Annals of the Former World

by John McPhee

  Moores spoke reflectively of “the joy of being alone with the geology,” of spending enough time walking such a scene to learn how some of it fits together, and then adding what you can to the scientific literature, “which is not like a solo but like an orchestral piece.” To Moores, what had happened to create California where no surviving rock had been before was much in evidence in the scene around us, as it had been in the rocks we saw along the road. As terrane—the homonym that refers not merely to surface configurations but to a full three-dimensional piece of the earth’s crust—this region had become known in geology as Sonomia. It reached from the Sonoma Range in central Nevada to the Sierra foothills west of where we stood. As plate theorists reconstruct plate motions backward through time, they see landmasses converging to form superterranes and breaking apart to form new continents. Swept up in these great events are islands and island arcs—Newfoundlands, Madagascars, New Zealands, Sumatras, Japans—that slide in or collide in toward continental cores. They become the outermost laminations of new landscapes.

  When terranes coming via the ocean attach themselves to a continent, they are said to have “docked.” Never shy about metaphors, geologists are not encumbered by the fact that they also call the docking place a suture. In early Triassic time, in the narrative according to present theory, Sonomia docked against western North America. The suture is on the longitude where Golconda, Nevada, is now. For a century or so before plate tectonics, the obviously overriding rock was known as the Golconda Thrust. It was an event that happened about two hundred and fifty million years before the present. Sonomia was an island arc. North to south, Moores said, it might have stretched two thousand miles. It brought with it those Paleozoic sandstones above Bear Valley and the quartz-rich sediments we could also see to the northeast. Volcanoes grew in the newly docked terrane. Bits of them would become the xenoliths in the granites of the summit. Along the western margin of Sonomia, where ocean crust was subducting in a trench, more volcanoes developed. Their rock was in peaks above us. Roughly where we stood, a coastal region of exceptional beauty had lain at the base of the volcanoes. Stratovolcanoes. Kilimanjaros and Fujis.

  Sonomia was actually the second terrane to attach itself to the western edge of ancestral North America. The first had arrived in Mississippian time. It had thrust itself almost to Utah. At this latitude, a third terrane would follow Sonomia in the Mesozoic, smashing into it with crumpling, mountain-building effects that would propagate eastward through the whole of Sonomia, metamorphosing its sediments—turning siltstones into slates, sandstones into quartzites—and folding them at least twice: the multicolored drapefolds we had seen beside the road. This was the country rock the batholith intruded.

  A granite batholith will not appear just anywhere. You will wait eons for one to develop under Kansas. A great tectonic event must come first. Then granite—or, rather, the magma that will cool and produce granite—comes in beneath the mountains. Volcanoes appear at the surface. Lava flows.

  To create the magma, you must in some way melt the bottom of the crust. Subduction—one plate sliding beneath another—will cause things to melt. And so will a collision that compresses and thickens terrane. After a continent-to-continent collision, the crust might double; a batholith will come up within thirty million years. In deep burial, the heat from such radioactive and universal elements as uranium, potassium, and thorium is trapped. The heat increases until the rocks melt themselves and their surroundings. Granite should be forming under Tibet at present, where India has hit the Eurasian Plate in a collision that is not yet over. Under California, both thickened crust and plate-under-plate subduction contributed to the making of the batholith, at first after Sonomia came in and sutured on and deformed itself, and again after Sonomia was hit from the west and further deformed.

  The Sierra batholith is melted crust of oceanic origin as well as continental. Most of the world’s great batholiths are not quite true granite but edge on down the darkening spectrum and, strictly speaking, are granodiorite. Too strictly for me. But that is the rock of the High Sierra, which almost everyone refers to as granite.

  After the batholith came nothing during the many millions of years of the Great Sierra Nevada Unconformity. At any rate, nothing from those years was left for us to see. The rock record jumps from the batholith to the andesite flows of recent time, patches of which Moores pointed out from the lookoff at Emigrant Gap. A few million years ago, when lands to the east of us began to stretch apart and break into blocks, producing the province of the Basin and Range, the Sierra Nevada was the westernmost block to rise, lifting within itself the folds and faults of the Mesozoic dockings, the roots of mountains that had long since disappeared. The chronology at Emigrant Gap ends with the signatures of glaciation on the new mountains—the bestrewn boulders and dumped tills, the horns, the aretes, the deep wide U of the Bear Valley.

  I remarked that geologists are like dermatologists: they study, for the most part, the outermost two per cent of the earth. They crawl around like fleas on the world’s tough hide, exploring every wrinkle and crease, and try to figure out what makes the animal move.

  Moores said he begged to differ. He said the whole earth is involved in plate tectonics. The earthquake slips of subducting plates could be read as deep as four hundred miles, and seismic data were now indicating that the plates’ cold ocean-crustal slabs may descend all the way to the core-mantle boundary. Bumps on the core may be related to the activity of hot spots like Hawaii, Yellowstone, and Iceland. He said he wouldn’t call that dermatology.

