Annals of the Former World

by John McPhee

  The Martinsburg seafloor and the underlying carbonate rocks had unquestionably been broken into thrust sheets and shuffled like cards. Uplifted with their Precambrian basement, they had, in perfect harmony with the Old Geology, become mountains that shed their sediments—shed their clastic wedges—and buried the Martinsburg deep enough to turn it into slate, buried the carbonates deep enough to turn them into marble. Thus, plate tectonics fit. Plate tectonics may have restyled the orogeny and dilapidated the geosyncline, but it fit the classical evidence.

  There were, to be sure, certain anomalies, which suggested further study. If the Brevard Zone was the suture, how come it was so short? It was evident for a hundred miles, dubious for a few hundred more, and nonexistent after that. If the Taconic, Acadian, and Alleghenian orogenies were subdivisional impacts of a single intercontinental collision, how come they took so long? In plate-tectonic theory, plates move at differing speeds, the average being two inches a year. The successive orogenic pulses that resulted in what we know as the Appalachian Mountains occurred across a period of about two hundred and fifty million years. In two hundred and fifty million years, at two inches a year, you can move landmasses a third of the way around the world. Geologists ordinarily require vast stretches of time to account for their theories. In this case, they have too much time. They have two continents in the act of collision for two hundred and fifty million years.

  In 1972, scarcely four years after the lithospheric plates had first been identified and the theory that described them had become news in the world, Anita and two co-authors published a set of papers offering strong evidence of plate tectonics in action, apparent proof that Sweden, or something like it, had once been in Pennsylvania. It was an inference drawn from conodont paleontology. The papers were widely cited for their support of plate-tectonic theory, and are cited still. North of Reading, in the Great Valley of the Appalachians, Anita had found early Ordovician conodonts of a type previously unknown in North America but virtually identical with early Ordovician conodonts from the rock of Scandinavia. All over North America were early Ordovician conodonts from tropical and subtropical seas. Their counterparts in Scandinavia were from cooler water, and so were these strangers Anita found in Pennsylvania. She found them in what is known as an exotic block, embedded in a fartravelled thrust sheet. They happened to be within a third of a mile of warm-water American conodonts in rock of about the same age which had moved hardly at all. The Scandinavian conodonts had apparently come to Pennsylvania with the closing of the proto-Atlantic ocean and been dropped ashore off the leading edge of the arriving plate. “Even I said, ‘Oh, this piece peeled off the oncoming European-African plate and got dumped in here along with the clastics,’” Anita said, telling the story. “Everybody cited those papers. To this day they are called ‘marvellous, landmark papers.’ I could eat my heart out. The papers have been used as prime backup proofs of plate-tectonic applications in the northern Appalachians. Even now, a lot of the people who use the plate-tectonic model for interpreting the Appalachians are completely unaware that those papers were based on a paleontological misinterpretation.”

  Working in Nevada three years later, Anita had found Scandinavian-style conodonts of middle Ordovician age. Her husband, Leonard Harris, savored the discovery not for its embarrassment to his wife, needless to say, but for its air-brake effect on the theory of plate tectonics. “Now, how could that be?” he would ask. “How did this happen? Europe can’t hit you in Nevada.” From the Toiyabe Range she had taken the cool-water fossils, and moving east to other mountains—from basin to range—she had come to middle Ordovician carbonates that contained a mixture of conodonts of both the cool-water and the warm-water varieties, the American and the Scandinavian styles. Farther east, in limestone of the same age in Utah, she had found only warm-water conodonts. She realized now the absoluteness of her error. Utah had been pretty much the western extremity of the vast Bahama-like carbonate platform that covered North America under shallow Ordovician seas. In western Utah, the continental shelf had begun to angle down toward the floor of the Pacific, and in central Nevada the continent had ended in deep cool water. The conodont types differed as a result of water temperatures, not as a result of their geographic origins. Shallow or deep, conodonts of northern Europe were the same, because the water was cool at all depths. But here in America, with the equator running through the ocean where San Francisco would someday be, Ordovician water temperatures varied according to depth. Those apparently Scandinavian fossils were forming in deep cool water, the American ones in warm shallows.

