At the center then, as today, was a ball of solid material, very heavy and incredibly hot, made up mostly of iron; this extended for about 770 miles. Around it was a cover about 1375 miles thick, which was not solid, but which could not be called liquid either, for at that pressure and that temperature, nothing could be liquid, as we know that word. It permitted movement, but it did not easily flow. It transmitted heat, but it did not bubble. It is best described as having characteristics with which we are not familiar, perhaps like a warm plastic.
Around this core was fitted a mantle of dense rock 1784 miles thick whose properties are difficult to describe, though much is known of them. Strictly speaking, this rock was in liquid form, but the pressures exerted upon it were such as to keep it more rigid than a bar of iron. The mantle was a belt which absorbed both pressure and heat from many directions and was, consequently, under considerable stress. From time to time throughout this story, the pressures will become so great that some of the mantle material will force its way toward the surface of the earth, undergoing marked change in the process. The resultant body of molten liquid, called magma, will solidify to produce the igneous rock, granite, but if it is still in liquid form as it approaches the surface, lava results. It was in the mantle that many of the movements originated which would determine what was to happen next to the visible structure of the earth, and although we shall not often refer again to the mantle, we must remember it deep beneath our feet, accumulating stress and generating enormous heat as it prepares for its next dramatic excursion toward the surface, producing the magma which will appear as either granite or lava.
At the top of the mantle, only twenty-seven miles from the surface, rested the earth’s crust, where life would develop. What was it like? It can be described as the hard scum that forms at the top of a pot of boiling porridge. From the fire at the center of the pot, heat radiates not only upward, but in all directions. The porridge bubbles freely at first when it is thin, and its motion seems to be always upward, but as it thickens, one can see that for every slow bubble that rises at the center of the pan, part of the porridge is drawn downward at the edges; it is this slow reciprocal rise and fall which constitutes cooking. In time, when enough of this convection has taken place, the porridge exposed to air begins to thicken perceptibly, and the moment the internal heat stops or diminishes, it hardens into a crust.
This analogy has two weaknesses. The flame that keeps the geologic pot bubbling does not come primarily from the hot center of the earth, but rather from the radioactive structure of the rocks themselves. And as the liquid magma cools, different types of rock solidify: heavy dark ones rich in iron settle toward the bottom; lighter ones like quartz move to the top.
The crust was divided into two distinct layers. The lower and heavier, twelve miles thick, was composed of a dark, dense rock known by the made-up name of sima, indicating the predominance of silicon and magnesium. The upper and lighter layer, fifteen miles thick, was composed of lighter rock known by the invented word sial, indicating silicon and aluminum. The subsequent two miles of Colorado’s rock and sediment would eventually come to rest on this sialic layer.
Three billion, six hundred million years ago the crust had formed, and the cooling earth lay exposed to the developing atmosphere. The surface as it then existed was not hospitable. Temperatures were too high to sustain life, and oxygen was only beginning to accumulate. What land had tentatively coagulated was insecure, and over it winds of unceasing fury were starting to blow. Vast floods began to sweep emerging areas and kept them swamp-like, rising and falling in the agonies of a birth that had not yet materialized. There were no fish, no birds, no animals, and had there been, there would have been nothing for them to eat, for grass and trees and worms were unknown.
There were in existence, even under these inhospitable conditions, elements like algae from which recognizable life would later develop, but the course of their future development had not yet been determined.
The earth, therefore, stood at a moment of decision: would it continue as a mass with a fragile covering incapable of sustaining either structures or life, or would some tremendous transformation take place which would alter its basic surface appearance and enlarge its capacity?
Sometime around three billion, six hundred million years ago, the answer came. Deep within the crust, or perhaps in the upper part of the mantle, a body of magma began to accumulate. Its concentration of heat was so great that previously solid rock melted partially. The lighter materials were melted first and moved upward through the heavier material that was left behind, coming to rest at higher elevations and in enormous quantities.
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 degree 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 titanic wrenching which 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 it rides, this infinitely aged platform, this permanent base for action. How do we know of its existence? From time to time, in subsequent events which we shall observe, blocks of this basement rock will be pushed upward, where they can be inspected, and tested, and analyzed, and even dated. At other memorable spots throughout Colorado this incredibly ancient rock will be broken by faults in the earth’s crust, and large blocks of it will be uplifted to form the cores of present-day mountain ranges.
It is beautiful to see, as it sticks its head into daylight, 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 bowels of the earth, once compressed by titanic 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, later than almost anywhere else in the United States, one of the most extraordinary events 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 which 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 small area around Centennial, 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 a 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 which would later form Pikes Peak were being assembled only a few miles to the south.
For hundreds of millions of years at a time Centennial must have lain at the bottom of the sea which 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 which 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 was repeated 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 much 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, it 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 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, generating 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 which left an identifiable record at Centennial, and with it our story begins. Within the mantle, forces developed which 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 in considerable majesty. They were the Ancestral Rockies, and since they left behind them rocks which can be analyzed, we can construct for them a logical history.
They were not soaring peaks, like their successors, but they did rise from sea level and would have seemed higher above their pediment than today’s Rockies, which although they lift far into the sky, take their start from plains already high.
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.