Annals of the Former World

by John McPhee

  Twenty thousand feet of rock was deroofed from the rising mountains. The entire stratigraphy from the Cretaceous down to the Precambrian was broken to bits and sent off to Natchez, as the mountains were denuded to their crystalline and metamorphic cores. In half a billion years of history, this was the great event. In the words of the Geologic Atlas of the Rocky Mountain Region, it was “tectonically unique in the Western Hemisphere and, therefore, it seems to require a somewhat unusual if not unique tectonic interpretation.” The foreland ranges, as the mountains east of the overthrust are called (the Wind Rivers, Uintas, Bighorns, Medicine Bows, Laramie Range, and so forth), came into the world with their own odd syncopation, albeit the general chronology went from west to east and the Laramie Range was among the last to rise. “The mountains were restless,” Love was saying now. “They didn’t all pooch out at once. They moved in fits and starts over a span of time. The Owl Creeks rose in the early Eocene, as did the Uintas. The Medicine Bows, which are farther east, came up before the Uintas. They are all separate mountains with the same general type of origin. They are cohesive in the way that a family is cohesive. They are part of the same event.”

  The event is known in geology as the Laramide Orogeny. Alternatively it is called the Laramide Revolution.

  Mountains always come down, of course, as they are coming up. In the contest between erosion and orogeny, erosion never loses. For a relatively short time, though, the mountains prevail by rising faster than they are destroyed. In what Love has called “some of the greatest localized vertical displacement known anywhere in the world,” the Wind Rivers rose sixty thousand feet with respect to the rock around them, the Uintas fifty thousand, others as much. Frequent rains and many streams helped melt them away. West of Wyoming, in the Eocene, there were no Coast Ranges, no Sierra Nevada. Warm winds off the Pacific brought rains to the Rockies, and a climate similar to the present climate of Florida. In the early Eocene, when the ranges in general looked much as they do today, the mountain building ceased. In the tectonic quiet, erosion of course continued, and the broad downwarps among the ranges continued to fill.

  Then came a footnote to the revolution. “In latest early Eocene, fifty-two million years ago, all hell broke loose again,” Love said. From thousands of fissures in northwest Wyoming, lava poured forth by the cubic mile. Torn apart by weather and rearranged by streams, it has since been etched out as the Absaroka Range. “After that, everything went blah,” he went on. “In the Oligocene, the tectonic activity was totally dead, and it stayed dead at least until the early Miocene. Thirty million years. Then, in the late Miocene, all hell broke loose again. And all hell has been breaking loose time and again for the last ten million years. This is not a static science.”

  During those thirty million years after things went blah, the Rockies were quietly buried ever deeper in their own debris—and, not so peacefully, in materials oozing overland or falling from the sky. Much came in on the wind from remote explosive volcanoes—stratovolcanoes of huge size in Idaho, Oregon, Nevada. “And maybe Arizona and California, for all we know,” Love said. “Clinical details are still inadequate. By the end of the Eocene, the Washakie and Owl Creek Mountains were so deeply buried that the Wind River and Bighorn Basins had coalesced above them. At the end of the Oligocene, only a thousand to four thousand feet of the highest mountains protruded above the aggradational plain. Streams were slow and sluggish and so choked with ash they were unable to erode.”

  Rhinoceroses lived through those changes, and ancestral deer and antelope, and little horses with three toes. As altitude and aridity increased, a subtropical world of figs, magnolias, and breadfruit cooled into forests of maple, oak, and beech. Altitude alone could not account for the increasing coolness. It foreshadowed the coming ice.

  The burial of the mountains continued far into the Miocene, with—as Love described it—“surprising thicknesses of sandstone and tuffaceous debris.” Volcanic sands, from Yellowstone and from elsewhere to the west, were spread by the wind, and in places formed giant dunes. Two thousand feet of sand accumulated in central Wyoming. Nineteen thousand—the thickest Miocene deposit in America—went into the sinking Jackson Hole. From the Wind River Mountains southward to Colorado and eastward to Nebraska, the plain was unbroken except for the tops of the highest peaks. Rivers were several thousand feet higher than they are now. The ranges, buried almost to their summits, were separated by hundreds of miles of essentially flat terrain. Mountains that were completely covered —lost to view somewhere below the water-laid sediments and deep volcanic sand—outnumbered the mountains that barely showed through. At its maximum, the broad planar surface occupied nearly all of Wyoming—upward of ninety per cent—and on it meandered slow streams, making huge bends and oxbows. As events were about to prove, the deposition would rise no higher. This—in the late Miocene—was the level of maximum fill.

