by John McPhee
In Precambrian, Cambrian, and much of Ordovician time, rivers ran southeastward off the American continent into the Iapetan ocean. Then the continental shelf bent low, and the Martinsburg muds poured into the depression from the east. Whether they were coming from Africa, Europe, or some accretionary, displaced, hapless Taiwan is completely unestablished, but what is not unestablished is the evidence preserved in the sediment—sand waves, ripple marks, crossbedded point bars—showing currents that flowed west and northwest. In later rock, such evidence is everywhere, showing eastern American rivers flowing toward what is now the middle of the continent all through the rest of Paleozoic time. As each successive orogeny produced another uplift in the east, fresh rivers would pour from it, building their depositional wedges, their minor and major deltas, but running always in a westerly direction. The last orogeny was pretty much spent about two hundred and fifty million years ago, in the Permian. For some tens of millions of years after that, the mountains were reduced by weather in a tectonically quiet world. Then, in early Mesozoic time, “earth forces” began to pull the terrain apart. According to present theory, the actual split, deep enough to admit seawater, came at some point in the Jurassic. The Atlantic opened. On the American side of the break, extremely short steep rivers flowed into the new sea, but for the most part the drainages of what is now the eastern seaboard continued to flow west. By Cretaceous time, the currents had reversed, assuming the present direction of the Penobscot, the Connecticut, the Hudson, the Delaware, the Susquehanna, the Potomac, the James.
Rivers come and go. They are younger by far than the rock on which they run. They wander all over their valleys and sometimes jump out. They reverse themselves and occasionally disappear—their behavior differentiated by textures in the solid earth below. The tightly folded Appalachians are something like the ribs of a washboard. The direction of the structure lies across the direction of scrubbing. In the Paleozoic era, when the tectonic washboard was made and repeatedly lifted from the east, falling rainwater, gathering in streams, found its way westward across the ribs.
With the coming of the Atlantic—the Mesozoic split—the principal drainages of the American East at first continued to flow toward the Midwest. A part of the plate-tectonic story is that a great deal of heat accompanies tectonic rifting and the heat lifts the two sides of the rift like trapdoors facing each other. The shores of the Red Sea look like that. On both sides are mountains, nine, ten, twelve thousand feet high. Extremely short steep rivers fall into the Red Sea. Principal drainages—the intermittent rivers of Arabia—run eastward almost from the east shore many hundreds of miles, and from near the west shore Egyptian rivers run west to the Nile. The world’s mid-ocean ridges—the spreading centers of plate tectonics —are configured like the rift of the Red Sea. Typically, the two sides are of gentle pitch, and gradually rise six thousand feet higher than the flanking abyssal plains. Groovelike down the ridgelines run sub-marine rift valleys. Into the rift valleys of eastern Africa pour extremely short steep rivers, while long ones, like the Congo, rise close to the rift but flow away westward a thousand miles to the sea. It was the discovery and confirmation of spreading centers that opened the story of plate tectonics—and this is still the aspect of the theory that provokes the least debate. Eastern America, in Jurassic time, gradually subsided. The present explanation would be that as the ocean grew wider and the heat of the spreading center became more distant, the region cooled like a collapsing soufflé, while the weight of water and accumulating sediments also pressed down on the continental shelf. In any case, the broad package of land that had tilted northwestward for approximately three hundred million years now seesawed and began again to tilt the other way.
