Now for the first time we come to the river that will command attention for the remainder of this story. It was born coincident with the rise of the new Rockies, called into being to carry rainfall and melting snow down from its heights. For millions of years it was not the dominant river of the region; in fact, five competing rivers led eastward from the Rockies, the long abandoned courses still discernible in the drylands. They lost their identity because of a peculiarity: an arm of our river began to cut southward along the edge of the mountain range, and in doing so, it captured one after another of the competing rivers, until they no longer ran eastward as independent rivers but coalesced to form the Platte.
When the Rockies were younger, and therefore higher than now, the river had to be of a goodly size. We can deduce this from the amount of material it was required to transport. The area covered by its deposits was about three hundred and twenty miles long and one hundred and forty miles wide. Depending upon how thick the overlay was, the river had to transport more than seven thousand cubic miles of rubble. In those early days it was wide and turbulent. It was capable of carrying huge rocks, which it disintegrated into fragments of great cutting power, but its main cargo was sand and silt. Its flow was irregular; at times it would wander fifty miles wide across plains, and for long periods it would hold to one channel. During these years it labored continuously at its job of building the plains of middle America.
About forty million years ago the river’s building process was aided by a cataclysmic event. To the southwest a group of volcanoes burst into action, and so violent were their eruptions, volcanic ash drifted across the sky for half a thousand miles, held aloft by great windstorms. The ash, blackening the sky as it passed, blanketed the area when it fell. Perhaps at some point an entire volcano may have exploded in one superburst, covering the heavens with its burden of fire and lava; eruptions continued over a period of fifteen million years, and the wealth of the ash that fell upon Colorado accumulated to a depth of thousands of feet. Combining with clay, it formed one of the principal rocks of the region.
It is difficult to comprehend the violence of this period. Twenty-three known volcanoes operated in Colorado, some of them much larger than Vesuvius in Italy or Popocatepetl in Mexico. Obviously, they could not have been in constant eruption; there had to be long periods of quiescence, but it does seem likely that some acted in concert, energized by a common agitation within the mantle. They deposited an incredible amount of new rock, more than fourteen thousand cubic miles of it in all.
They glowed through the nights, illuminating in ghostly flashes the mountains and plains they were creating. At times they sponsored earthquakes, and then for some mysterious reason, possibly because the molten magma was exhausted, they died, one after another, until there were no active volcanoes in the region. Only the clearly defined calderas still stand to mark this age of violence.
* * *
About fifteen million years ago the area underwent a massive dislocation in a process that extended for ten million years. The entire central portion of America experienced a massive uplifting. Perhaps the continental plate was undergoing some major adjustment, or there may have been a sizable disruption within the mantle. At any rate, the surfaces—both mountains and valleys to the west, and the low-lying flat plains to the east—rose. Colorado was uplifted to its present altitude. Rivers like the Missouri, which then ran north to the Arctic Ocean, began to take form and run south, and the outlines of our continent assumed more or less their present shape. Many subsequent adjustments of a minor nature would still occur—for example, at this time North and South America were not yet joined—but the shapes we know were discernible.
About one million years ago the Ice Age began to send its rapacious fingers down from the north polar ice cap. Because of intricate changes in the climate, triggered perhaps by variations in the carbon dioxide content of the Earth’s atmosphere or by accumulations of volcanic dust which intercepted the sun’s heat that otherwise would have reached the Earth, large sheets of ice began to accumulate where none had been before.
The glaciers invading North America reached so far south and were so thick, they imprisoned water that normally belonged to the oceans, which meant that shorelines which had lain submerged for the preceding millions of years now lay exposed. The great western glacier did not quite reach the Centennial area; it halted some distance to the north. But at high elevations in the Rockies, small glaciers did form and filled the valleys, and as they moved slowly to lower levels they gouged out the valley bottoms and carved the standing rocks, so that much of the beauty of the new Rockies stems from the work of the glaciers.
They arrived in the mountains at spaced intervals, the first major one appearing about three million years ago; the last, only fifteen thousand years ago. But of course, at high, cold altitudes like the topmost new Rockies small glaciers persisted and still exist.
As the mountain glaciers melted they produced unprecedented amounts of water, which created floods of gigantic proportions. They cascaded down with fierce velocity and submerged traditional rivers, causing them to expand many times their customary width. Much detritus was borne down from the mountains, most of it with sharp cutting edges, and it was this mixture of copious water and cutting rock that planed down the lands to the east.
Sometimes, high in the Rockies, the glacier would impound a temporary rock-and-ice barrier, and behind it an enormous lake would be formed. It would exist for decades or centuries. Then one day there would be a violent cracking sound, and with one vast rush the contents of the lake would surge forth, miles wide until it roared into some confining canyon, where it would compress into a devastating liquid missile, shooting along with terrifying force, uprooting every living thing and ripping away huge boulders from the walls of the canyon before rushing at last onto the plains.
There it would reach the river. A wall of water would fan out across the plains, engulfing both the river and its tributaries. Churning, roaring, twisting, it would scour everything before it as it scratched and clawed its way eastward. In the space of an afternoon, such a flood might carve away deposits that had required ten million years to accumulate.
