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.
Enter page number
PreviousNext
Page 70 of 167
Words from 35549 to 36062
of 85893
