Annals of the Former World

by John McPhee

  The Gros Ventre River entered the valley almost opposite the high Teton peaks. A short way up the Gros Ventre was a denuded mountainside, where seventy-five million tons of rock had recently avalanched and dammed the river. He saw glacial grooves running north-south, and remembered the levees that kept the Snake from spilling west. This suggested to him that the valley floor had tilted westward since the glacier went by. Curvilinear pine-covered mounds cupped the valley’s various lakes and held them close to the Tetons. Each lake was at the foot of a canyon. Evidently, alpine glaciers had come down the canyons to drop their moraines in the valley and, melting backward, fill the lakes. Some of the effects of ice were as fresh as that; others were less and less discernible, dating back from one episode of glaciation to another, separated by tens of thousands of years. Love’s son Charlie, who teaches geology and anthropology at Western Wyoming Community College, was hiking one day in 1967 along the ridgelines of the Gros Ventre Mountains when he discovered boulders whose source bedrock was fifty miles away in the Absaroka Range. If they were glacial, as they seemed to be, they recorded an episode until then unknown, and of greater magnitude than any other. The evidence remains scant, but what else could have carried those boulders fifty miles and set them down on mountain summits at ten thousand feet? David answered the question by coining the term “ghost glaciation.”

  From lookoffs in and around Jackson Hole, the view to the north concluded with a high and essentially level tree-covered terrain that seemed to be advancing from the direction of Yellowstone, as indeed it had done, spreading southward, concealing the earlier topography, filling every creek bed, pond, and gulch. When he first rode in that terrain, he saw with no surprise that the rock was rhyolite, which has the same chemistry as granite but not its crystalline texture, because rhyolite cools quickly as a result of coming out upon the surface of the earth. This rhyolite, in a fiery cloud rolling down from Yellowstone, had buried the north end of the Teton Range, where it split and flowed along both sides.

  From one end to the other of the valley were outcrops that from a distance looked like snow. Close up, they were white limestone, white shale, and white ash. After noting strikes and dips, and compiling the data, he calculated the thickness of the deposit as approximating six thousand feet. Top to bottom, it was full of freshwater clams and snails, and some beavers, aquatic mice, and other creatures that live in shallows. So the valley had been filled with a lake. The lake was always shallow. Yet its accumulated sediments were more than a mile thick. There was no rational explanation—unless the floor of the valley was steadily sinking throughout the life of the lake.

  Volcanic rocks around the valley were white, brown, red, purple, and numerous hues of yellow and green. Quartzite boulders—stream-rounded, and scattered far and wide—had come from a source far to the northwest, in Idaho, and could not have been transported by ice. In the Mt. Leidy Highlands and along the eastern edge of Jackson Hole he saw other boulders, larger than human heads. Like the quartzites, they asked questions that, for the time being, he could not answer. He found black and gray sediments of the Cretaceous seas. He measured them, and they were two miles thick. Just above them in time, he found coal. In red and salmon rock nearby were the small tracks and tiny bones of dinosaurs. Larger ones, too. There were beds of marine phosphate. He collected cherty black shales, pure dolomites, dark dolomites, the massive sandstones of an ocean beach. He went into blue-gray caves in beautiful marine limestone. He found mud-crack-bearing shales. He saw mounds resembling anthills, which had been built by blue-green algae.

  Chipping with his hammer, he bagged folded and fractured schist, amphibolite, and banded gneiss—and granite that had come welling up as magma, intruding these older rocks at a time when they were far below the earth’s surface, a time that was eventually determined by potassium-argon dating. The time was 2.5 billion years before the present. Therefore, the rock that the granite invaded was a good deal older, but it had been metamorphosed, and there was no telling how long it had existed before it was changed —how far it reached back toward the age of the oldest dated rock on earth: a number approaching four billion years.

  In these lithic archives—randomly assembled, subsequently arranged and filed—was a completeness in every way proportionate to the valley’s unexceedable beauty. From three thousand million years ago to the tectonically restless present, a very high percentage of the epochs in the history of the earth were represented. It was no wonder that a geologist would especially be drawn to this valley. As he moved from panorama to panorama and outcrop to outcrop —relating this rock type to that mountain, this formation to that river—David gradually began to form a tentative regional picture, and after thirty years or so had placed in sequential narrative the history of the valley. When new evidence and insight came along, what had once seemed logical sometimes fell into discard. When plate tectonics arrived, he embraced its revelations, or accommodated them, but by no means readily accepted them. He wrote more than twenty professional papers on subjects researched in the vicinity, and, with his colleague John Reed, published a summary volume for the general public called Creation of the Teton Landscape. When the Department of the Interior honored him with a Citation for Meritorious Service, it said, in part, that he had “established the fundamental stratigraphic and structural framework for a region.” In short, he had put the petals back on the flower.