  For something began to elevate the region—the whole terrane, the complete interred family of underthrust, upthrust, overthrust mountains—to lift them swiftly about a mile. “The uplift was not absolutely uniform everywhere,” Love said. “But nothing ever, is.” What produced this so-called epeirogeny is a subject of vigorous and sometimes virulent argument, but the result, continuing to this day, is as indisputable as it has been dramatic. It is known in geology as the Exhumation of the Rockies.

  From around and over the Wyoming ranges alone, about fifty thousand cubic miles have been dug out and taken away, not to mention comparable excavations in the neighboring cordillera. Though the process has been going on for ten million years, it is believed to have been particularly energetic in the past million and a half, in part because of the amount of rain that fell on the peripheries of continental ice. In response to the uplift, the easygoing streams that had aimlessly wandered the Miocene plain began to straighten, rush, and cut, moving their boulders and gravels in the way that chain saws move their teeth. The streams lay in patterns that had no relationship to the Eocene topography buried far below. Some of them, rushing along through what is now the Wyoming sky, happened to cross the crests of buried ranges. After they worked their way down to the ranges, they sawed through them. Some effects were even odder than that. If a river happened to be lying above a spur of a buried range, it would cut down through the spur, and seem, eventually—without logic, with considerable magic—to flow into a mountain range, change its mind, and come back out another way. “Eventually,” of course, is now. The North Platte River now flows into the rocks of the Medicine Bow Mountains, comes out again to cross the Hanna Basin, and then runs through the Seminoe Mountains and the Granite Mountains. It is joined by the Sweetwater River on the crest of the Granite Mountains. Irrespective of modern topography, the pattern of the rivers is Miocene. On the Laramie Plains, the Laramie River behaves for a while in a deceptively conventional manner. It establishes itself as the centerpiece of the basin, pretending to be the original architect of the circumvallate scene, but then takes a sharp right and, like a bull with lowered head, charges the Laramie Range. The canyon it has made is deep and wild. Water roars through it. When, in the exhumation, the river got down to the mountains, it packed the abrasive power to cut them in half.

  In fact, there is no obvious relationship between most of the major rivers in Wyoming and the landscapes they traverse. While rivers elsewhere, running in their dendritic patterns like the veins in a leaf, shape in harmony the landscapes they dominate, almost all the rivers of the Rockies seem to argue with nature as well as with common sense. At Devil’s Gate, on the Oregon Trail, the Sweetwater River flows into a hill of granite and out the other side. The Wind River addresses itself to the Owl Creek Mountains and flows right at them. It, too, breaks through and comes out the other side. It, too, flowed across the totally buried mountains in the Miocene, and descended upon them during the exhumation. The anomaly is so startling that early explorers, and even aborigines, did not put one and one together. To the waters on the south side of the mountains they gave the name Wind
River. The waters on the north side they called the Bighorn. Eventually, they discovered Wind River Canyon.

  On the east flank of the Laramie Range is a piece of ground that somehow escaped exhumation. Actually contiguous with Miocene remains that extend far into Nebraska, it is the only place between Mexico and Canada where the surface that covered the mountains still reaches up to a summit. To the north and south of it, excavation has been deep and wide, and the mountain front is of formidable demeanor. Yet this one piece of the Great Plains—extremely narrow but still intact—extends like a finger and, as ever, touches the mountain core: the pink deroofed Precambrian granite, the top of the range. At this place, as nowhere else, you can step off the Great Plains directly onto a Rocky Mountain summit. It is known to geologists as the gangplank.

