by John McPhee
Anita turned on the windshield wipers and wiped an April shower. Beside the interstate, the Pocono Devonian roadcuts were of much the same age and character as ones we had seen before. We passed them by. “Better not to do geology in the rain,” Anita said. “It’s unfair to the rocks.” With regard to the possibility of exotic terranes having added themselves to eastern North America, she said, “If you stretch out the overthrusts in the Appalachians, they show that—before the mountain building began—the continent was much larger than it is now, not smaller.”
I remembered Leonard Harris—one day at their home in Laurel, Maryland—saying, “The Brevard Zone is the sort of fault you would see in any thrust belt. With the plate-tectonic model, anybody can write a history of an area without having been there. These people have no way to evaluate what they’re doing. They just make up stories.”
“Plate-tectonic interpretations often start where data stop,” Anita had said. “These people will just float microplates around. If the West is made of microplates, where the hell was the landmass that produced the pieces?”
“They want to be science-fiction writers,” Leonard said. “That’s what they want to do. They really look at it in a science-fiction mode. I have never been able to do that. If you don’t know what caused something, you don’t know; and that’s the way it is.”
“Yeah, but it’s a much more romantic way to look at things,” she said. “And it certainly does turn students on.”
“People love it.”
“It allows them to play all kinds of games without the necessity and painstaking dogwork of gathering facts. It allows them to write papers without killing themselves getting data.”
“People want the science-fiction story. It’s easier to believe that pieces of the world move than it is to see a sand grain move. The principal problem about interpreting the Appalachians is that there have been no available subsurface data in the Blue Ridge and Piedmont. All interpretations, up until 1979, were based on what people thought was a rooted system. Their ideas were based on offshore data, where they had 3-D—you know, seismic data, magnetic data —and these data were more or less applied onshore. The concepts were developed from the ocean to the land. Now that we are beginning to get subsurface data on land, we are testing their concepts. A lot of what people have been saying is not hanging together. Some of what they have said has hung together.”
I said to them, “One would gather from the seismic lines that for a continent-to-continent collision you’d have to go pretty far east to find the suture.”
“I don’t think you can go far east enough,” Anita said. “The oceanic basin is out there.”
“When you start working on the shore and you look offshore, you’ve got an immediate problem,” Leonard said. “They tell us that the oldest ocean crust that has ever been found is Jurassic. Onshore, we have everything that’s ever been built—from the Precambrian on up. We have a continuum. We have something that has been preserved much longer. We have rock that is nearly four billion years old. So we have a problem relating. If all the ocean crust is Jurassic, or younger, there’s a lot happening here onshore that is never preserved out there. It’s difficult to compare the two.”
Anita said, “I believe in plate tectonics—just not in the way they’re perpetrating it for places like the East Coast. It shouldn’t be used as the immediate answer to every problem. That’s what I object to. Now that their suture zones have disappeared, people are going to microplates.”
“They seem to be saying that you don’t have to see any order,” Leonard said. “Because it’s all chaos, and if it’s chaos why worry about it?”
“What we try to do is pull the thrust plates apart and make them into some sort of recognizable geologic model,” Anita said.
Leonard said, “You pull something apart to see what it might have been, not what you think it was in advance. It might have been a shelf, a basin. You work at it, and see what it was.”
“The plate-tectonics boys make no attempt to do this, because they see no reason to,” Anita said. “There are too many pieces missing. Each existing piece is an entity unto itself. Everything is random pieces.”
“Most people have never had an opportunity to work with thrust-faulted areas. We’ve lived with them all our lives. If we go along a fault system far enough, we can actually see the next thrust plate. Maybe I’ll have to go a hundred miles until I find out what it really looked like. You do that by making a model. You pull the thrusts apart and see what the country originally looked like. But until you’ve done that, and been faced with that problem, it’s natural to say, ‘God, these are so different. They could be microcontinents.’ You can reconstruct a large flat piece out to the east as an original depositional basin. You can see volcanic terrane that was partly onshore, partly offshore. You can look at that as a basin, too, just sitting there, a continuous thing. You see the same thing from Georgia north. The Appalachian belts are almost continuous basins, showing different kinds of depositional patterns. They’re not exotic pieces.”
“Not at all.”
“Science is not a detached, impersonal thing. People will be influenced as much by someone who is a spellbinder as by someone with a good, logical story. It is spellbinding to say that these belts are exotic and were built through time by micro or macro pieces aggregated to the continent. But the fact that you’ve got seismic lines without any apparent suture lines makes you wonder what really happened. Where are those Devonian and Taconic sutures? Are they just not being recognized? Or are they in fact thrust plates?”