  And it was some flower. The Teton landscape contained not only the most complete geologic history in North America but also the most complex. (“One reason I’ve put in a part of my life here is that we have so much coming together. I don’t want to waste my time. I can make more of a contribution by concentrating here than on any other place.”) After more than half a century with the story assembling in his mind, he can roll it like a Roman scroll. From the Precambrian beginnings, he can watch the landscape change, see it move, grow, collapse, and shuffle itself in an intricate, imbricate manner, not in spatial chaos but by cause and effect through time. He can see it in motion now, in several ways responsively moving in the present—its appearance indebted to the paradox that while the region generally appears to have been rising the valley has collapsed.

  Splitting the wall of the Tetons is a diabase dike a hundred and fifty feet wide, running like a dark streak of warpaint straight up the face of the mountains. Diabase: a brother of gabbro, a distant relative of granite. Four miles below the surface of the earth, the space occupied by this now solid dike was once a fissure through which the dark rock flowed upward as magma. At the same point in the narrative—1.3 billion years before the present, in the age of the Precambrian called Helikian time—marine beaches are not far to the west, and beyond them is a modest continental shelf. There is no Oregon, no Washington, not much Idaho—instead, blue ocean over ocean crust. Down toward the beaches flow sluggish rivers across a featureless plain. Folded and faulted schists and gneisses are bevelled under the plain, preserving in their deformation compressive crustal movements that have long since driven skyward uncounted ranges that have worn away. The Helikian beaches in their turn disappear, in burial becoming sandstone, which in the heat and pressure of more folding mountains is altered to quartzite. The mountains dissolve, and still another quiet plain vanishes below waves. The water advances into this piece of the world that will one day form as Jackson Hole. It lies close to the latitude of Holocene Sri Lanka—or Malaya, or Panama—and is moving toward the equator. The water is warm but not always quiet or clear. Blue-green algae build mounds in the shallows. There is a drop in sea level. Polygonal mud cracks become ceramic in the tropical sun. The sea returns. The water is virtually transparent, and the skeletons of billions of creatures form a pure blue-gray limestone. Like Debussy’s engulfed cathedral, the site comes up now and again into the light and air, but for the most part seas stay over it. Sands accumulate—broad, deep sands—but they preserve almost no fossil record, so not even David Love will ever say with certainty whether they are underwater or out in the air. (What he
cannot say with certainty he will readily say without certainty, provided the difference is clear. He prefers not to be, as he likes to put it, “a man walking with one foot on each side of a fence.” He thinks that some of those sands were terrestrial dunes and coastlines, reddened as oxides in the air.) Jackson Hole is close to the equator, and phosphates form in the shallow evaporating sea. Tidal flats appear—wide red flats, thickened by slow rivers coming from an uplift far to the east. In the muds are small tracks and tiny bones of dinosaurs. Rapidly—and possibly as a result of the breaking up of the earth’s only continent—the region travels north, moving about a thousand miles in thirty million years. Big dunes form upon the flats: dry, windblown dunes—a Sahara in salmon and red, at the precise latitudes of the modern Sahara. The red sands in turn are covered by the Sundance Sea. Coming from the north, it not only buries the big dunes under mud and sand but covers them with galaxies of clams. When the water drops, floodplains emerge, and flooding rivers band the country—pink, purple, red, and green. Dinosaurs wander this chromatic landscape—a dinosaur as large as a corgi, a dinosaur as large as a bear, a dinosaur larger than a Trailways bus. Seas return, filled with a viciousness of life. Black and gray sediments pour into them from stratovolcanoes off to the west. In these times, the piece of sea bottom which is the future site of Jackson Hole overshoots the latitude of modern Wyoming and continues north to a kind of apogee near modern Saskatoon.

  The land arches. Deep miles of sediments lying over schists and granites rise and bend. The seas drain eastward. The dinosaurs fade. Mountains rise northwest, rooted firmly to their Precambrian cores. Braided rivers descend from them, lugging quartzite boulders, and spreading fields of gold-bearing gravel tens of miles wide. Other mountains—as rootless sheets of rock—appear in the west, sliding like floorboards, overlapping, stacking up, covering younger rock, colliding with the rooted mountains, while to the east more big ranges and huge downflexing basins appear in the random geometries of the Laramide Revolution.

  For all that is going on around it, the amount of activity at the site of Jackson Hole is relatively low. Across the future valley runs a northwest-trending hump that might be the beginnings of a big range but is destined not to become one. Miles below, however, a great fault develops among the Precambrian granites, amphibolites, gneisses, and schists—and a crustal block moves upward at least two thousand feet, stopping, for the time being, far below the surface.