Rivers turned around, pooled temporarily against the ribs of the washboard, and ran over them, seeking weaknesses in the rock. Anew, the running water began to etch out the country. It was a process analogous to photoengraving, wherein acid differentially eats pictures into treated sheets of metal. The new and reversed eastern rivers differentially eroded the Appalachian structures. Where they got into the shales and the carbonates, they dug deep and wide. Where they found quartzite and other metamorphic rock, they encountered tough resistance. Sometimes, working down into the country, they came to the arching quonset roofs of anticlines, and slicing their way through quartzite found limestones within. It was like slicing into the foil around a potato and finding the soft interior. The water would remove the top of the arch, dig a valley far down inside, and leave quartzite stubs to either side as ridges flanking the carbonate valley. Streams eroding headward ate up the hillsides back into the mountainsides, digging grooves toward the nearest divide. On the other side was another stream, doing the same. Working into the mountain, the two streams drew closer to each other until the divide between them broke down and they were now confluent, one stream changing direction, captured. In this manner, some thousands of streams—consequent streams, pirate streams, beheaded streams, defeated streams—formed and re-formed, shifting valleys, making hundreds of water gaps with the general and simple objective of finding in the newly tilted landscape the shortest possible journey to the sea. A gap abandoned by its streams is called a wind gap. In the regional context, the water gap of the Delaware River is a little less phenomenal than it once appeared to be.
Until about 1970, the picture in vogue of the early Cenozoic American East was of a vast peneplain, a flat world of scant relief, with oxbowed meandering rivers heading almost nowhere. The assault of water on the ancestral mountains was thought to have worn down the whole topography close to sea level. The peneplain then rose up, according to the hypothesis, and rivers dissected it, flushing out the soft rock and leaving hard rock high, in the form of remarkably level ridges—as flat as the peneplain, of which they were thought to be remnants. Where the rivers of the peneplain had flowed across the tops of buried ridges, they cut down through them as the ridges came up—making gaps. That was the history as it was taught for three-quarters of a century. It was known as the hypothesis of the Schooley Peneplain, after Schooley Mountain, in New Jersey, which looks like an aircraft carrier. The Schooley Peneplain is out of vogue. It is an emeritus idea. It has been replaced by a story out of steady-state physics having to do with the relationship of level ridgelines to certain degrees of slope. A graduate student once remarked to me that old hypotheses never really die. He said they’re like dormant volcanoes.
Under the carbonate valleys and quilted farms, the rock was buried from view. The beauty of the fields against steep-rising forests, the shimmer of April green, was not doing much for Anita. She was in need of a lithic fix. Her fingers tapped the wheel. She reminded me of a white-water fanatic on a meandering stretch of flat river. “No wonder I never did geology in this part of Pennsylvania,” she said again. It had been a long time between rocks. “I’d really like to go to Iran someday,” she went on, desperately. “The Zagros Mountains are another classic fold-and-thrust belt. The thing about the Zagros is that there’s no vegetation. You can see everything. They’re a hundred per cent outcrop.”
She had scarce uttered the words when the road jumped to the right and through a nameless gap and past a roadcut twenty metres high—Bald Eagle quartzite—and then more and higher roadcuts of Juniata sandstone in red laminations dipping steeply to the west. “I take it back. This is one hell of a series, let me tell you,” Anita said. More rock followed, rock in the median, rock right and left, and we ran on to scout it, to take it in whole. The road was descending now through gorges of red rock—the results of precision blasting, of instant geomorphology. Their depth increased. They shadowed the road. And in their final bend was the revealed interior of a mountain, geographically known as Big Mountain. There had been a natural gap, but it had not been large enough, and dynamite had contributed three hundred thousand years of erosion. The entire mountain had been cut through—not just a toe or a spur. “Holy Toledo! Look at that son of a bitch!” Anita cried out. “It’s a hell of an exposure, a hell of
a cut.” More than two hundred and fifty feet high and as red as wine, it proved to be the largest man-made exposure of hard rock on Interstate 80 between New York and San Francisco. It was an accomplishment that might impress the Chinese Geological Survey. “When you’re doing geology, look for the unexpected,” Anita instructed me, forgetting the Zagros Mountains.
