The stage was now set for an event which would elevate the rocks into a mountain range. It occurred when the subterranean plate on which rested the crust that was later to become part of the continent of Africa began to move slowly westward. In time the migration of this plate became so determined—and perhaps it was matched by a comparable movement of the American plate eastward—that collision became inevitable. The predecessor of the Atlantic Ocean was squeezed so severely that it was entirely eliminated. The continents came into actual contact, so that such living things as then existed could move from America to Africa and back again over land.
As the inexorable collision continued, there had to be some kind of dislocation along the edges that were bearing the brunt. It seems probable that the edge of the African plate turned under, its rocky components returning to the crust and perhaps even back into the mantle. We know that the edge of the American plate was thrust upward to produce the Appalachian Mountains, not some ancestral Appalachians, but the roots of the very mountains we see today.
After some twenty million years of steady growth the Appalachians stood forth as a more considerable range than the Ancestral Rockies had been. They were, of a certainty, some of the world’s most impressive mountains, soaring thousands of feet into the air.
Inevitably, as soon as they began to emerge, the tearing-down process commenced. First the continental plates drifted apart, with Africa and the Americas winding up in roughly the positions they occupy today. The Atlantic Ocean as it exists today started to develop, its deep inclines providing a basin for the catchment of rock and silt eroded from the heights. Volcanoes operated and at intervals enormous fractures occurred, allowing vast segments of the range to rise while others fell.
As early as a hundred million years ago the Appalachians—only a truncated memory of their original grandeur—began to assume their present shape; they are thus one of the oldest landscape features of the United States. At this time the Appalachians had no competition from the Rocky Mountains, for that range had not yet emerged; indeed, most of America from the Appalachians to Utah was nothing but a vast sea from which substantial land would rise only much later.
The Appalachians play no further part in this story—except that a stubborn Dutchman who grew up along their Bank will travel westward to Centennial in his Conestoga—and in their present condition they seem a poor comparison to the Rockies. They are no longer high; they contain no memorable landscape; they do not command great plains; and they are impoverished where minerals like gold and silver are concerned. But they are the majestic harbingers of our land; they served their major purpose long before man existed, then lingered on as noble relics to provide man with an agreeable home when he did arrive. They are mountains of ancient destiny, and to move among them is to establish contact with a notable period of our history.
They have been mentioned here to provide a counterbalance to the great things that were about to happen in the west. About seventy million years ago much of the western part of America lay beneath a considerable sea, and if this configuration had persisted, the eastern United States would have been an island much like Great Britain, but dominated by the low-lying Appalachians.
But beneath the surface of the inland sea great events portended. The combined weight of sediment and water, pressing down upon a relatively weak basin area, coincided with an upsurge of magma from the mantle. As before, these magmatic pressures from below pushed upward huge blocks of the basement, and bent the more flexible layered rocks above the basement until a massive mountain range had been erected. The range, running from northern Canada almost to Mexico, was both longer and wider than the Ancestral Rockies had ever been and placed somewhat farther east. Its major elevations stood very high, and as these areas were uplifted, the inland sea was drained off.
The mountain range was composed in part of rock that had formerly been utilized in the Ancestral Rockies—which is why we know so much about those ancient mountains we have never seen—and formed one of the world’s major structural forms, which it still does.
The Rockies are therefore very young and should never be thought of as ancient. They are still in the process of building and eroding, and no one today can calculate what they will look like ten million years from now. They have the extravagant beauty of youth, the allure of adolescence, and they are mountains to be loved.
Their history is reasonably clear. Not all were born as a result of basement block uplift, for certain mountains were squeezed upward by vast forces acting laterally. Others may have arisen as a result of some movement of the American plate. And we have visible proof that some of the southern mountains were built by spectacular action.
About sixty-seven million years ago volcanic activity of considerable range and intensity erupted throughout Colorado. As the mountains rose, the crust cracked and allowed lava to rise to the surface in great quantity. Lava flows were extensive, but so were explosions of gaseous ash, which sometimes accumulated to a depth of several hundred feet, compressing itself finally into rock which still exists.
Especially awesome were the vast clouds of gaseous matter which drifted eastward, with internal temperatures rising to thousands of degrees. Whatever they passed over they killed instantly through the exhaustion of oxygen, and when their temperatures fell, the clouds fell too. Their contents then solidified to form crystalline rock, one cloud producing enough to blanket large areas to a depth of seven or eight feet. In other areas, lakes were formed, dammed by lava flows from volcanic fields.
Now for the first time we come to the river which will command our 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 the heights. For millions of years it was not the dominant river of the region; in fact, five competing rivers led eastward from the Rockies, their 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 carry. The area covered by its deposits was about three hundred and twenty miles long and one hundred and forty 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 burden was sand and silt. Its flow was irregular; at times it would wander fifty miles wide across plains; 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 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 super burst, commanding the heavens with its burden of fire and lava; eruptions continued over a period of fifteen million years, and the wealth of 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 or Popocatepetl. Obviously, they could not have been in constant eruption; there had to be periods of long 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 thousan
d 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 which 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 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 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 Centennial; it halted some distance to the north. But at high elevations in the Rockies, small glaciers did form and filled the valleys, and when 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 proportion. 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 which 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, when 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 which had required ten million years to accumulate.
