the island arc drifted eastward and the continent westward, nearly all the intervening ocean crust was consumed, but some broke off and came to rest on the continental margin, announcing the collision.
North-south, the third terrane probably came near to being a thousand miles long. What remains of it is closer to a hundred. Its width, including the part that is under the Great Valley, is about a hundred miles, too. This ten-thousand-square-mile piece of ground, named for a gold camp some twenty-five miles north of Auburn, is known in geology as the Smartville Block.
If you look at a map of the Mother Lode and lode-gold belts related to it—a narrow band, north-south, lying under Grass Valley, Forest Hill, Placerville, Plymouth, Mokelumne Hill, Angels Camp, Carson Hill—you are, for practical purposes, looking at a map of the Smartville suture. As a geologically immediate result of the collision, the nearby rock developed the numerous high-angle faults that now appear on the geologic map along the Mother Lode. The voluminous magmas of the batholith came into the country. Water moving down through the faults would have circulated close to—or actually in—the magma, dissolving high-temperature gold compounds, and carrying them upward to precipitate the gold in fissures. In this manner, the Smartville Block, docking in the Jurassic, not only doubled the size of central California but created its Mother Lode.
If you could pull up an acre of abyssal plain anywhere in the world—lift into view a complete column of the ocean floor, from the accumulated sediments at the top to mantle rock at the base—you would find the sheeted dikes about halfway down. In contrast to the rock columns you find all over the continents—giddy with time gaps among lithologies of miscellaneous origin and age—this totem assemblage from the oceans tells a generally consistent story. At its low end is peridotite, the rock of the mantle, tectonically altered in several ways on departure from the spreading center. Above the mantle rock lie the cooled remains of the great magma chamber that released flowing red rock into the spreading center. The chamber, in cooling, tends to form strata, as developing crystals settle within it like snow—olivine, plagioclase, pyroxene snow—but above these cumulate bands it becomes essentially a massive gabbro shading upward into plagiogranite as the magmatic juices chemically differentiate themselves in ways that relate to temperature. Just above the granites are the sheeted dikes of diabase, which kept filling the rift between the diverging plates. Above the sheeted dikes, where the fluid rock actually entered the sea, the suddenly chilled extrusions are piled high, like logs outside a sawmill. Because these extrusions have convex ends that bulge smoothly and resemble pillows, they are known in geology as pillow lavas. Above the pillows are the various sediments that have drifted downward through the deep sea: umbers, ochres, cherts, chalk. Unlike the rest of the crust-and-mantle package, the sediments may hint at the surrounding world. Water that gets down through all this and into the mantle rock—at the spreading center or anywhere else—will change the nature and appearance of that rock. Through an alteration of minerals, the rock takes on a silky lustre and a very smooth texture, becomes fibrous, and develops color—occasional streaks and spots of white, but mainly chrome green, myrtle green, Nile green, in patterned shapes within the mantle black. Because the patterns strongly suggest the skin of a snake, this rock has been known—for nearly six hundred years in the English language—as serpentine. Geologists—in their strange, synecdochical way—have named the entire oceanic assemblage for this one component rock. But not directly. In their acute sense of time, they were not content to settle for a term of Latin derivation. Instead, they extracted from a deeper stratum φς—ophis—the Greek word for snake. From the mantle upward, the complete column of ocean-floor rock is collectively known in geology as an ophiolite. The generally consistent differences within it are the ophiolitic sequence.
On the American River under the bluffs of Auburn, in 1852, a single pan of gravel might be worth a hundred dollars. In 1857, after the lone miners had worked the place over, the American River Ditch Company built a dam there, to impound water for hydraulic mining. The dam eventually crumbled. The dam site did not. As environmentalists have discovered to their eternal chagrin, a dam site is a dam site forever, no matter what the state or the nation may decide to do about it in any given era. On present road maps of California, that part of the American River is marked “Auburn Dam and Reservoir (Under Construction).”
The dam site is scarcely a mile from the shattered ophiolite of Interstate 80, so Moores and I went to see how the dam might relate to the Smartville collision, and we have returned there since. The river’s deep canyon is walled with sheared foliated rock—broken, disrupted, deformed lithologies, Bruchgeitrochen, tortured rocks—as one would expect of a place where an oceanic island arc had sutured onto the continent. There were sheeted dikes, serpentines, plagiogranit
es, gabbros, and other items from the ocean suite. The type of dam chosen in 1967 by the Department of the Interior’s Bureau of Reclamation was a thin arch of concrete rising six hundred and eighty-five feet from channel to crest. Its purpose was to store winter runoff for use in summer, supplementing the storage behind Folsom Dam, fifteen miles downstream. The new reservoir, Lake Auburn, would reach twenty miles into the Sierra, filling two forks of the river—up the North Fork past Codfish Creek and Shirt Tail Creek beyond Yankee Jim’s almost to Iowa Hill, and up the Middle Fork over New York Bar and Murderers Bar and the Ruck-a-Chucky Rapids to Volcanoville. The lake would cover ten thousand acres and be twice as deep as the Yellow Sea.
When Moores and I first visited the site, in 1978, it resembled one of the huge excavations flushed out by the hoses of hydraulic mining. Benched roadways descended switchbacks a thousand feet down the canyon walls. A cofferdam had tucked the river to one side. Reaching eleven hundred and fifty feet across the canyon floor lay the white concrete of the dam’s base. From the outset, the construction project had had to deal with the inconvenience of the faulting that had followed the arrival of the Smartville Block. The dam site was squarely in the suture zone. Under the dam’s foundation ran a fracture known to engineers as the F-1 Fault. A tectonic event on the scale of an arc-to-continent docking will not result in every fissure’s being filled with quartz and gold. Countless empty cracks remain. In order to secure the dam’s basement, the Reclamation engineers had performed what they described as “dental work,” a “root canal.” They had sealed in the Smartville fault zone with three hundred and thirty thousand cubic yards of grout.