View to the
southwest with Rider Canyon in the right foreground, North
Canyon near the center, and the Roaring Twenties Rapids in the
distance (where the river bends slightly out of sight to the
right).
The Colorado River continues to cut ever deeper
into the Marble Platform. The surface layer here is still the
Kaibab Limestone. Within the canyon, the Kaibab, Toroweap, and
Coconino Formations form the upper cliff. Then the Hermit
Shale forms the slope half way down while the Supai forms the
cliff down to river level. Just beyond North Canyon, the total
depth of the canyon passes 2,000 feet.
The distinct cliff-slope-cliff pattern that is
characteristic of the Grand Canyon (including the side canyons
as well as the central gorge) is a product of direct cutting
by the river augmented by gradual erosional widening into the
multiple horizontal rock layers. Abrasive action by sand and
rocks carried by the river produces a "sand blasting" effect
that gradually deepens the channel. However, the main function
of the river is to transport sand and gravel downstream and
redeposit this silt in its delta (Tries to fill the gradually
widening rift that has produced the Gulf of California).
As soon as river down-cutting exposes new surface
area to atmospheric conditions, rock falls, flash floods
(excavate side canyons), creep, slumps, etc. go to work to
widen the canyon. (When eroded material gets down to the river
it is removed as above.) Erosion is easiest in the shale
layers as these have the weakest chemical bonds. If a shale
layer initially has a steep slope, then the top of the shale
layer will erode rapidly while any underlying, harder
limestone or sandstone layer will support the base of the
shale. Thus the steepness of the shale layer becomes less
(which decreases the erosion rate) until the rate of erosion
is in equilibrium with other layers. Since this weakness
within the shale layer is more or less constant wherever it is
exposed, the resulting surface steepness (slope) of the shale
has approximately the same angle everywhere it is exposed
whether it is in the main gorge or a side canyon.
View to the
southwest with North Canyon in the right foreground, the
Roaring Twenties Rapids in the center where the river
curves to the right, and Shinumo Wash entering from the
left in the distance.
The river gradient in this section reaches 15
feet per mile centered at mile marker 24 - the steepest
portion of the entire Marble Canyon section of the Grand
Canyon. There are rapids at mile markers 23, 23.5, 24,
24.5, 25, and 25.5 as noted in Belknap's Grand Canyon
River Guide.
The steeper gradient is probably a result of
a small fault (the "Fence Fault") that can been seen
entering near the center of the left edge and continuing
diagonally upward to near the center of the top edge. The
far side of this fault has been down dropped some 50 to
100 feet (probably fairly recently). The fault itself
intersects the river just beyond mile 30. The strata
displacement allows the river to drop more rapidly as it
crosses the fault, and this additional drop quickly works
it way back upstream to enhance canyon cutting.
Return
to river miles 8 to 16
Continue
to
river miles 24 to 32
Return
to
the Index Page for the Grand Canyon Tour
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