  Now the Bronco began to rise through the snow, and Love remarked that we were on the gangplank. The land fell away on either side, and in the low visibility we seemed indeed to be on a plank going up into the sky. As we continued to climb, the strip of earth became narrower and narrower. We pulled over onto the shoulder, shut off the motor, and squinted. We appeared to be on a bridge—built of disassembled Rockies and travelled ash—crossing a great excavation through flapping veils of snow. “There are twelve inches of precipitation per annum here, and it’s mostly snow,” he said. “The mean temperature is thirty-eight degrees. The growing season is less than ninety days. Conditions are about the same in this part of Wyoming as at the Arctic Circle.” With that, we gave up the geology and crawled off to his home in Laramie, defeated by the snow. We went back to the gangplank in clear weather.

  It was half a mile long. To the north and south, the land fell away along the mountain front in profound excavation of the sediments that once had been there. The excavation had exposed the broken, upturned ends of Pennsylvanian sandstones, dipping steeply eastward and leaning on the mountains. They rested there like lumber stood against a barn. These red sandstones lean against the Laramie Range on both sides. By themselves, they tell the story of the Laramide Orogeny, for they are a part of what was deroofed. They are a part of the Paleozoic package that once rested flat on the deep Precambrian granite. They are thought by some to have been Pennsylvanian beach sands. Whatever they may have been, they were indubitably horizontal, and for roughly two hundred and fifty million years remained horizontal while layer after layer of sediment accumulated above them, finally including the floors of the Cretaceous seas. Then all hell broke loose, and the granite rose beneath them. The granite core came up like a basement elevator that rises through a city sidewalk, pushing to either side a pair of hinged doors. That was the chronology of numerous ranges—the old hard stuff from far below breaking upward through roofrock and ultimately standing highest, while the ends of the roofrock lean on the flanks in gradations of age that are younger with distance from the core. The broken ends of that Pennsylvanian sand—the outcropping edges of the tilted strata—had weathered out as a rough, serrate ridge along the border of the range. Rocky Mountain ranges are typically flanked by such hogbacks. Boulder, Colorado, is backdropped with hogbacks (the Flatirons), which are more of the same Pennsylvanian strata leaning against the Front Range. Now, on the gangplank, Love said parenthetically, “You are seeing Paleozoic rocks for the first time since the Mississippi River. They go all the way through—under Iowa and Nebraska—but they’re buried.”

  In the fall of 1865, Major General Grenville Dodge and his pack trains and cavalry and other troops were coming south along the St. Vrain Trail, under the front of the Laramie Range. The Powder River campaign, behind them, had been, if not a military defeat, a signal failure in its purpose: to cow the North Cheyennes and the Ogallala Sioux. General Dodge, though, was preoccupied with something else. President Lincoln, not long before he died, had instructed Dodge to choose a route for the Union Pacific Railroad. Dodge, like others before him, had sought the counsel of Jim Bridger, the much celebrated trapper, explorer, fur trader, commercial entrepreneur, and all-around mountain man. Bridger, who was sixty by then, had preceded almost everybody else into the West by two or three decades and knew the country as few other whites ever would. It was he who discovered the Great Salt Lake, reporting his find as the Pacific Ocean. It was he whose descriptions of Jackson Hole, Yellowstone Lake, Yellowstone Falls, the Fire Hole geysers, and the Madison River had once been known as “Jim Bridger’s lies.” His father-in-law was Chief Washakie. And now this bluecoat general wanted to know where to put a railroad. The Oregon Trail went around the north end of the Laramie Range and up the Sweetwater to South Pass—to say the least, an easy grade. But for a competitive transcontinental railroad the Sweetwater was a route of wide digression and no coal. Bridger mentioned Lodgepole Creek and said the high ground above it was the low point on the crest of the Laramie Range (a fact that theodolites would in time confirm). The route could go there.

  So Dodge, in 1865, coming south from the Powder River, left his pack trains and cavalry on the St. Vrain Trail and led a small patrol up Lodgepole Creek. At the top, he turned south and did reconnaissance of the summit terrain. In the small valley of a high tributary of Crow Creek—five or ten miles south of Bridger’s recommendation—he surprised a band of Indians. His report does not say of what tribe. They were hostiles—or at least became so after Dodge started firing at them. At the moment of mutual surprise, they were between him and his main column, and that made him tactically nervous. The patrol dismounted and walked due east —“holding the Indians at bay, when they came too near, with our Winchesters.” In this manner, the gangplank was discovered. As Dodge kept going east, expecting to reach the escarpment from which he would signal with smoke, he reached no escarpment. Instead, he reached the remnant of the high ancient surface—this interfluvial isthmus between Crow Creek and Lone Tree Creek—touching the mountain summit.

