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.
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, the break
in the strata. To the left it is fully one hundred and fifty to two hundred
feet higher than it is on the right. The same depression may be observed in
driving out to Hopi, Point, or returning. The stratum on which the road is
made should be at the same level as the stratum on which El Tovar rests.
Fault at Bass Camp. This fault is but one of a score or more on the
plateau. At Bass Camp there was a fault which displaced the strata on each
side of the "break" to the extent of four thousand feet. Later, another
fault occurred, which readjusted the displacement somewhat, and reduced the
difference to two thousand feet, yet left the evidences of the former wide
divergence. It was also during these uplift periods that the volcanic
mountains of the region came into existence, as the San Francisco Range,
Mounts Kendricks, Sitgreaves, Williams and Floyd on the south, and the
Uinkarets - Mounts Trumbull, Logan, Emma - on the north.
Lava Flows. In one place, south of Mount Emma, Powell's party saw where
vast floods of lava had flowed from it into the river. They declare that "a
stream of molten rock has run up the Canyon three or four miles, and down,
we know not how far. The whole north side, as far as we can see, is lined
with the black basalt, and high up on the opposite wall are patches of the
same material, resting on the benches, and filling old alcoves and caves,
and giving to the wall a spotted appearance." All these volcanic mountains
can be seen from Hopi or Yavapai points, near El Tovar.
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