It was the river that laid down the new land; it was the river that took it away. The endless cycle of building up, tearing down and rebuilding, using the same material over and over, was contributed to by the river. It was the brawling, undisciplined, violent artery of life and such it would always be.
Some sixty-five million years ago—shortly after the emergence of the new Rockies—the river began hauling down an extraordinary amount of rock, gravel and sand, which it deposited in a thick overlay on the flat plains to the east; the deposit was eventually more than two hundred feet thick.
When this process was completed, thirty-eight million years ago, the plains to the east were so built up that they blended harmoniously into the lower reaches of the new Rockies, creating a lovely sweep that extended in unbroken beauty several hundred miles into Nebraska and Kansas. This symmetry did not endure, for the new Rockies experienced a massive uplifting, which raised them above the gentle sweep. As a result, the river now dropped more steeply from the mountains, carrying with it many cutting rocks. It surged eastward and for twelve million years dominated the foothills, cutting them away, scraping down hillocks, and depositing on the plains new layers of rocky, infertile soil.
The great inland sea that had once dominated this area had long vanished, so that the building of this new rock had to be accomplished in open air. The river would bring down deposits, which would spread out in fans. Sun and wind would act upon them, and new deposits would form over them. Gradually, disparate components would begin to solidify, and as heavier forms accumulated on top, those at the bottom would coalesce to form conglomerates.
Each year the plains grew a little higher, a little more stable in their footing. Finishing touches were applied about eleven million years ago when a sandstone rock was laid down, sealing the entire region. This
final rock had a peculiar characteristic; at the spot we are talking about, north of Centennial, some variation occurred in the cement that bound the granular elements together. Different from the cement operating in nearby regions, it had been formed perhaps from volcanic ash that had drifted in; at any rate, it created an impermeable caprock that would protect the softer sandstone that rested beneath. At last the vast job of building was ended. From the period when the new Rockies underwent their secondary uplift, some three hundred and twenty feet of solid rock and soil had been laid down, all protected by the caprock, and had there been an observer at the time, he could have been excused had he concluded that what he saw then, eight million years ago, would be the final structure of the plains.
But it was still the river that determined what the surface of the land would be, and starting eight million years ago, it once more began to tumble out of mountains with greatly increased velocity, cutting and swirling and spreading far across the plains. It was engaged upon a gargantuan task, to scour away every vestige of the enormous quantity of land that had been contributed by the new Rockies. In some places it had to remove up to a thousand feet of burden; from extensive areas it had to cut away at least three hundred feet. But it succeeded—except where that extra-hard caprock protected its monolith.
No matter how wild the torrents that raged down from the mountains, nor how powerful the flash floods that cascaded across the plains after some torrential downpour, the monolith persisted. It covered an area no more than a quarter of a mile long, two hundred yards wide, but it resisted all assaults of the river. For millions of years this strange and solitary monolith maintained its integrity.
Neighboring sandstone covers were breached, and when they were gone, the softer areas they had protected were easily cut down by the river. Winds helped; meltwater from ice did its damage; and as the eons passed, the river completed its task—all remnants of the land deposited by the new Rockies were swept away, except the solitary monolith.
And then, about two million years ago, the central portion of the caprock weakened, cracked during a heavy winter and broke away. The softer rock that it had been protecting quickly deteriorated—say, over a period of two hundred thousand years—until it was gone.
Two pillars remained, about a quarter of a mile apart, each somewhat elongated in shape; the western was over five hundred feet long and two hundred feet wide, the eastern only three hundred and eighty feet long and a hundred and ninety feet wide. The western pillar was taller, too, standing three hundred and twenty feet above its pediment; the eastern only two hundred and eighty.
They were extraordinary, these two sentinels of the plains. Visible for miles in each direction, they guarded a bleak and silent empire. They were the only remaining relics of that vast plain that the new Rockies had deposited; each bit of land the sentinels surveyed dated back to ancient times, before the mountains were born.
And so the stage is set. One billion seven hundred million years of activity, including the building of at least two high mountain ranges and the calling into being of vast seas, have produced a land that is ready at last to receive and nurture living things.
DIPLODOCUS, THE DINOSAUR
Toward dusk on a spring evening one hundred and thirty-six million years ago a small furry animal less than four inches long peered cautiously from low reeds growing along the edge of a tropical lagoon that covered much of what was to be the present state of Colorado. It was looking across the surface of the water as if waiting for some creature to emerge from the depths, but nothing stirred. From among the fern trees to the left there was movement, and for one brief instant the little animal looked in that direction.
Shoving its way beneath the drooping branches and making considerable noise as it awkwardly approached the lagoon for a drink of water, came a medium-sized dinosaur, lumbering on two legs and twisting its short neck from side to side as it watched for larger animals that might attack.