  Since Moores had learned geology in the late nineteen-fifties and early nineteen-sixties, when the theory of plate tectonics was still in a formative unheralded stage, I asked him what he had been taught. How had his teachers at the California Institute of Technology explained—in what is now known as the Old Geology—the building of mountains, the rise of volcanoes, the construction of North America west of Salt Lake? And how large had the transition been from what he learned then to what he knew now?

  He said that the science’s understanding of mountain-building mechanisms took its first great step forward in the second half of the nineteenth century, when James Hall, the state geologist of New York, conceived of the geosynclinal cycle, and so put in place the geology that prevailed until 1968, when plate tectonics was nailed to the church door. Since mountain belts tended to rise at the margins of continents and to contain, among other things, folded marine sediments and intruding batholiths, Hall imagined the long wide seafloor trough, the deep dimple, in which vast amounts of sediment would pile up and where magmas would intrude until the material was ready to rise as mountains. That is what Moores was taught.

  The geosyncline, like any admirable and serviceable fiction, contained a lot of truth. From stratigraphy to structure, geology was understood in terms of geosynclines for about a hundred years. You found gold with your knowledge of geosynclines. You found silver, antimony, and oil. You started conceptually with a geosyncline and projected events forward in time until you saw the geosyncline shuffled up in the mountains before you. Or you started with the mountains, disassembled them in your mind, and made palinspastic reconstructions, backward in time, as far as the geosyncline. The entire procedure—from the making of rock to the making of mountains to the destruction of mountains to the making of fresh formations of rock—was the geosynclinal cycle.

  Inevitably, the concept was improved, refined, unsimplified. The archetypical geosyncline was deep in the middle and shallow at the sides, and grew different kinds of rocks in various places. The German tectonicist Hans Stille proposed the names miogeosyncline and eugeosyncline for the shallows and the deeps. The vocabulary was universally accepted. Miogeosynclines were the source of shallow-water sediments (limestone, for example) and no volcanics. In the eugeosynclines, volcanism occurred, and deepwater sediments, like chert, collected. In the twentieth century, as the science matured and thickened, mio- and eu- became inadequate to prefix all the differing syn
clinal scenes that new generations of geonovelists were describing. The germinant term was soon popping like corn. The professional conversation came to include parageosynclines, orthogeosynclines, taphrogeosynclines, leptogeosynclines, zeugogeosynclines, paraliageosynclines, and epieugeosynclines.

  Moores had entered Caltech in 1955. “In the Old Geology, one learned of the eugeosyncline and miogeosyncline of western North America, which started in the late Precambrian and went through the Cretaceous,” he said. “Rock deformed by orogeny—folding and thrusting—from the center of the eugeosyncline out toward the continental shelf. The mechanism was ‘orogenic forces.’ Here in the Sierra, for example, you had a eugeosyncline and a miogeosyncline, and the eugeosyncline was thrust on the miogeosyncline. And that was the Golconda Thrust. No one knew how this ‘orogeny’ happened.”

  If California rock was disassembled on paper and palinspastically reassembled as the original geosyncline, there were shallow-water sediments followed by deepwater material, but there was no other side. “That was never explained,” Moores went on. “Also, the geosynclinal cycle was said to be about two hundred million years. In the Overthrust Belt in Montana, forty thousand feet of Precambrian sediment had been thrust over Cretaceous sediment. As students, we wondered why all that Precambrian was still there. What had the source geosyncline been doing sitting there for a billion years when the cycle was two hundred million? There was no answer.”

  Hall’s idea was not preposterous. It was incomplete. There was, after all, marine rock in mountains. Between the geosyncline and the mountains, though, something was missing, and what was missing was plate tectonics.

  We continued west from Emigrant Gap through cuts in unsorted glacial till, buff and bouldery, and past the many blue doors of the pink garage of the Transportation Department’s mountain center for snowplows and road maintenance, situated, with its cavalry of trucks, within a slowly moving earthflow, a creeping descent of unstable moraine, a sedate landslide. “The engineers strike again,” Moores said, but in scarcely three miles his contempt went into a subduction zone, melted, and came back up as appreciation for a long high competent roadcut that exposed bright beds of rhyolite tuff. Twenty-nine million years ago, this air-fall ash came out of a volcano in what is now Nevada, he said, as he pulled over to the side of the road, got out, and put his nose on the engineered outcrop. While he examined the tuff through his hand lens, an eighteen-wheeler that had also come from Nevada was smoking down the mountain grade. Its brakes were furiously burning, and emitted a dark cloud. Long after the truck had gone, the cloud hung stinking in the air. The ash had been launched in several eruptive episodes. Blown west, it had landed hot, and had welded solid in successive bands. Here, more than sixty miles from the source volcano, a single ash fall was more than a metre thick. The ash had settled, of course, horizontally. Having risen with the Sierra, it was now tilting west. We descended past the four-thousand-foot contour, moving on among volcanic rocks five times older than the tuff and of more proximate origin: rock of the Sonomia Terrane altered in the heat and pressure of the assembly of California and weathered along the interstate into an abstract medley of red and orange and buff and white.

