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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