It was about three feet tall at the shoulders and not more than six feet in length. Obviously a land animal, it edged up to the water carefully, constantly jerking its short neck in probing motions. In paying so much attention to the possible dangers on land, it overlooked the real danger that waited in the water, for as it reached the lagoon and began bending down in order to drink, what looked like a fallen log lying partly submerged in the water lunged toward him.
It was a crocodile, armored in heavy skin and possessing powerful jaws lined with deadly teeth. But it had moved too soon. Its well-calculated grab at the reptile’s right foreleg missed by a fraction, for the dinosaur managed to withdraw so speedily that the great snapping jaw closed not on the bony leg, as intended, but only on the soft flesh covering it.
There was a ripping sound as the crocodile tore off a strip of flesh, and a sharp guttural click as the wounded dinosaur responded to the pain. Then peace returned. The dinosaur could be heard for some moments retreating. The disappointed crocodile swallowed the meager meal it had caught, then resumed its log camouflage.
As the furry little animal returned to its earlier preoccupation of staring at the surface of the lagoon, it became aware, with a sense of panic, of wings in the darkening sky. At the very last moment it threw itself behind the trunk of a gingko tree, flattened itself out and held its breath as a large flying reptile swooped down, its sharp-toothed mouth open, and just missed its target. Still flat against the moist earth, the little animal watched in terror as the huge reptile banked low over the lagoon and then came straight at the crouching animal, but, abruptly, it had to swerve away because of the gingko roots. Dipping one wing, the reptile turned gracefully in the air, then swooped down on another small creature hiding near the crocodile, unprotected by any tree. Deftly it opened its beak and caught its prey, which shrieked as it was carried aloft. For some moments the little animal hiding behind the gingko tree watched the flight of its enemy as the reptile dipped and swerved through the sky like a falling feather, finally vanishing with its catch.
The little watcher could breathe again. Unlike the great reptiles, which were cold-blooded and raised their babies from hatching eggs, it was warm-blooded and came from the mother’s womb. It was a pantothere, one of the earliest mammals and progenitor of later types like the opossum, and it had scant protection in the swamp. Watching cautiously lest the flying hunter return, it ventured forth to renew its inspection of the lagoon, and after a pause, spotted what it had been looking for.
About ninety feet out into the water a small knob had appeared on the surface. It was only slightly larger than the watching animal itself, about six inches in diameter. It seemed to be floating on the surface, unattached to anything, but actually it was the unusual nose of an animal that had its nostrils on top of its head. The beast was resting on the bottom of the lagoon and breathing in this unique manner.
Now, as the watching animal expected, the floating knob began slowly to emerge from the waters. It was attached to a head, not extraordinarily large but belonging obviously to an animal markedly bigger than either the first dinosaur or the crocodile. It was not a handsome head, nor graceful either, but what happened next displayed each of those attributes.
The head continued to rise from the lagoon, higher and higher in one long beautiful arch, until it stood twenty-five feet above the water, suspended at the end of a long and graceful neck. It was like a ball extended endlessly upward on a frail length of wire, and when it was fully aloft, with no body visible to support it, the head turned this way and that in a delicate motion, surveying the world that lay below.
The head and neck remained in this position for some minutes, sweeping in lovely arcs of exploration. Apparently what the small eyes on either side of the projecting nose had seen reassured the beast, for now a new kind of motion ensued.
From the surface of the lake an enormous mound of dark flesh began slowly to appear, muddy waters falling from it as it rose. The body of the great reptile looked as if it were about twelve feet tall, but how far into the wa
ter it extended could not be discerned; it surely went very deep. Now, as the furry animal on shore watched, the massive beast began to move, slowly and rhythmically. Where the neck joined the great dark bulk of the body, little waves broke and slid along the flanks of the beast. Water dripped from the upper part of the animal as it moved ponderously through the swamp.
The reptile appeared to be swimming, its neck probing in sweeping arcs, but actually it was walking on the bottom, its huge legs hidden by water. And then as it drew closer to shore and entered shallower water, an enormous tail emerged. Longer than the neck and disposed in more delicate lines, it extended forty-four feet, swaying slightly on the surface of the lagoon. From head to tip of tail, the reptile measured eighty-seven feet.
Up to now it had looked like a snake, floundering through the lagoon, but the truth was revealed when the massive legs became visible. They were enormous, four pillars of great solidity but attached to the torso by such crude joints that although the creature was amphibious, it could not easily support itself on dry land, where water did not buoy it up.
With slow, lumbering strides the reptile moved toward a clear river that emptied into the swamp, and now its total form was visible. Its head reared thirty-five feet; its shoulders were thirteen feet high; its tail dragged aft some fifty feet; it weighed nearly thirty tons.
It was a diplodocus, not the largest of the dinosaurs and certainly not the most fearsome. This particular specimen was a female, seventy years old and in the prime of life. She lived exclusively on vegetation, which she now sought among the swamp waters. Moving her head purposefully from one plant to the next, she cropped off such food as she could find. This was not an easy task, for she had an extremely small mouth studded with minute peglike front teeth and no back ones for chewing. It seems incomprehensible that with such ineffectual teeth she could crop enough food to nourish her enormous body, but she obviously did.