We stopped on the shoulder in the shadow of the rock. “Holy Toledo, look at that son of a bitch,” Anita repeated, with her head thrown back. “Mamma mia!” The bedding was aslant in long up-sweeping lines, of which a few were green. Almost due south of Lock Haven and thirty-one miles west of the Susquehanna River, it was Juniata sandstone, brought down off the Taconic uplift and spread to the west by the same system of rivers that transported the rock of the Delaware Water Gap. “This would be a beautiful place to measure the thickness of the section,” Anita said. “It’s completely exposed. It’s consistent. There are no faults. The thin green bands are where deposition was too rapid for oxidation to take place.” Evidence of geologists was everywhere. They had painted numbers and letters on the rock. They had removed countless paleomagnetic plugs. The bedding, seen close, was not monotonously even, as rock would be that formed in still water. Instead, it was full of the migrating channels, feathery crossbedding, natural levees, and overbank deposits of its thoroughly commemorated river. There were little maroon mud flakes. They were plucked off flats in a storm.
We went back a few miles and slowly reviewed the rock. When again we approached the huge roadcut, Anita said, “In Illinois, this would be a state park.”
The bedding planes of the Holy Toledo cut, as I would ever after refer to those enormous walls of red stone, were dipping to the east. Over the past few miles, the rock of the country had been folded ninety degrees. To the immediate west, therefore, we would be going down in time and predictably would descend in space to a Cambro-Ordovician carbonate valley, which is what happened, as the road fell away bending left and down into Nittany Valley, where ribs of dolomite protruded here and there among rich-looking pastures turning green, gentle streamcourses, white farms. “Penn State sits on Nittany dolomite,” Anita said. “It’s twenty miles down this valley.”
Some remnant Cambrian sandstone formed a blister in the valley. The interstate drifted around it in a westerly way and toward the foot of still another endless mountain—Bald Eagle, the last ridge of the deformed Appalachians. After the Cambrian sandstone, the Ordovician dolomite, there was Silurian quartzite in the gap that broke through the mountain. Its strata dipped steeply west. The rock had bent again, and again we were moving upward through history. Now, though, the dip of the strata would reverse no more. In a dozen miles of ever younger rock, we climbed through the Paleozoic era almost from beginning to end. We went up through time at least three hundred million years and up through the country more than a thousand vertical feet, the last ten miles uphill all the way, from Bald Eagle Creek to Snow Shoe, Pennsylvania—the longest steady grade on I-80 east of Utah—while light, wind-driven snow began to fall.
We had come to the end of the physiographic province of the folded-and-faulted mountains, and the long ascent recapitulated Paleozoic history from the clean sands of the pre-tectonic sea to the dense twilight of Carboniferous swamps. We came up through the debris of three cordilleras, through repetitive sandstones and paper shales—Silurian paper shales, Devonian paper shales, Mississippian paper shales—crumbling on their shelves like acid-paper books in libraries. The shales were so incompetent that they would long since have avalanched and buried the highway had they not been benched—terraced in the manner of Machu Picchu. In other roadcuts, Catskill Delta sandstones, beet-red and competent, were sheer. We had gone through enough hard ridges and soft valleys for me not just to sense but to see the Paleozoic pageant repeatedly played in the rock. For all the great deformity and complexity, the mountains now gone had left patterns behind. The land rising and falling, the sea receding and transgressing, the ancestral rivers losing power through time had not just obliterated much of what went before but had always imposed new scenes, and while I, for one, could not hold so many hundreds of pictures well related in my mind I felt assured beyond doubt that we were moving through more than chaos.
The strata at the foot of the ten-mile hill had been nearly vertical. Gradually, through the long climb, they levelled out. They leaned backward, relaxed, one degree every two million years, until in the end they were flat—at which moment the interstate left the deformed Appalachians and itself became level on the Allegheny Plateau.
The rock was now Pennsylvanian—massive river sandstones of Pennsylvanian time. Flat, deck-like, it was comparatively undisturbed. It had been shed, to be sure, from eastern mountains, but had not been much affected by their compressive drive. Crazed streams had disassembled the plateau, leaving half-eaten wedding cakes, failed pyramids, oddly polygonal hair-covered hills. Pittsburgh was built upon such geometries, its streets and roads faithful to the schizophrenic streams, its hills separating its people into socio-racial ethno-religious piles—up this one the snobs, up that the Jews, up this the tired, up that the poor.