  Moving east from the Toiyabe Range and into Utah, Anita had gone from outcrop to outcrop through the Ordovician world, from ocean deeps to the rising shelf into waist-deep limestone seas. She could see now that the thrusting involved in the eastern orogenies had shoved the cool-water conodonts and their matrix rock from the deep edge of the continental rise into what would be Pennsylvania. They had travelled, to be sure—but they had more likely come from Asbury Park than from Stockholm. In the thrusting and telescoping of the strata, the transition rocks of the American continent’s eastern slope had been deeply buried. In them, almost surely, would be a mixture of cool-water and warm-water conodont types. To the east of the Toiyabe Range, there had been less telescoping, and the full sequence was traceable—from the cool deep continental edge up the slope to the warm far-reaching platform. “The change had nothing to do with moving plates,” Anita concluded. “Nothing to do with plate tectonics. I blew it. It was an environmental change, an environmental sequence.”

  More recently, working in Alaska, she had seen the sequence again, this time in tightly banded concentration, for the “American” conodonts were from reefs around Ordovician volcanic islands with steeply plunging sides and the “Scandinavian” conodonts were from cold deeps nearby.

  Swerving to avoid a pothole, Anita said, “The plate-tectonics boys look at faunal lists and they go hysterical moving continents around. It’s not the paleontologists doing it. It’s mostly the geologists, misusing the paleontology. Think what geologists would make of the present east coast of the United States if they did not understand oceanography and the resulting distribution of modern biota. Put yourself forty or fifty million years from now trying to reconstruct the east coast of the United States by looking at the remains in the rock. God help you, you would probably have Maine connected to Labrador, and Cape Hatteras to southern Florida. You’d have a piece of Great Britain there, too, because you see the same fauna. Well, did you ever hear of ocean currents? Did you ever hear of the Gulf Stream? The Labrador Current? The Gulf Stream brings fauna north. The Labrador Current brings fauna south. I think that a lot of the faunal anomalies you see in the ancient record, and which are explained by invoking plate tectonics, can be explained by ocean currents bringing fauna into places they shouldn’t be. In the early days of plate tectonics, a lot of us, including me, jumped on the bandwagon in order to explain the distribution anomalies we were seeing not only in the eastern Appalachians but in North America as a whole. When we better understood the paleoecologic controls on the animals some of us were working on, there was no reason to invoke plate tectonics.”

  The experience was cautionary, to say the least. It did not close her mind to plate tectonics, but it opened a line of suspicion and made her skeptical of the theory’s insistent universality. Her discomfort varies with distance from the mobile ocean floors. She likes to describe herself as a “protester.” The protest is not so much against the theory itself as against excesses of its application—up on the dry land. “A number of these people took very interesting ideas that apply to ocean floors and tried to apply them to everything,” she remarked. “They tried to extrapolate plate tectonics through all geologic time. I don’t know that that holds. My husband has blown some of their ideas apart.”

  Leonard Harris, sometimes known as Appalachian Harris, was very much a protester, too. Tragically, he died in 1982, a relatively early victim of cancer and relate
d trouble. He was a genial and soft-spoken, almost laconic man with a lean figure that had walked long distances without the help of trails. He liked to build ideas on studied rock, and was not easily charmed by megapictures global in their sweep. He referred to the long deep time before the Appalachian orogenies as “the good old days.” With regard to plate tectonics, he looked upon himself as a missionary of contrary opinion—not flat and rigid but selective, where he had knowledge to contribute. His wife has compared him to Martin Luther, nailing theses to the door of the castle church.