  It led down to the plains without a break. I then said to my guide that if we saved our scalps I believed we had found the crossing.

  General Dodge went back east, and in the spring of 1867 returned with his route approved. The Union Pacific at that time ended in the middle of Nebraska. He got off the train, went up the North Platte, up the Lodgepole, and, as he approached the mountains, went directly overland to Crow Creek, where he staked out the western end of the railroad’s next division. Without much pleasing anybody, he named the place Cheyenne. In no time, he was defending himself against furious Cheyennes. They killed soldiers and laborers, pulled up survey stakes, stole animals, and destroyed equipment. When some politicians, bureaucrats, and financiers arrived on a see-it-yourself junket west, the Cheyennes attacked them. With drawn revolver, General Dodge told his visitors, “We’ve got to clean these damn Indians out or give up building the Union Pacific Railroad. The government may take its choice.”

  The narrowest point on the gangplank is wide enough for the Union Pacific and nothing else. The interstate highway clings to one side. The tracks and lanes are so close that the gangplank resembles the neck of a guitar. A long coal freight slid by us. “The coal isn’t piled higher than the tops of the gondolas,” Love commented. “It’s an environmental move—to keep the dust from blowing downwind.” He said it was a good idea, no doubt, but he had experienced so many cinder showers earlier in his life that he could not help thinking that this latter-day assault on dust was “like bringing a fire under control at timberline.” A cinder shower was what happened when an old-time locomotive pulled into a town and blew its stack. He also said that this could not have been an important emigrant route, because there was a lack of grass and water—absolute necessities for animal-powered travel. To the Union Pacific, however, the gangplank offered speed, efficiency, and hence predominance with respect to the competition. When the Denver & Rio Grande was laboring up switchbacks in a hampering expenditure of money and time—and the Santa Fe was struggling not only with mountains but also with desert terrain—the Union Pacific had already run up the gangplank, opened the West, and become everybody’s Uncle Pete. Love said,
“Out here, Uncle Sam is a gnat under a blanket compared to Uncle Pete. The Union Pacific had the best of it. This Miocene Ogallala formation was the youngest of the high-plains deposits that lapped onto the mountain front. It’s subtle and seems academic until you try to build a railroad. This is the only place in the whole Rocky Mountain front where you can go from the Great Plains to the summit of the mountains without snaking your way up a mountain face or going through a tunnel. This one feature had more to do with the building of the West than any other factor. I don’t diminish the importance of the Oregon Trail, but here you had everything going for you. This point hasn’t been made before.”

  When the railroad was built, it was given (by the federal government) fifty per cent of the land in a forty-mile swath along its route—in checkerboard fashion, one square mile in every two. Uncle Pete is so big that he has spun off, among many things, the Rocky Mountain Energy Company, the Upland Industries Corporation, the Champlin Petroleum Company, and enough unmined uranium to send Wyoming to the moon. In Cheyenne, the Union Pacific station and the state capitol face each other at opposite ends of Capitol Avenue. The Union Pacific station came out of the Laramie Range, forty miles west, and, like the range itself, is sheathed in the russet Pennsylvanian sandstone and has a foundation of Precambrian granite. At least as imposing as the capitol, it is a baronially escutcheoned mountain of grandeur.

  Indians, of course, had used the gangplank for who knows how long before General Dodge surprised them on the Laramie summit. They had crossed it on their journeys from the Great Plains to the Laramie Basin and on up to hunting grounds in the Medicine Bow Mountains. And the Indians, from the beginning, were themselves following a trail. Buffalo discovered the gangplank. “It was a buffalo trail,” Love said. “Buffalo were the real trailmakers—trails you wouldn’t believe. They were as good as the best civil engineers. It remains true today. If you’re in Yellowstone, in the backcountry, and you have trouble finding your way across swamps, mountains, and thermal areas, you look for a buffalo trail and you’ll get through.” Beside Interstate 80 on the gangplank, a sign said, “GAME CROSSING.”