The Coconino sun., 1925-10-30
GEOLOGICAL FORMATION OF
GRAND CANYON BIG ITEM IN
THE SELECTION OF DAM SITES

The unschooled visitor to the Grand
Canyon is apt to stand on the rim
anywhere and, being told that a surg
ing stream of water flows in the bot
tom, believe that a dam thrown across
the river at any point, would do the
business of harnessing the river, says
Engineer LaRue.
How lar this is from the truth is
revealed in the geological studies of
the canyon, contributed to the LaRue
report by R. c. Moore, geologist who
accompanied the Birdseye expedition
down the river two years ago. Mr..
Moore has studied the formations
from one end of the Grand Canyon to
the other. But on his boat trip with
the adventuresome party in 1923, he
had the particular object of looking
the ground over with reference to
possible dams. He therefore took
special sort of pains to get informa-
tion about the lay of the rock forma-
tions, the character of the strata as
affecting pofosity and the quality of
building material at all possible dam
sites.
Not all of the Grand Canyon would
make good reservoir foundation.
There are sections of the gorge that
are perhaps as close to the ideal as
can be found anywhere on the globe.
But geology, which has stood in won
der and admiration on the brink of
this chasm, viewing at glance strata
which in flatter country requires
years of surveying to define, has its
most important contribution to be
made to the formation of the Colo
rado river development program.
What Geology Does
The science of the rocks contributes
these things to the consideration of
the Colorado:
/. Relative values of damsites for
holding both the dam foundations and
the stored water.
2. Character of the reservoir area
for storing water, both from the view
point of porosity and capacity.
g. Convenience of easily available
ard useable building material at the
various damsites.
1t is peculiar to some rock that it
does not erode easily, but cleaves off
on both sides of an eroding stream to
form vertical or nearly vertical walls.
Wherever the Colorado river runs
through narrow gorges, it is because
it is there cutting through refractory
rock layers. In these gorges, hap
PLY enough, there are the best dam
sites for the reason that the channel
is in bedrock without lot of loose
material, such as deposited silt, sand
and boulders to dig through before
laying the foundations for a struc-
ture On the other hand, some of the
narrow damsites are unfavorable for
the reasonl that there is not the avail
able basin back of them for storing
huge quantities of flood water.
At other points in the Grand Can
yOn, the walls widen out, the river
bed is in soft detritus material, often
thrown up in the form of bars. Here
the soil through which the river is
cutting is soft and tends to shape it
self in slopes, rising more or less
steeply away from the stream to the
spot where next the hard stratum is
encountered. 1t is the alienation Of
softer and harder rock or soils in the
nearly level beds of the Grand Can
yon country that gives to that gorge
its spectacle qualities, allowing the
river and its tributaries and local ero-
sion to form the fantastic shapes of
its spires, buttes, walls and escarp
ments.
There are in the canyon, successive
layers of rock that give good holding
ground for dams and reservoirs. The
highest stratum, the one found in the
upper part of the Grand Canyon in
the vicinity of Lees Ferry, is the
Kaibab limestone Unlike small local
occurrences of limestone, this shows
little tendency TO solution by ground
waters: Mr.. Moore therefore recon-
mends dansites in this material and
ventures the opinion that although
there are occasional springs, he does
not believe any of the reservoirs will
leak through clefts in the strata.
Below the Kaibab limestone verti-
cally and found lower in the gorge
there is the Coconino sandstone, form
ing erect walls, but somewhat perme-