  For some years he assisted oil companies in the training of geologists and geophysicists in southern-Appalachian geology, and in return the companies made available to him their proprietary data from seismic investigations of the Appalachian crust. Later on, these data were supplemented by the seismic thumpings of the U.S.G.S. and several university consortiums, whose big trucks go out with devices that literally shake the earth while vibration sensors record wave patterns reflected off the rock deep below. The technique is like computed axial tomography—the medical CAT scan. The patterns reveal structure. They reveal folds, faults, laminations, magmatic bodies both active and cooled. They report the top of the mantle. They also reveal density, and hence the types of rock. Moving cross-country, the machines make subterranean profiles known as seismic lines. Seismic shots with explosives have been used for years in the search for oil. Alaska is crisscrossed with all but indelible “seismic” disruptions of tundra. The reserves of Prudhoe Bay were discovered in this manner. Dynamite in the populous East could irritate the public, so the universities and the U.S.G.S. use a behemoth called Vibroseis to shiver the timbers of the earth. One of the first discoveries the vibrations reported was that the Brevard Zone is relatively shallow and the crust below it is American rock that does not in any vague way reflect a continent-to-continent suture. Africa was nowhere in the picture. The Brevard Zone proved to be the toboggan-like front end of a large and essentially horizontal thrust sheet.

  Plate-tectonic theorists accommodated this news by moving the suture fifty miles east. The new edge of Africa was under Kings Mountain. Seismic shots took the stitches out of Kings Mountain. “When we got the data for the Brevard”—as Leonard Harris liked to tell the story—“they pushed the suture to Kings Mountain, and when we got data for that they said the suture must be under the coastal plain, and now that we are getting data for the coastal plain they say it must be in the continental shelf. Well, we’ve got data out there, too.” Up and down the Appalachians, wherever such data were collected, thrust sheets were seen to have moved in a northwesterly direction, and much of the thrusting had never been suspected before. Conventional thought had been that the old rock of the Green Mountains, the Berkshires, the New Jersey Highlands, the Catoctin Mountains, and the northern Blue Ridge was in place, firmly rooted—autochthonous, as geologists are wont to say. It may have been crushed and pounded in the various orogenies, and metamorphosed, too, but it was nonetheless thought to be securely glued where it first had formed as rock. The belt was supposed to have been the fixed starting block from which, somehow, thrusting had proceeded northwest. The idea had come up through the Old Geology and been incorporated into the substance of plate tectonics. Then, in 1979, Vibroseis rumbled into the country and showed that from Quebec to the Blue Ridge the entire belt was deracinated. The Great Smokies and the Skyline Drive, Camp David and the Reading Prong, the Berkshires and the Green Mountains—all of it had moved, at least a few tens of miles and as much as a hundred and seventy-five miles, northwest.

  Using the new data, Leonard meticulously drew a palinspastic reconstruction of North American rock, showing it as it had appeared before it was shoved and deformed. He chose a cross section that had been shot more or less from Knoxville to Charleston and out to sea. The reconstruction showed that the rock of the Ridge and Valley—the folded-and-faulted, deformed Appalachians—had been squeezed so much that its breadth had been reduced about sixty miles. The supposedly rooted Blue Ridge had been moved inland from what is now the coast. Rock of the present Piedmont had come from three hundred miles out in the present sea. This left Africa out in the cold and plate-tectonic theory in no small need of a substitute for what had been—and in many classrooms would continue to be—the world’s most “classical” example of a continent-to-continent suture.

  With patience geological, the believers restyled their belief, apparently according to the criterion “If at first it doesn’t fit, fit, fit again.” There was suggestive help from the West. A great deal of land out there had not been there when the carbonate rocks sloped away to ocean-crustal deeps in Ordovician time. California, Oregon, Washington, British Columbia had appeared where there was no continental structure of any kind. Up and down the western margin, in fact, there was an unaccounted-for swath of land averaging four hundred miles wide. There was also the whole of Alaska. How did all that country come to be where it is? What compressed the western mountains? If Europe were on the international date line, these questions would have a ready answer, but inconveniently it was not. No one was enthusiastic enough to suggest a hit-and-run visit from China. Where, then, since Ordovician time, had the North American continent acquired nine hundred million acres of land?

