Centennial (6 page)

From the moment of their birth they participated in a startling series of events. No sooner had they pushed their crests above the flat surface of the land than small streams began to nibble at their flanks, eating away small fragments of rock and sand. High winds tore at their low summits, and freezing winters broke away protuberances. At intervals earthquakes toppled insecure rocks; at other times inland seas lashed at their feet, eroding them further.

As the mountains increased in age, the small streams grew into rivers, and as they increased in volume, they also increased in carrying power, and soon they were conveying broken bits of mountain downward, cutting as they went and forming great alluvial fans along the margins of the range.

In a beautiful interrelationship, the mountains continued to push upward at about the rate at which the eroding forces were tearing them down. Had the mountains been permitted to grow unimpeded, they might have reached heights of twenty thousand feet; as it was, the system of balances kept them at some undetermined elevation, perhaps no more than three or four thousand feet.

And then, for some reason, the upward pressures ceased, and over a period of forty million years this once formidable range was razed absolutely flat by erosion, with not a single peak remaining as a memento of what had been one of the earth’s outstanding features. The fabled Ancestral Rockies, a masterpiece of landscape, vanished, its component rocks reduced to rubble and scattered across the growing plains of eastern Colorado, Kansas and Nebraska. Mountains that had commanded the landscape had become pebbles.

Later, as if to seal off even the record of their existence, the land upon which they had stood was submerged spasmodically over a period of eighty to ninety million years in the Jurassic and Cretaceous periods, the era of the dinosaurs. Clay, silt and sand were moved in by rivers emptying into the inland sea, filtering down slowly, silently in the darkness, accumulating in soft layers. But with the passage of time and the weight of water and sediment pressing down, it gradually solidified into layers of rock thousands of feet thick. Thus the roots of the once great mountains were sealed off, as if the forces which had erected them in the first place had reconsidered, erased them and then buried the evidence.

It is essential to comprehend the meaning of time. When a mountain ten thousand feet high vanishes over a period of forty million years, what has happened? Each million years it loses two hundred and fifty feet, which means each thousand years it loses three inches. The loss per year would be minuscule and could not have been detected while it was happening.

This extremely slow average rate does not preclude occasional catastrophes like earthquakes or floods which might compress into one convulsion the losses for an average millennium. Nor does it mean that the debris could be easily removed. These mountains covered an extensive land area, and even a trivial average loss, if applied over that total area, would require much riverine action to carry the eroded materials away.

The fact remains that an enormous mountain range had vanished.

Since this seems a prodigal action, extremely wasteful of motion and material, a caution must be voiced. The rocks that were lifted from the depths of the earth to form the Ancestral Rockies had been used earlier in the construction of other mountain ranges whose records have now vanished. When those predecessor ranges were eroded away, the material that composed them was deposited in great basins, mainly to the west.

The earth was much like a prudent man who has an allotted span of life and a given amount of energy. Using both wisely, conserving where possible, he can enjoy a long and useful life; but no matter how prudent, he will not escape ultimate death. The earth uses its materials with uncanny thrift; it wastes nothing; it patches and remodels. But always it expends a little of its heat, and in the end—at some unpredictable day billions of years from now—that fire will diminish and earth, like man, will die. In the meantime, its resources are conserved.

While the Ancestral Rockies were disappearing, an event which was to leave still-visible consequences was reaching its climax along the eastern shore of what would later be known as the United States. The time was about two hundred and fifty million years ago; during preceding periods, reaching very far into the past, a building process of beautiful complexity had been operating. Into the deep ocean depressions east of the wandering shoreline, prehistoric and very ancient mountains had deposited sediments that had accumulated to a remarkable depth; at some places they were forty thousand feet thick. With the passage of time and in the presence of great pressure, they had of course formed into rock. Thrust and compression, uplift and subsidence had crumpled these rocks into contorted shapes.

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 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 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.