  New volcanoes rise to the north and east. Fissures spread open. Materials ranging from viscous lavas to flying ash obliterate the existing topography. Streams disintegrate these materials and rearrange them in layers a few miles away. So far, these scenes—each one of which is preserved in the rock of Jackson Hole—have advanced to a point that is 99.8 per cent of the way through the history of the earth, yet nothing is in sight that even vaguely resembles the Tetons. The Precambrian rock remains buried under younger sediments. At the surface is a country of undramatic hills. The movements that brought the Overthrust Belt to western Wyoming—and caused the more easterly ranges to leap up out of the ground—have all been compressional: crust driven against crust, folded, faulted, and otherwise deformed. Now the crust extends, the earth stretches, the land pulls apart—and one result is a north-south-trending normal fault, fifty miles long. On the two sides of this fault, blocks of country swing on distant hinges like a facing pair of trapdoors—one rising, one sagging. The rising side is the rock of the nascent Tetons, carrying upward on its back the stratified deposits of half a billion years. One after another, erosion shucks them off. Even more rapidly, the east side falls—into a growing void. Magma, in motion below, is continually being drawn toward volcanoes, vents, and fissures to the north. Just as magma moving under Idaho is causing land to collapse and form the Snake River Plain, magma drawn north from this place is increasing the vacuity of Jackson Hole. As the magma reaches Yellowstone, it rises to the surface, spreads out in all directions, and in a fiery cloud rolls down from Yellowstone to bury the north end of the Tetons, where it splits and flows along both sides. The descending valley floor breaks into blocks, like ice cubes in a bucket of water. Some of them stick up as buttes. A lake now fills the valley—shallow, forty miles long—and in it forms a limestone so white it looks like snow. There are white shales as well, and water-laid strata of white volcanic ash. As these sediments thicken to a depth approaching six thousand feet, the lake that rests upon them is always shallow, and full of freshwater clams and snails, and some beavers and aquatic mice. While the lake is accepting sediment, the bottom of its bottom is sinking at the same rate. With a loud terminal hissing, lavas flowing down from Yellowstone cool in the lake as obsidian. Fiery billows of sticky fog come down the valley as well. It cools as tuff. The big lake vanishes. In successive earthquakes, there is more valley faulting, damming the valley streams to form deep narrow lakes, which appear suddenly and as quickly go. Off the fast-rising block of mountains, erosion has by now removed fifteen thousand feet of layered sediments, and the Precambrian granites—with their attendant amphibolites, schists, and gneisses, and a vertical streak of diabase—are the highest rock below the sky. Bent upward against the flanks of the Precambrian are the broken-off strata of the Paleozoic era, and the broken-off strata of the Mesozoic era—serrated, ragged hogbacks, continually pushed aside. Perched on the granite at the skyline is a bit of Cambrian sandstone that the weather has yet to take away. On the opposite side of the Teton Fault, the same sandstone lies beneath the valley. The vertical distance between the two sides of this once contiguous formation is thirty thousand feet.

  That brings the chronicle essentially to the present, but still the blockish mountains look more like hips than breasts. Now off the Absarokas, off the Wind Rivers, off the central Yellowstone Plateau—and, to a lesser extent, down the canyons of the Tetons—comes a thousand cubic miles of ice. A coalesced glacier more than half a mile thick enters and plows the valley. The west side of this glacier scrapes along the Tetons above the level of the modern timberline. Melting away, the glacier leaves a barren ground of boulders. More ice comes—a lesser but not insignificant volume—and a third episode, which is smaller still. The ice cuts headward up canyons into the mountains, making cirques. As rings of cirques further erode, they form the spires known as horns. The ice signs the valley with lakes, and as it shrinks back into the mountains human beings have come to watch it go. Long after it is gone, the valley floor, continuing to be unstable as magmas are drawn north from below, drops even more. Big spruce go down with it—trees with diameters of five feet—and are enveloped by the water of Jenny Lake. The mountains jump upward at the same time, many feet in a few seconds near the end of the fourteenth century, emphasizing the fact that they are active in our time. In 1925, seventy-five million tons of rock fall into the Gros Ventre River. In 1983, the year that the trees are discovered at the bottom of Jenny Lake, an earthquake halfway up the Richter scale rumbles through Jackson Hole.

  A geologic map is a textbook on one sheet of paper. In its cryptic manner—its codes of color and sign—it reflects (or should reflect) all the important research that has been done on any geologic topic within its boundaries. From broad formational measurements down to patterns in the fabric of the rock, a map should serve as an epitome of what is known and not known about a region, up to date. Regional maps have traditionally been presented state by state, and the dates they are up to vary: Nevada 1978, New York 1970, New Jersey 1910. On a geologic map, as on any scientific publication, the name of the person primarily responsible appears first. The job involves so many years and such a prodigious bibliography that the completion of a state geologic map can be regarded as the work of a lifetime, and David Love is only the second person in the history of American geology who has served as senior author of a state map twice (Wyoming 1955, Wyoming 1985). Geology is a descriptive, interpretive science, and conflict is commonplace among its practitioners. Where two or more geologists have come to divergent conclusions, Love has had to go out and
rehearse their field work, in order to decide what to show on the map. People tend to become ornery when the validity of their assertions is challenged, and figuratively some of his colleagues have reached for their holsters, which may have been a mistake, as the buttons fell off their shirts and they felt a little breeze.