I thought also of field trips in the company of geologists trying to puzzle out the details of plate-tectonic theory. Metamorphic details. Geophysical details. The dialogue is not without crescendos. They debate in a language exotic in itself, and shuffle like a blackjack deck the stratigraphic units of the world. In Vermont, say, walking the hard-packed dirt roads among Black Angus meadows and roll-mop hay, over plank bridges—“LEGAL LOAD LIMIT 24,000 POUNDS” —and down through the black spruce to Cambrian outcrops jutting up as ledges in fast, clear streams, they argue.
“You’ve got the right first approximation, but you’ve got to go ahead and prove that it’s the correct approximation.”
“We’re talking about developing fabrics.”
“It’s pretty clear now that fabrics don’t develop that way.”
“For an anisotropic crystal, I don’t think you can say what you just said. You’ve got to put in another sentence there to justify using that approximation.”
“I don’t see what you’re saying. Anisotropic or not, that’s the definition of being stressed.”
“When you say the thermodynamic stability of the phase that’s growing is sigma 1, sigma 2, sigma 3, divided by 3, it’s proof for an isotropic crystal. For garnets growing, that’s fine. For mica, that’s not fine.”
The hills roundabout are decidedly footloose and no one knows how far they have moved. The rock they are made of has flopped over in recumbent folds and is older than the rock it rests on. In the Old Geology, these hills were described as large pieces of the high Taconic mountains, which had slid downhill by gravity and come to rest in the westward seas. Now they are seen variously as remnants of thrust sheets or as a possible exotic terrane.
“You can perhaps picture for yourself that the allochthon was at one time more extensive. It was coming this way through a sea of black mud, and here is the record of it. This is where it touches North America—at least that’s a possibility.”
“But there are no remnants of the western side of the ocean.”
“Evidence would be in the rest of the conglomerate, little bits of limestone debris. Evidence would be in the seismic line.”
“But that evidence could be … You could imbricate the stuff that’s coming off North America.”
“Yes. Yes, you can.”
“So I don’t think that’s definitive.”
“I’m not saying it is. I’m just saying here’s another pos
sibility. And I’m going to stick to that for the time being, as well as the Chain Lakes ophiolite.”
It doesn’t matter that you don’t understand them. Even they are not sure if they are making sense. Their purpose is trying to. Everyone has crowded in. The science selects these people—with their jeans and boots and scuffed leather field cases and hats of railroad engineers. To them, just being out here is in no small measure what it’s all about. “The three key things in this science are travel, travel, and travel,” one says. “Geology is legitimized tourism.” When geologists convene at an outcrop, they see their own specialties first, and sometimes last, in the rock. People listen closely for techniques applicable to areas they work in elsewhere. If someone is a specialist in little bubbles that affect cleavage planes, others will turn to the specialist for comment when cleavage is of interest in the rock. The conversation runs in links from specialty to specialty, from minutia to minutia—attempting to establish new agreement, to identify problems not under current research. From time to time, details compose. The picture vastly widens.
“Aren’t we in North America?”
“You are in North America. Yes.”
“And you are in Europe.”
“That’s one possibility. Yes.”
“You are standing across the ocean.”
“No. I’m not standing across the ocean. I’m transplanted here. Is the Atlantic between me and you? No.”
“You are allochthonous.”
“You’re damned right I am.”
“You are rootless.”
“Not to mention recumbent.”
“Only after hours.”
“There may be another suture.”
“There may be another suture, but this is the only one we’ve got.”
“No, no, you’ve got another one, which goes up through Quebec.”
“No, that’s not in place. The Canadian seismic line proves it. You will remember also that Laval—way back, 1965—came out with late Ordovician fossils in the Sherbrooke anomalies. Where else do you find a continuous sedimentation from the middle Ordovician up to the Silurian—from Rangeley Lake up to New Brunswick in one belt? The Sherbrooke thing, restored, would come from where it ought to come from. So I’m suggesting either that two continents collided, and you have one basin there, receiving continuous sedimentation, or …”
“You may have a double arc.”
“You might have a double arc.”
“There’s another solution.”
“Sure, but I’m saying let’s take the simplest configuration.”
“Why not just have one arc with basins on both sides of it?”
“No. No. You have the Bronson Hill anticlinorium, and then you have the Ascot-Weedon.”
“You have a volcanic arc on the stable side of the subduction zone, an expected arc above the downgoing slab.”
