Slowly but with irresistible power it broke through the earth’s crust and burst into daylight. In some cases, the sticky, almost congealed magma may have exploded upward as a volcano whose ash would cover thousands of square miles or, if the magma was of a slightly different composition, it would pour through fissures as lava, spreading evenly over all existent features to a depth of a thousand feet.
As the magma spread, the central purer parts solidified into pure granite. Most of it, however, was trapped within the crust, and slowly cooled and solidified into rock deep below the surface.
What amount of time was required for this gigantic event to complete itself? It almost certainly did not occur as a vast one-time cataclysm although it might have, engulfing all previous surface features in one stupendous wrenching that shook the world. More likely, convective movements in the mantle continued over millions of years. The rising internal heat accumulated eon after eon, and the resultant upward thrust still continues imperceptibly.
The earth was at work, as it is always at work, and it moved slowly. A thousand times in the future this irresistible combination of heat and movement would change the aspect of the earth’s surface.
This great event of three billion six hundred million years ago was different from many similar events for one salient reason: it intruded massive granite bodies which, when the mountains covering it were eroded away, would stand as the permanent basement rock. In later times it would be penetrated, wrenched, compressed, eroded and savagely distorted by cataclysmic forces of various kinds. But through three billion six hundred million years, down to this very day, it would endure. Upon it would be built the subsequent mountains; across it would wander the rivers; high above its rugged surface animals would later roam; and upon its solid foundations homesteads and cities would rest.
A relatively short distance below the surface of the earth lies this infinitely aged platform, this permanent base for action. How do we know of its existence? From time to time, in subsequent events, blocks of this basement rock were pushed upward, where they could be inspected, and tested and analyzed, and even dated. At other memorable spots throughout Colorado this ancient rock was broken by faults in the earth’s crust, and large blocks of it uplifted to form the cores of present-day mountain ranges.
This rock is beautiful to see—a hard, granitic pink or gray-blue substance as clean and shining as if it had been created yesterday. You find it unexpectedly along canyon walls, or at the peaks of mountains or occasionally at the edge of some upland meadow, standing inconspicuously beside alpine flowers. It is a part of life, an almost living thing, with its own stubborn character formed deep in the earth, once compressed by enormous forces and heated to hundreds of degrees. It is a poem of existence, this rock, not a lyric but a slow-moving epic whose beat has been set by eons of the world’s experience.
Often the basement rock appears not as granite but as unmelted gneiss, and then it is even more dramatic, for in its contorted structure you can see proof of the crushing forces it has undergone. It has been fractured, twisted, folded over to the breaking point and reassembled into new arrangements. It tells the story of the internal tumult that has always accompanied the genesis of new land forms, and it reminds us of the wrenching and tearing that will be required when new forms rise into being, as they will.
It must be understood that basement rock is not a specific kind of rock, for its components change from place to place. It has been well defined as ‘the layer of rock below which lies ignorance.’ In some places it hides far below sea level; at others it marks the tops of mountains fourteen thousand feet high. Throughout most of the United States it lies hidden, but in Canada it is exposed over large areas, forming a shield. Nor was it all laid down at the same time, for variations in its dating are immense. In Minnesota it was deposited more than three and a half billion years ago; in Wyoming, only two and a half billion years ago; and in Colorado, only a few miles to the south, at the relatively recent date of one billion seven hundred million years ago.
After the basement rock had been accumulated at Centennial, Colorado, later than almost anywhere else in the United States, an extraordinary event occurred. About two billion years of history vanished, leaving no recoverable record. By studying other parts of the West, and by making shrewd extrapolations, we can construct guesses as to what must have happened, but we have no proof. The rocks that should have been at hand to tell the story have either been destroyed beyond recognition or were never deposited in the first place. We are left in ignorance.
This situation is not confined to the region around Centennial, a small area in northwest Colorado, although there the gap is spectacular. At no spot in North America have we been able to find an unbroken sequence of rocks from earliest basement to recent sediment. Always there is some tantalizing gap. Over short distances it can have amazing variations in time and extent; for example, during the missing years at Centennial, massive accumulations of granite that would later form Pikes Peak were being assembled only a few miles to the south.
For hundreds of millions of years at a time the Centennial area must have lain at the bottom of the sea that at intervals covered much of America. The grains of sediment, eroded from earth masses remaining above sea level, would drift in silently and fall upon the basement, building with infinite slowness a sedimentary rock that might ultimately stand five thousand feet thick.
At other intervals the new-forming land would rise from the sea, to be weathered by storm and wind and creeping rivers long vanished. This cycle of beneath-sea, above-sea occurred at least a dozen times; repeatedly magma sent upward by the mantle broke through the crust and crept over the land; repeatedly erosion cut it away and left new forms quite different from their predecessors.
The time required! The slow passage of years! The constant alterations! Now part of an uplifted mountain, now sunk at the bottom of some sea, Centennial experienced wild fluctuations. Because of the erratic wanderings of the earth, the site stood sometimes fairly close to the equator, with a baking sun overhead; at subsequent times it might be closer to the North Pole, with ice in winter. It was a swamp during one eon, a desert the next. Whenever it came to temporary rest, it should have been exhausted, a worn-out land, but always new energies surged up from below, creating new experiences.
Those lost two billion years lie upon the consciousness of man the way vague memories or ghosts survive in the recollections of childhood. When man did finally arrive on the scene, he would be the inheritor of those vanished years, and everything he did would be limited to some degree by what had happened to his earth in those forgotten years, for it was then that its quality was determined, its mineral content, the value of its soil and the salinity of its waters.
About three hundred and five million years ago occurred what can be called the first event that left an identifiable record at Centennial. Within the mantle, forces developed that produced a penetration of the earth’s crust. The basement broke into discrete blocks, some of which were pushed upward higher than their surroundings, to relieve the pressure from below.
The resulting mountains covered much of central Colorado, following fairly closely the outlines that the historical Rocky Mountains would later occupy, and at the conclusion of five or ten million years they constituted a major range.
It was not born in cataclysm. There was no dramatic opening of earth from which fully formed mountains emerged. Nor was there any excess of volcanism. Instead, there was the slow, unceasing uplift of rock until the new mountains stood forth with majesty. They were the ancestral Rockies, and since they left behind them rocks that can be analyzed, we can construct for them a logical history.
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 fragments of rock and sand. High winds tore at their low summits, and freezing winters broke away protuberances. At interva
ls 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 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 the water and sediment pressing down, the silt and sand 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 that erected them in the first place had decided to erase them and bury the evidence.
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.
This extremely slow average rate does not preclude occasional catastrophes like earthquakes or floods that 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 is much like a prudent person 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 is, he will not escape ultimate death. The Earth loses 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 that 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, erosion of prehistoric and very ancient mountains had brought about deposits of sediment that had accumulated to a remarkable depth; at some places the layers were forty thousand feet thick. With the passage of time and in the presence of great pressure, they had formed into rock. Thrust and compression, uplift and subsidence had crumpled these rocks into contorted shapes.
The stage was now set for an event that 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 overland.
As the inexorable collision continued, there had to be some kind of dislocation along the edges that were bearing the brunt of the action. 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. Certainly, they were 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 now exists 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.
In their present condition the Appalachians 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 there was the portent of great events. 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 a rock that had formerly appeared in the ancestral Rockies�
�which is why we know so much about these 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 the explosions of gaseous ash, which sometimes accumulated to a depth of several hundred feet, compressing itself finally into a rock that still exists.
Especially awesome were the vast clouds of gaseous matter that 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 flow from volcanic fields.
Creatures of the Kingdom: Stories of Animals and Nature Page 4