  Now thirty miles west of Donner Summit, we were well into the country rock of California gold—the rock that was there when, in various ways, the gold itself arrived. The most obvious place to look for it was in fluviatile placers—the rubble of running streams. In such a setting it had been discovered. Placer, which is pronounced like Nasser and Vassar, was a Spanish nautical term meaning “sandbank.” More commonly, it meant “pleasure.” Both meanings seem relevant in the term “placer mining,” for to separate free gold from loose sand is a good deal easier than to crack it out of hard rock. Some of the gold in the running streams of the western Sierra was traceable to the host formations from which it had eroded—traceable, for example, to nearby quartz veins that had grouted ancient fissures. Within two years of the discovery of gold in river gravel, gunpowder was blasting the hard-rock fissures. Into the quiet country of the low Sierra—between the elevations of one thousand and four thousand feet—gold seekers spread more rapidly than an explosion of moles. Their technology was as rampant as they were, and in its swift development anticipated the century to come. In 1848, the primary instrument for mining gold was a sheath knife. You pried yellow metal out of crevices. Within a year or two, successively, came the pan, the rocker, the long tom, and the sluice—variously invented, reinvented, and introduced.

  There was also a third source of gold. It was found in dry gravel far above existing streams—on high slopes, sometimes even on ridges. The gravel lay in discontinuous pods. Geologists, with their dotted lines, would eventually connect them. In cross section, they were hull-shaped or V-shaped, and in some places the deposits were more than a mile wide. They had the colors of American bunting: they were red to the point of rutilance, and white as well, and, in their lowest places, navy blue. They were the beds of fossil rivers, and the rivers were very much larger than the largest of the living streams of the Sierra. They were Yukons, Eocene in age. Fifty million years before the present, they had come down from the east off a very high plateau to cross low country that is now California and leave their sorted bedloads on a tropical coastal plain. Forty million years later, when the Sierra Nevada rose as a block tilting westward, it lifted what was left of that coastal plain. It included the beds of the Eocene rivers, which were fated to become so celebrated that they would be known in world geology less often as “the Eocene riverbeds of California” than as, simply, “the auriferous gravels.” Fore-set, bottom-set, point bar to cutbank, under the suction eddies—gold in varying assay was everywhere you looked within the auriferous gravels: ten cents a ton in the high stuff, dollars a ton somewhat lower, concentrated riches in the deep “blue lead.”

  To separate gold from gravel, you wash it. But you don’t wash a bone-dry enlofted Yukon with the flow of little streams bearing names like Shirt Tail Creek. Mining the auriferous gravels was the technological challenge of the eighteen-fifties. The miners impounded water in the high country, then brought it to the gravels in ditches and flumes. In five years, they built five thousand miles of ditches and flumes. From a ditch about four hundred feet above the bed of a fossil river, water would come down through a hose to a nozzle, from which it emerged as a jet at a hundred and twenty miles an hour. The jet had the diameter of a dinner plate and felt as hard. If you touched the water near the nozzle, your fingers were burned. This was hydraulic artillery. Turned against gravel slopes, it brought them down. In a contemporary account, it was described as “washing down the auriferous hills of the gravel range” and mining “the dead rivers of the Sierra Nevada.” A hundred and six million ounces of gold—a third of all the gold that has ever been mined in the United States—came from the Sierra Nevada. A quarter of that was flushed out by hydraulic mining.

  The dry bed of an Eocene river carries Interstate 80 past Gold Run. The roadside records the abrupt change. As if you were swinging off a riverbank and dropping into the water, you go out of the metavolcanic rock and into the auriferous gravels. We stopped, stood on the shoulder, and looked about a hundred feet up an escarpment that resembled an excavated roadcut but had not been excavated by highway engineers. It was capped by a mat of forest floor, raggedly overhanging. The forest, if you could call it that, was a narrow stand of ponderosas, above an understory of manzanita with round fleshy leaves and dark-red bark. The auriferous gravels were russet, and were full of cobbles the size of tomatoes—large stones of long transport by a most impressive river.

  To the south, across the highway, the scene dropped off into a deep mountain valley. The near end of the valley was three hundred feet below the trees above us. The far end of the valley was nearly twice as deep. A mile wide, this was a valley that had not been a valley when wagons first crossed the Sierra. All of it had been water-dug by high-pressure hoses. It was man-made landscape on a Biblical scale. The stand of ponderosas at the no
rthern rim was on the level of original ground.

  The interstate was on a bench more than halfway up the gravel. Above us, behind the trees, were the tracks of the Southern Pacific. In the eighteen-sixties, when the railroad (then known as the Central Pacific) was about to work its way eastward across the mountains, it secured the rights to this ground before the nozzles reached it. Moores and I made our way up to the tracks, where the view to the north was over a hosed-out valley nearly as large as the one to the south, and bordered by white hydraulic cliffs. The railroad, with the interstate clinging to its hip, ran across a septum of the old terrain, an isthmus in the excavation, an unmined causeway hundreds of feet high made of gravel and gold.