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Grand
Canyon attracts the attention of the world for
many reasons, but perhaps its greatest significance
lies in the geologic record preserved and exposed
here. The rocks at Grand Canyon are not inherently
unique. Similar rocks are found throughout the
world. What is unique about the geologic record
at Grand Canyon is the variety of rocks present,
the clarity with which they are exposed, and
the complex geologic story they tell.
Two
separate geologic stories exist at Grand Canyon.
The older story is the one revealed in the thick
sequence of rocks exposed in the walls of the
canyon. These rocks provide a remarkable record
of the Paleozoic Era (550-250 million years
ago). Scattered remnants of Precambrian rocks
as old as 2000 million years can also be found
at the bottom of the canyon. The story these
rocks tell is far older than the canyon itself.
Mesozoic and Cenozoic rocks (250 million years
old to the present) are largely missing at Grand
Canyon. They have either been worn away or were
never deposited.
The
second geologic story at Grand Canyon concerns
the origin of the canyon itself: when and how
did it come to be? On one level the answer is
simple: Grand Canyon is an erosional feature
that owes its existence to the Colorado River.
Of equal importance are the forces of erosion
that have shaped and continue to shape the canyon
today. These include running water from rain,
snowmelt, and tributary streams which enter
the canyon throughout its length. The climate
at Grand Canyon is classified as semi-arid.
The South Rim receives 15 inches/38 cm
of precipitation each year. The bottom of the
canyon receives 8 inches/20 cm. The rain
comes suddenly in violent storms, particularly
in the late summer of each year. The power of
erosion is therefore more evident here than
in other places which receive more rain.
Grand
Canyon owes its distinctive shape to the different
rock layers in the canyon walls. Each responds
to erosion in a different way: some form slopes,
some form cliffs, some erode more quickly than
others. The vivid colors of many of these layers
are due mainly to small amounts of various minerals.
Most contain iron, which imparts subtle shades
of red, yellow, and green to the canyon walls.
Climate plays an important role in the appearance
of the canyon. If there was a higher amount
of precipitation at Grand Canyon, the plants
and trees that grow here would be very different.
The canyon walls might be covered with lush
vegetation, rather than the cacti and shrubs
growing there today.
How
old is the canyon itself? The early history
and evolution of the Colorado River (of which
Grand Canyon is only a part) is the most complex
aspect of Grand Canyon geology. We know that
the erosion which has shaped the canyon has
occurred only in the past five to six million
years. This is only yesterday, considering the
age of the rocks through which the river has
carved.
Grand
Canyon continues to grow and change. As long
as rain and snow continue to fall in northern
Arizona, the forces of erosion will continue
to shape Grand Canyon.
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