A hundred miles northeast of Pittsburgh in the flurrying snow there were numerous roadcuts now, and in them were upward-fining sequences of sandstones, siltstones, shales—Allegheny black shales —underlying more levels of sandstone, siltstone, and shale. “If you were a prospector for coal, you’d go bananas when you saw these black shales,” Anita said. “There ought to be coal in these roadcuts. This is Pennsylvania in the Pennsylvanian—the home office of the rock.”
Pennsylvania in the Pennsylvanian was jungle—a few degrees from the equator, like southern Indonesia and Guadalcanal. The freshwater swamp forests stood beside the nervously changing coastline of a saltwater bay, just as Sumatran swamps now stand beside the Straits of Malacca, and Bornean everglades beside the Java Sea. This was when glacial cycles elsewhere in the world were causing sea level to oscillate with geologic rapidity, and the swamps pursued the shoreline as the sea went down, and marine limestone buried the swamps as the sea returned. In just one of these cycles, the shoreline would move as much as five hundred miles—the sea transgressing and regressing through most of Pennsylvania and Ohio. There were so many such cycles at close intervals in Pennsylvanian time that Pennsylvanian rock sequences are often striped like regimental ties—the signature of glaciers half the world away. They existed three hundred million years ago, and glacial patterns of that kind have not been repeated until now, when the measure of our own brief visit to the earth is being recorded as a paper-thin stripe in time.
On both sides of the interstate, above the silhouettes of screening trees, we saw the tops of draglines—the necks and heads of industrial giraffes. They and predecessor machines had been working for fifty years, altering the topography, stripping the coal beds of Pennsylvania—in all, a mineral deposit worth a great deal more than the diamond mines of Kimberley and the goldfields of the Klondike. Coal was in the roadcuts now and would continue to be for many tens of miles—in layers that were not the dull deep gray of the Allegheny shale but truly black and shining. Layered light and dark, the roadcuts looked like Hungarian tortes. Reading up from the bottom, there was sandstone, siltstone, shale, coal, sandstone, siltstone, shale, coal. We would see limestones farther on, capping the coal where sea had covered the swamps. The present sequence was built behind a coastline—as is happening now, for example, in the bayous of the Mississippi Delta—by rivers meandering to and fro, covering with sand the matted vegetation. “These roadcuts are a textbook on the making of coal,” Anita said. Buried and compressed, vegetal debris first becomes peat—a mélange of spores, seed coats, wood, bark, leaves, and roots which looks like chewing tobacco and burns about as well. Peat bears much the same relation to coal that snow does to glacier ice. As snow is ever more buried and compacted, it recrystallizes and becomes ice—on the average ten times as dense as the original snow. As peat is buried, compacted, subjected to geothermal heat, it gradually g
ives up much of its oxygen, hydrogen, and nitrogen, and concentrates its content of carbon. The American Geological Institute’s Glossary of Geology defines coal as “a readily combustible rock.” By weight, any rock that is half carbonaceous material is coal. Its density is roughly ten times the density of peat. In the United States, there is enough peat to keep Ireland warm for a thousand years. The United States uses almost none of it, because the United States also happens to have a great deal more coal than any other country in the world. Peat that remains near the surface will never become coal. Buried three-quarters of a mile, it becomes bituminous. With a microscope, you can see wood and bark, leaves and roots, seed coats and spores in bituminous coal—and even identify the plants they came from. Buried deeper and folded severely under pressure, it becomes anthracite. Anthracite is roughly ninetyfive per cent carbon and is so hard that it fractures conchoidally, like an arrowhead. Anthracite is iridescent, and bums with a clear blue flame. Coal is a record of tectonics. In late Pennsylvanian time, when the third set of mountains came up in the east and shed still another wedge of debris, kneading it into what had gone before, the great pressure, deep burial, and severe folding produced the anthracites of eastern Pennsylvania, the pod-shaped coalfields of the folded-and-faulted mountains, which erosion and isostasy have lifted from the depths. Anthracite seams are often upside down or standing on end. Here in the Allegheny Plateau, burial was reasonably deep but tectonic pressures were minor, and the result is a lesser grade of coal.