… In late Cretaceous and early Tertiary time, mountains began to rise beneath the wide seas and marsh flats of Wyoming. The seawater drained away to the Gulf of Mexico, to the Arctic Ocean. And in [geologist] David Love’s summary description, “all hell broke loose.” In westernmost Wyoming, detached crustal sheets came planing eastward — rode fifty, sixty, and seventy-five miles over younger rock — and piled up like shingles, one overlapping another. In the four hundred miles east of these overthrust mountains, other mountains began to appear, and in a very different way. They came right up out of the earth. In Love’s phrase, they simply “pooched out.”
… The spines of the ranges trended in as many directions as a weathervane. The Laramie Range trended north-south. The Wind Rivers and Bighorns northwest-southeast. The spectacularly anomalous Uintas, lining themselves up at right angles to the axis of the Western cordillera, ran east-west, and so did the Owl Creeks. All these mountain ranges were coming up out of the craton — heartland of the continent, the Stable Interior Craton. It was as if mountains had appeared in Ohio, inboard of the Appalachian thrust sheets, like a family of hogs waking up beneath a large blanket. An authentic enigma on a grand scale, this was one of the oddest occurrences in the tectonic history of the world. It would probe anybody’s theories. It happened rapidly. As David Love at one point remarked about the Medicine Bow Mountains, “It didn’t take very long for these mountains to come up, to be deroofed, and to be thrust eastward. Then the motion stopped. That happened in maybe ten million years, and to a geologist that’s really fast.”
… 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.
… 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 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.
… 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.
the gangplank [image from the USGS]
[ … ]
… To the Union Pacific [Railroad Co.] … the gangplank offered speed, efficiency, and hence predominance with respect to the competition. When the Denver & Rio Grande [Railroad Co.] 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.”
… 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.”