The Grand Canyon of Arizona [49]
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. M.
Present Conditions. With this general view of the great plateau in our mind's eye, we are prepared to examine present conditions at any given spot in the Canyon. Let us, therefore, take a seat at El Tovar, and try to read a few pages of the stone book of Creation as opened there. Suppose all this vast region at about sea level, and the uplift just beginning. The course of the Colorado River is already well defined. As the uplift continues, the cherty limestone and possibly the cross-bedded sandstone are both cut through, as the plateau slowly emerges. Whether the process of uplift is slow or rapid, as soon as a stratum emerges, it becomes subject to the influences of weathering, and the uppermost strata appearing first, they are weathered most. Hence the recession of the uppermost cliffs is greater than that of the cliffs lower down. The differences in hardness and resistance to weathering are alone responsible for the step-like profile of cliffs and terraces. The lower platform owes its width entirely to the rapid weathering and recession of the soft shales, which overlie the Tonto sandstones. The red-wall limestone, on the other hand, remains standing out as a cliff because of its exceeding durability.
The Faults. During the final uplift, the river cut through the Cambrian and Algonkian strata, and into the Granite Gorge as we find it to-day, and the process is still slowly going on. During these various periods of uplift, there were other changes occurring. Sometimes the uplift was uneven, certain parts of the plateau being lifted more rapidly than other parts; then occurred breaks in the strata, called faults. There are a great number of these faults in the plateau country, most of them crossing the Canyon from north to south. This faulting, as is readily seen, would produce cracks, and as the uneven uplift continued; the strata on one side of the crack would be lifted higher than the strata on the other side. Or, the strata on one side of the crack would be uplifted, while the other would subside.
Bright Angel Fault. El Tovar rests directly upon the strata affected by the Bright Angel Fault line. On going down the Bright Angel Trail, one cannot fail to see, as he passes the tap of the cross-bedded sandstone,
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. M.
Present Conditions. With this general view of the great plateau in our mind's eye, we are prepared to examine present conditions at any given spot in the Canyon. Let us, therefore, take a seat at El Tovar, and try to read a few pages of the stone book of Creation as opened there. Suppose all this vast region at about sea level, and the uplift just beginning. The course of the Colorado River is already well defined. As the uplift continues, the cherty limestone and possibly the cross-bedded sandstone are both cut through, as the plateau slowly emerges. Whether the process of uplift is slow or rapid, as soon as a stratum emerges, it becomes subject to the influences of weathering, and the uppermost strata appearing first, they are weathered most. Hence the recession of the uppermost cliffs is greater than that of the cliffs lower down. The differences in hardness and resistance to weathering are alone responsible for the step-like profile of cliffs and terraces. The lower platform owes its width entirely to the rapid weathering and recession of the soft shales, which overlie the Tonto sandstones. The red-wall limestone, on the other hand, remains standing out as a cliff because of its exceeding durability.
The Faults. During the final uplift, the river cut through the Cambrian and Algonkian strata, and into the Granite Gorge as we find it to-day, and the process is still slowly going on. During these various periods of uplift, there were other changes occurring. Sometimes the uplift was uneven, certain parts of the plateau being lifted more rapidly than other parts; then occurred breaks in the strata, called faults. There are a great number of these faults in the plateau country, most of them crossing the Canyon from north to south. This faulting, as is readily seen, would produce cracks, and as the uneven uplift continued; the strata on one side of the crack would be lifted higher than the strata on the other side. Or, the strata on one side of the crack would be uplifted, while the other would subside.
Bright Angel Fault. El Tovar rests directly upon the strata affected by the Bright Angel Fault line. On going down the Bright Angel Trail, one cannot fail to see, as he passes the tap of the cross-bedded sandstone,