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
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
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