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Grand Canyon
Immense gorge cut by the Colorado River into the high
plateaus of
northwestern Arizona, U.S., noted for its fantastic shapes and
coloration.
The broad, intricately sculptured chasm of the Grand
Canyon contains
between its outer walls a multitude of imposing peaks, buttes, canyons,
and
ravines. It ranges in width from about 0.1 to 18 miles (0.2 to 29 km)
and
extends in a winding course from the mouth of the Paria River, near the
northern boundary of Arizona, to Grand Wash Cliffs, near the Nevada
line, a
distance
of
about 277 miles (446 km). The canyon includes many tributary side
canyons and
surrounding plateaus. The deepest and most impressively beautiful
section, 56
miles (90 km) long, is within Grand Canyon National Park, which
encompasses the
river's length from Lake Powell to Lake Mead. In its general colour,
the canyon
is red, but each stratum or group of strata has a distinctive hue--buff
and
gray, delicate green and pink, and, in its depths, brown, slate-gray,
and
violet. At 8,200 feet (2,500 m) above sea level, the North Rim is 1,200
feet
(350 m) higher than the South Rim.
The first sighting of the Grand Canyon by a European is
credited to
the Francisco Coronado expedition of 1540 and subsequent discovery to
two
Spanish priests, Francisco Garcés and Silvestre Vélez de Escalante, in
1776. In
the early 1800s trappers examined it, and sundry government expeditions
exploring and mapping the West began to record information about the
canyon. By
the 1870s, following the exploration of John Wesley Powell and others,
extensive reports on the geography, geology, botany, and ethnology of
the area
were being published.
Grand Canyon National Park, now containing 1,904 square
miles (4,931
square km), was created in 1919. Its area was greatly enlarged in 1975
by the
addition of the former Grand Canyon National Monument and Marble Canyon
National Monument and by portions of Glen Canyon National Recreation
Area, as
well as other adjoining lands. The North and South rims are connected
by a
215-mile- (346-kilometre-) long paved road and by a transcanyon trail.
Scenic
drives and trails lead to all important features. Mule-pack trips down
the
canyon and rides down the river in rafts and power-driven craft are
intensively
sought-after ways of viewing and experiencing the vast beauty of the
canyon.
Many pueblo and cliff-dweller ruins, with accompanying artifacts,
indicate
prehistoric occupation. There are five Indian tribes living on nearby
reservations.
Geologic history
Although its awesome grandeur and beauty are the major
attractions
of the Grand Canyon, perhaps its most vital and valuable aspect lies in
the
time scale of Earth history that is revealed in the exposed rocks of
the canyon
walls. No other place on Earth compares with the Grand Canyon for its
extensive
and profound record of geologic events. The canyon's record, however,
is far
from continuous and complete. There are immense time gaps; many
millions of
years are unaccounted for by gaps in the strata in which either vast
quantities
of Earth materials were removed by erosion or there was little or no
deposition
of Earth materials. Thus rock formations of vastly different ages are
separated
only by a thin, distinct surface that reveals the vast unconformity in
time.
Briefly summarized, the geologic history of the canyon
strata is as
follows. The crystallized, twisted, and contorted unstratified rocks of
the
inner gorge at the bottom of the canyon are granite and schist about
two
billion years old. Overlying these very ancient rocks is a layer of
limestones,
sandstones, and shales that are more than 500 million years old. On top
of
these are rock strata composed of more limestones, freshwater shales,
and
cemented sandstones that form much of the canyon's walls and represent
a
depositional period stretching over 300 million years. Overlying these
canyon
rocks is a thick sequence of Mesozoic Era rocks (245 to 66.4 million
years old)
that form precipitous butte remnants and the vermilion, white, and pink
cliff
terraces of southern Utah but which have been entirely eroded away in
the area
of the Grand Canyon proper. Of relatively recent origin are overlying
sheets of
black lava and volcanic cones that occur a few miles southeast of the
canyon
and in the western Grand Canyon proper, some estimated to have been
active
within the past 1,000 years.
