The Grand Canyon of Arizona [45]
as the Grand Canyon Country. It was at the time when life was beginning to dawn, for in the remnants of the strata are found fossils of the earliest known life. These strata, therefore, are of immense interest to the geologist, as they are the first known rocks containing life to emerge from the primeval sea. Within the last few years, they have been called the Algonkian Series, and later I shall speak of them more freely.
Prior to the deposition of these Algonkian strata, the Laurentian rocks (the granite) upon which they rest were subject to a long period of "planation,"--as the grinding down and leveling of rock surfaces is termed. After this planation was complete, a subsidence occurred; the whole area became the bed of an inland sea, and upon the planed-down granite, the debris that formed the Algonkian strata was washed.
While they were being deposited, the whole region was the scene of several seismic and volcanic disturbances, for great dykes and "chimneys" of lava are found, showing clearly that, by some means or other, the strata were broken and shattered, cracked and seamed, and that through these cracks the molten lava oozed--forced up from the interior of the earth. It spread out over the Algonkian rocks in small sheets or blankets, which here and there are still to be found to-day.
Tilting of the Algonkian Strata. Slowly this twelve thousand feet of strata emerged into the sunlight. In the uplifting processes, the surface of the earth, where they were, became tilted, and these strata therefore "dipped" or "tilted" away from the perpendicular. As they emerged, weathering and erosion began. It is most probable that this process of degradation began and continued while the topmost strata were at or near sea level, so that it was a simultaneous process with the uplift.
Erosion of the Algonkian. How many centuries this weathering and washing away process consumed no one knows. At the close of this epoch, however, the Algonkian strata had been eroded almost away, owing to its tilted condition, so that in some places even the surface of the Archaean was exposed, and suffered the planing-down process. Figure 1 on plate facing page 98 is a suggestion as to the possible appearance of the rocks at this time.
Even then, in those far-away, early ages of history, if one had been present to measure these strata, he would have discovered the astounding fact that, although he had measured them and found twelve thousand feet before they began to emerge from the ocean, there were but about five hundred feet of them left. This is one of the interesting facts in geology,--that an observant reader can deduce so much from so little.
The twelve thousand feet deposit. "But," asks the layman, "I cannot possibly see how, if only five hundred feet of strata are left, any one could ever tell that there were once twelve thousand feet. If eleven thousand five hundred feet are gone, how do you know they ever existed?"
A very reasonable question and one very easily answered. Refer to the sketch. Let the bracket on the right show the present width of the remaining strata, viz: five hundred feet. Now observe the tilted condition of the remnants. To get the original height of the depositions begin with No. 1, the stratum nearest the Archaean and measure that. Suppose it gives us five hundred feet. No. 2 gives two hundred feet; No. 3, five hundred feet; No. 4, one hundred and seventy-five; and so on up to No. 14. As these strata were deposited horizontally, all we have to do is to mentally replace them in their horizontal position. Throw the tilted strata back again into their original condition, and by this method of measurement it is seen that the twelve thousand feet can be made up. Figure 2, facing page 98.
Another interesting question here arises: "What became of the vast quantity of sand and silt and pebbles that formed and were carried away during such a gigantic process? For, think of it, eleven thousand five hundred feet of strata, or rock, two miles high, almost three times as high a mass as the present distance
Prior to the deposition of these Algonkian strata, the Laurentian rocks (the granite) upon which they rest were subject to a long period of "planation,"--as the grinding down and leveling of rock surfaces is termed. After this planation was complete, a subsidence occurred; the whole area became the bed of an inland sea, and upon the planed-down granite, the debris that formed the Algonkian strata was washed.
While they were being deposited, the whole region was the scene of several seismic and volcanic disturbances, for great dykes and "chimneys" of lava are found, showing clearly that, by some means or other, the strata were broken and shattered, cracked and seamed, and that through these cracks the molten lava oozed--forced up from the interior of the earth. It spread out over the Algonkian rocks in small sheets or blankets, which here and there are still to be found to-day.
Tilting of the Algonkian Strata. Slowly this twelve thousand feet of strata emerged into the sunlight. In the uplifting processes, the surface of the earth, where they were, became tilted, and these strata therefore "dipped" or "tilted" away from the perpendicular. As they emerged, weathering and erosion began. It is most probable that this process of degradation began and continued while the topmost strata were at or near sea level, so that it was a simultaneous process with the uplift.
Erosion of the Algonkian. How many centuries this weathering and washing away process consumed no one knows. At the close of this epoch, however, the Algonkian strata had been eroded almost away, owing to its tilted condition, so that in some places even the surface of the Archaean was exposed, and suffered the planing-down process. Figure 1 on plate facing page 98 is a suggestion as to the possible appearance of the rocks at this time.
Even then, in those far-away, early ages of history, if one had been present to measure these strata, he would have discovered the astounding fact that, although he had measured them and found twelve thousand feet before they began to emerge from the ocean, there were but about five hundred feet of them left. This is one of the interesting facts in geology,--that an observant reader can deduce so much from so little.
The twelve thousand feet deposit. "But," asks the layman, "I cannot possibly see how, if only five hundred feet of strata are left, any one could ever tell that there were once twelve thousand feet. If eleven thousand five hundred feet are gone, how do you know they ever existed?"
A very reasonable question and one very easily answered. Refer to the sketch. Let the bracket on the right show the present width of the remaining strata, viz: five hundred feet. Now observe the tilted condition of the remnants. To get the original height of the depositions begin with No. 1, the stratum nearest the Archaean and measure that. Suppose it gives us five hundred feet. No. 2 gives two hundred feet; No. 3, five hundred feet; No. 4, one hundred and seventy-five; and so on up to No. 14. As these strata were deposited horizontally, all we have to do is to mentally replace them in their horizontal position. Throw the tilted strata back again into their original condition, and by this method of measurement it is seen that the twelve thousand feet can be made up. Figure 2, facing page 98.
Another interesting question here arises: "What became of the vast quantity of sand and silt and pebbles that formed and were carried away during such a gigantic process? For, think of it, eleven thousand five hundred feet of strata, or rock, two miles high, almost three times as high a mass as the present distance