  There was an answer in the concept of microplates, also known as exotic terranes. New Guineas, New Zealands, New Caledonias, Madagascars, Kodiaks, Mindanaos, Fijis, Solomons, and Taiwans had come over the sea to collect like driftwood against the North American craton. The first such terrane identified was Wrangellia, named for the stratovolcanic Wrangells, some of the Fujis of Alaska. Dozens of other exotic terranes have since been named—Sonomia, Stikinia, the Smartville Block. If a piece of country is possibly exotic and possibly not—if it is so enigmatic that no one can say whether it has come from near or far—it is known as suspect terrane. I returned one time from a visit to the country north of the Tanana River, in eastern interior Alaska, where streams that resemble gin come down from mountains and into the glacial Yukon. A geologist in New Jersey welcomed me home with an article from Nature which described the Alaskan region of the upper Yukon. “The terrane is probably composite,” said Nature, “with nappes of upper Palaeozoic oceanic assemblages thrust across a quartzo-feldspathic and silicic volcanicrich protolith of probable Precambrian to known Palaeozoic age and of unknown continental affinity.” I was appalled to discover that that was where I had been, and mildly disturbed to learn that terrain long familiar to me had now become suspect.

  Taiwan, at this writing, is evidently on its way to the Chinese mainland. Taiwan is the vanguard of a lithospheric microplate and consists of pieces of island arc preceded by an accretionary wedge of materials coming off the Eurasian Plate and materials shedding forward from the island’s rising mountains. As the plate edges buckle before it, the island has plowed up so much stuff that it has filled in all the space between the accretionary wedge and the volcanic arc, and thus its components make an integral island. It is in motion northwest. For the mainland government in Beijing to be wooing the Taiwanese to join the People’s Republic of China is the ultimate inscrutable irony. Not only will Taiwan inexorably become one with Red China. It will hit into China like a fist in a belly. It will knock up big mountains from Hong Kong to Shanghai. It is only a question of time.

  As an exotic terrane on the verge of collision with a continent, Taiwan is a model not only for the building of the American West but for the application of microplate-tectonic theory to the eastern orogenies and the closing of the proto-Atlantic. In this respect, a plane fare to Taiwan has been described as “a ticket to the Ordovician,” a time when something or other, beyond question, produced the Taconic Orogeny, and if it was not the slamming-in of a continent against North America, then possibly it was the arrival of an exotic island like Taiwan. The analogy becomes wider. South of Taiwan are Luzon, Mindanao, Borneo, Celebes, New Guinea, Java, and hundreds of dozens of smaller islands from the Malay Peninsula to the Bismarck Archipelago. Coming up below them is Au
stralia, palpably moving north, headed for collision with China, with a confusion of microplates lying between. According to microplate theory, as Europe, Africa, and South America closed in upon North America through Paleozoic time, there rode before them an ocean full of Javas, New Guineas, Borneos, Luzons, Taiwans, and maybe hundreds of dozens of smaller islands. The Avalon Peninsula of Newfoundland appears to have been a part of such an island, and the Carolina Slate Belt, and a piece of Rhode Island east of Providence, and Greater Boston. A schedule of arrivals of incoming exotic terranes will account—as a simple continent-to-continent collision cannot do—for the long spreads of time between one and another of the Appalachian mountain-building pulses. As someone once compacted it for me, “you sweep the New Zealands and Madagascars out of the ocean and then you close it with the Alleghenian Orogeny.” Disagreeing interpreters see terranes of highly varied dimension. Nominated as the terrestrial remains of one exotic block is the whole of New England from Williamstown eastward, arriving in the Ordovician to lift the Taconic mountains.

  Exotic terranes and their effects represent only one of the responses of plate-tectonic theorists to the embarrassment caused by the failure of Exhibit A among intercontinental collisions to exhibit a finite suture. Another response has been the notion that when two continents collide there is every possibility that one will split the other, like an axe blade entering cedar; if so, you would find the invaded country rock both above and below the invader. The concept is known as flake tectonics. Its message to Vibroseis is to stop shaking and go home. With a little erosion and flake tectonics, you can have the native rock reaching far under the rock from across the sea. Even so, the bunching of exotic terranes seems to solve more problems than flake tectonics does. Exotic terranes not only explain the intervals of time involved in the Taconic, Acadian, Alleghenian orogenies, they suggest as well why Taconic deformation occurred in the northern but not the southern Appalachians. Shortening collisional boundaries, they restore some dignity to the Brevard suture.