“You have a short-lived slab going down below the Ascot-Weedon and the one of longer duration that’s on the other side. I would somehow think that there has to be something in these rocks, in the limestones, that you’d be able to hopefully connect to that platform.”
“The only thing I can say is …”
“What about the blue quartz?”
“What about the blue quartz? The stratigraphy of the Taconic rock matches unit for unit with Cambrian rocks of Avalon, and the fossils look alike. That’s all I can tell you. Nowhere else do we find this sort of thing except Wales.”
A structural geologist with a foot on each continent looks up and aside from this contentious scene. “While geologists argue, the rocks just sit there,” he remarks. “And sometimes they seem to smile.”
The car hit an erosional vacuity that almost threw it off the road. Geology versus the State of Pennsylvania. Geology wins. In eastern weather, the life expectancy of an interstate is twenty years. Mile after mile, I-80 had been heaved, split, dissolved, and cratered. A fair amount of limestone is incorporated in the road surface in Pennsylvania. Limestone is soluble in distilled water, let alone in acid rain. “Acid rains eat the surface, then water goes in and freezes, thaws, freezes again, and fractures the hell out of the road,” Anita said, easing down toward minimum speed. “That, of course, is exactly how water works on bedrock. But an interstate can’t be compared to bedrock. An interstate has no soil protecting it. And it’s mostly carbonate. It’s not very resistant stuff.”
We were sixty miles into Pennsylvania and had descended from the Pocono Plateau, generally running backward through time and down through the detritus of two great ranges of mountains. Now the country was familiar—valley, ridge, valley, ridge. We were again in the deformed Appalachians. While the Delaware Water Gap had been a part of the main trunk of the foldbelt, this was an offshoot that curled around the western Poconos—a broad cul-de-sac whose long ropy ridges ended like fingers, gesturing in the direction of New York State. It was rhythmic terrain, predictable and beautiful, the quartzite ridges and carbonate valleys of the folded-and-faulted mountains, trending southwest, while the interstate negotiated with them for its passage toward Chicago. Looked at in continental scale on a physiographic map or a geologic map—on almost any map that doesn’t obscure the country with exaggerated human improvements—the sinuosity of the deformed Appalachians is as consistent as the bendings of a moving serpent. In Alabama, the mountains come up from under the Gulf Coastal Plain and bend right into Georgia and then left into Tennessee and right into North Carolina and left in the Virginias and right in Pennsylvania and left in New Jersey and New York and right in Quebec and New Brunswick and left in Newfoundland. Some people believe that in this Appalachian sinuosity we are seeing the coastline of the Precambrian continent —North America in the good old days, when the Taconic Orogeny was off in the future and these big, scalloped bends were the bays of Iapetan seas. Signify what they may, their repetitive formality through two thousand miles does not suggest the random impacts of exotic terranes, nor, for that matter, does it suggest the ragged margins of crashing continents. The Great Valley, as the most prominent axial feature of the Appalachians, also seems inconvenient to the narrative of colliding lands, because the soft black slates and shales and the dissolving carbonates that make up the valley from end to end were all moved a great many miles northwest, and if random New Zealands and the odd Madagascar were shoving at different times in different places, one would expect the formations to be considerably offset. One would imagine that miscellaneously disturbed rock—folded, faulted, shuffled, thrusted, disarranged—would be much too chaotically bulldozed to emerge through erosion as an integral, elongate, and geometrically formal valley. Similar thoughts come to mind with regard to the Precambrian highlands in their sinuous journey from the Great Smokies to the Green Mountains, and the Piedmont as well: the consistent, curving, parallel stripes of the Appalachian ensemble.
Anita, for the moment, was more interested in tangible limestone than in how it had been shoved and deformed. Eight miles west of Bloomsburg, she saw a limestone outcrop that looked good enough to sample. It was a quarter of a mile off the interstate, and we walked to it. She dropped some acid on it. Vigorously, the outcrop foamed. “It’s upper Silurian limestone,” she said. “I shouldn’t be able to tell you that without running the conodonts, but I know.”
“What if you’re wrong?”
“Then I’m wrong, aren’t I? They pay me to do the best I can. Geologists are detectives. You work with what you have.”
She swung full force with her sledgehammer. The stone did not crack. “This profession is very physical,” she complained, and belted the outcrop again.
Her knees sometimes turn black-and-blue when she carries samples down from mountains. She once handed a suitcase to a Greyhound bus driver who said, “What have you got in here, baby —rocks?” She was content to have them ride in the baggage compartment. A geologist I know in California would be unnerved by that. When he travels home from far parts of the world, he buys two airline seats—one for himself and one for his rocks.