It was the river which had laid down the new land; it was the river which 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 would always be.
The major characteristics of land around Centennial were now fairly well determined and there is little more to report. There were, however, four special places of no great consequence in the grand scheme of things but around which much of this story will revolve.
The first was a chalk cliff running north-south some miles northwest of Centennial. Its basic components had been laid down some two hundred and seventy million years ago during that period when the Ancestral Rockies were disintegrating and washing out to sea. At the bottom of this sea huge beds of limestone accumulated in flat layers, one above the other like sheaves of paper in a pile. This limestone was infinitely older than the New Rockies and constituted yet another example of how the earth’s material was used, broken down, accumulated, conserved and reissued in new form.
For about one hundred million years this limestone bed lay flat, sometimes exposed but usually at the bottom of some sea. Then internal turbulence within the mantle uplifted the area so that it stood as high as some mountains. No sooner had it moved into its elevated position than it was visited by a racking accident: a large fault wrenched the surface of the earth, depressed the area and cracked the limestone along its north-south axis. The eastern portion dropped some eighty feet below its previous level, while the western half rose twenty, forming a white chalk cliff about one hundred feet high.
There it stood, one hundred and thirty-six million years ago, a chalky white cliff with a rain forest on its upper plateau and a great swamp bathing its feet, ready for the dramatic incidents which would occur along its margin.
The second place was a moderately high mountain valley to the west of Centennial and slightly to the south. A small stream of water ran the length of the valley before joining the river; it had been the cause of the valley’s existence. The valley was not ancient, for it developed only in the later stages of mountain building; it could not have been any more than forty million years old, but throughout its brief life it had always been a place of exceptional beauty.
It ran almost due east and west and was only a few miles long. Its sides were formed by steep mountains which hemmed it in; it was not wide, the mountain walls being less than a mile apart, and it had a gentle fall, the higher end being at the west. Its beauty was gemlike rather than expansive.
During its existence it had undergone little change. It had started at an elevation of only four thousand feet, but at the great uplifting fifteen million years ago it had been raised to an elevation of ten thousand feet. Subsequent erosion had lowered this to eight thousand, just low enough to make possible one of the features which made it memorable.
On the north wall, which of course received the sun, a thick grove of aspen trees developed about one million years ago to form a thing of joy. The trees heralded spring as it was about to appear, their small gray-green leaves shimmering in the sun. In midsummer their leaves were exquisite, for they were attached to their branches in a peculiar manner which left them free to flutter constantly; the slightest breath of air set the aspen shaking so that at times the entire north wall of the valley seemed to be dancing. It was in the autumn, however, that the aspen came into its true glory, for then each leaf turned a brilliant gold, so that a single tree seemed an explosion of vibrating loveliness. In autumn this valley was a place of beauty unmatched across the entire continent.
But curiously, the valley would take its name from a much different kind of tree, which clustered on the dark south wall where the sun did not shine. It was an evergreen. Now, there were many types of evergreens in the New Rockies; they might be considered the symbolic tree of the region, but the one that grew in this valley was different, for it was not green but a spectacular blue. It was the blue spruce, a tree of dignified proportions and splendid color. It grew much taller than its neighbor across the stream, and larger. And it was not deciduous, so that in late autumn when the aspen stood bare, its golden leaves having vanished one by one, the glory of the blue spruce came into its own. In winter, when clinging snow covered the spruce, allowing patches of blue to show, the valley was a quiet, d
reamlike place, so lovely that even passing animals instinctively found refuge here. Throughout the balance of the year the tall spruce trees showed noble coloring, from powdery blue to indigo.
In historic times, the place would be named Blue Valley, a notable setting, well proportioned in all things. If it did not lie at the top of the mountains, it did not hide at the bottom, either. Its stream was capable but never turbulent, and although snow sometimes crowded the floor of the valley, it rarely fell so deep that the valley itself became inaccessible. Under any circumstances this delectable valley, with its gold and blue trees, would have been memorable, but two events made it even more so.
When permanent ice began to form in the highest mountain valleys, it became only a matter of time until some glacier shoved its icy snout into Blue Valley. This happened, and the front edge of the glacier gouged out the bottom lands, widened the base of the valley and scoured the walls of the mountains that enclosed it. Of course, every tree within the valley was destroyed, but ages later, when the ice had receded, the trees reestablished themselves as if nothing serious had interrupted them, and the valley was now much more pleasant than it had been before, since the glacier had carved out a spacious meadow filled on the north bank with aspen and on the south with blue spruce.