The cutting of the mile-deep Grand Canyon by the
Colorado River is an event of relatively recent geologic history that
began not
more than six million years ago, when the river began following its
present
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course. The Colorado River's rapid velocity and large
volume and the
great amounts of mud, sand, and gravel it carries swiftly downstream
account
for the incredible cutting capacity of the river. Prior to the building
of the
Glen Canyon Dam, the sediments carried by the Colorado River
weremeasured at an
average of 500,000 tons per day. Conditions favourable to vigorous
erosion were
brought about by the uplift of the region, which steepened the river's
path and
allowed deep entrenchment. The depth of the Grand Canyon is due to the
cutting
action of the river, but its great width is explained by rain, wind,
temperature, and chemical erosion, helped by the rapid wear of soft
rocks, all
of which steadily widened it. Amazingly, the canyon was cut by a
reverse
process, for the river remained in place and cut through the rocks as
the land
moved slowly upward against it. Only thus can be explained the canyon's
east-to-west course across a south-facing slope and the presence of
plateaus
that stand across the river's course without having deflected it.
The most significant aspect of the environment that is
responsible
for the canyon is frequently overlooked or not recognized. Were it not
for the
arid climate in the surrounding area, there would be no Grand Canyon.
Slope
wash from rainfall would have removed the canyon walls, the stairstep
topography would long ago have been excavated, the distinctive
sculpturing and
the multicoloured rock structures could not exist, the Painted Desert
would be
gone, and the picturesque Monument Valley would have only a few rounded
hillocks.
Biological past and
present
Plant and animal fossils are not abundant in the Grand
Canyon's
sedimentary rocks and are confined mostly to primitive algae and
mollusks,
corals, trilobites, and other invertebrates. Animal life in the Grand
Canyon
area today is varied and abundant, however. The common animals are the
many
varieties of squirrels, coyotes, foxes, deer, badgers, bobcats,
rabbits,
chipmunks, and kangaroo rats. Plant life is also varied. In the bottom
of the
canyons are willows and cottonwoods, which require abundant water
during the
growing season. At the other end of the moisture scale are
drought-resistant
plants such as the yucca, agave, and numerous species of cactus.
On
the canyon
rims, north and south, there is a wide assortment of plant life.
Typical of the
South Rim is a well-developed ponderosa pine forest, with scattered
stands of
piñon pine and juniper. Bush vegetation consists mainly of scrub oak,
mountain
mahogany, and large sagebrush. On the North Rim are magnificent forest
communities of ponderosa pine, white and Douglas fir, blue spruce, and
aspen.
Under less optimum conditions the plant life reverts to the desert
varieties.
Grand Canyon Series
Major division of rocks in northern Arizona dating from
Precambrian
time (about 3.8 billion to 540 million years ago). The rocks of the
Grand
Canyon Series consist of about 3,400 m (about 10,600 feet) of quartz
sandstones, shales, and thick sequences of carbonate rocks. Spectacular
exposures of these rocks occur in the Grand Canyon of the Colorado
River in
northwestern Arizona, where they overlie the strongly deformed and
contorted
Vishnu Schist, the angularity of which stands in bold contrast to the
almost
horizontal bedding of the Grand Canyon Series. The Grand Canyon Series
actually
dips slightly eastward and is separated from the overlying Cambrian
sandstones
by a major erosion surface unconformity. A conglomerate was deposited
on the
eroded surface of the Vishnu Schist. Limestones, shales, and sandstones
occur
over the conglomerate and are thought to represent shallow water
deposits. The
area of deposition was probably a large deltaic region that was slowly
subsiding, allowing great thicknesses of sediment to accumulate near
sea level
. The presence of Precambrian organisms is indicated by calcareous
algaelike
structures in the carbonate rocks, as well as by tracks and trails of
wormlike
creatures in other rocks. Initially, in a generalized outline of the
Precambrian history of the region, the Vishnu Schist was upraised,
folded, and
metamorphosed and then slowly eroded and worn down to a flat surface.
The Grand
Canyon Series was deposited perhaps as part of a slowly subsiding
geosynclinal
trough. The region was then subjected to uplift and tilting, and a
Precambrian
period of erosion for the Grand Canyon Series began. This action was
later
followed by a long period of deposition during the Paleozoic Era (540
to 245
million years ago) and then further erosion during the Cenozoic Era
(beginning
66.4 million years ago) until the region assumed its modern
configuration.
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