Eocene Lake. We know this was an inland sea, and had no connection with the ocean, for all the fossils and sediments deposited in it reveal that they are fresh-water organisms. In this sea, as in the earlier oceans, vast deposits of sediment were made in the early Eocene period, and another period of subsidence occurred. Then the great lake was drained, and the uplift began, slow and sure; then, and not before, were the conditions existent that have made the Canyon country as we see it to-day. Peaks and islets received the rainfall, tiny rivers were formed that grew larger and cut their way in deeper, as the uplift continued. The principal stream, which was then born, was the Colorado. It is supposed, from various evidences, that the rainfall was very much more abundant then than now, and consequently the rivers had greater flow, and more eroding and carrying capacity. The uplift continued, and the geologists tell us it did not cease until about fifteen thousand feet, deposited since Cretaceous times, were thrust up into the air. As almost all this mass of deposition has disappeared from the immediate Canyon region, we are compelled to believe that it has been swept away down the Colorado River to join the sands of the Carboniferous and later periods in the Colorado Desert, the Salton Basin, the great low region of Lower California, and the Gulf itself.
Less by Erosion in the Canyon Region. Now figure out for a few moments the results of these different erosive periods. Eleven thousand five hundred feet of Algonkian gone; a small amount of erosion in the Cambrian epoch, the depth of which is unknown; and then the great denudation of the Eocene period sweeping away upwards of fifteen thousand feet of strata, give us a total of twenty-six thousand five hundred feet that have totally disappeared from the Canyon region. A vertical mile is five thousand two hundred and eighty feet. Mount Washington is about six thousand five hundred feet above the sea,--a trifle higher than Mount Lowe, near Pasadena, California. Take off from this six thousand five hundred feet, say one thousand five hundred feet, for the level of the country at the base of these two mountains, and then imagine a region five times as high as both of them, covering an area of country of possibly thirteen thousand to fifteen thousand square miles, slowly planed off by the erosive forces of nature.
Formation of River Beds. How was it done? I have spoken of the peaks and islets that first emerged from the Eocene Sea, and received the rains. Down their slopes ran the earliest watercourses, first as rills, then as creeks, finally as rivers. The higher the peaks ascended, the more the accompanying land was lifted up, and therefore the longer and deeper became the rivers.
The course of a river once established, it is exceedingly difficult to change it--hence the law that geologists call "the persistence of rivers."By and by, the uplifted country appeared as one vast area of river valleys, separated by stretches of plateau. Little by little, working by laws that are pretty well understood, the swift flowing avers cut downwards. When their velocity ceased, the widening of the river courses began, and progressed with greater rapidity, so that, in time, the divides that intervened between the rivers were worn away,--a process rudely shown in Fig. 5 A. B. C. and D. of plate on page 110.
The Formation of the Canyon. Now, in imagination, let us hark back to the day when this plateau was in the condition thus described. Nearly everything in the way of strata has been planed down to the Carboniferous rocks. The plateau is about at sea level. One great river already exists, with two arms, now called the Green and the Grand, the main river some day to be known as the Colorado. Slowly the uplift begins. It is a fairly even process, and yet there is slightly more pressure brought to bear under the southern portion, so that the whole mass has a slight tilt to the north.
Professor Salisbury found certain beds of rock at seven thousand eight hundred feet above sea level at the base of the San Francisco Mountains near Flagstaff. Forty-five miles north, at the Grand Canyon, these same beds are only six thousand four hundred feet above sea level, while at the Vermilion Cliffs, another forty-five miles to the north, they are but four thousand four hundred feet above the sea.
Yet in spite of this northward tilt, when the eye ranges over the country to the south and west, from the upper porch of El Tovar, a large area of depression can clearly be seen, showing that surface erosion has planed away much of the upper crust.
The Plateau Region. Now we are ready to take a look at the borders of the plateau region. On the north, it extends into Utah, where still higher plateaus bound it. To the west, it extends by gigantic steps into the desert region. The main step is along the Grand Wash, near the one hundred and fourteenth meridian. To the south, there is one glorious step, known as the Mogollon Escarpment (locally the Red Rock Country), some three thousand feet high, which extends for a number of miles east and west, and then breaks down. This step and broken levels lead to the irregular lands of Central and Southern Arizona. On the east, the plateau extends to the Echo Cliffs beyond Marble Canyon, and as far as the ridge of the Continental Divide, where the Santa Fe crosses the Zuni Mountains, east of Gallup, N.
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