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The Grand Canyon is the world's most spectacular example of erosion and most remarkable assemblage of exposed rock in sequence and intact.

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The Grand Canyon officially measures 277 river miles from Lee's Ferry to the Grand West Cliffs.

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Today the Colorado River is bracketed at either end by dams, Glen Canyon Dam, Lake Powell on the upstream end and Hoover Dam, Lake Mead, at the lower end.

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As a result, the dynamics of the Colorado River have changed considerably.

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Prior to its impoundment, the river carried a sediment load many times what it carries today.

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today.

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Compared to the rocks in which it is carved, the Grand Canyon is geologically young, occurring

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only in the past six million years.

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The Grand Canyon is the result of erosion, primarily by water.

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While the Colorado River has played the primary role in creating the canyon's present depth,

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Runoff from rain and snow and the streams that flow into the canyon from both rims also help shape and size the canyon.

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1,000 vertical feet below the south rim of the Grand Canyon, the Colorado River flows. From its origins, high in the Colorado Rockies, more than 1,400 miles tour to the Gulf of California and passes through a series of remarkable canyons, of which the Grand Canyon is only one.

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Due in part to the semi-arid climate which exists in this part of the U.S., erosion proceeds in a very dramatic fashion. Each of the rock layers within the Grand Canyon erodes in its own manner, giving the canyon its characteristic steppe pyramid appearance.

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Many of the stunning colors of the remarkable features of this landscape are due to the presence of small amounts of iron and other minerals which stain the surface of the canyon walls.

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The measured thickness of rock in the Grand Canyon is about 6,000 feet.

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Each layer represents an interval of time during which a particular environment of deposition prevailed,

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but many of the layers are separated by gaps of unrecorded time and missing rock layers referred to as unconformities.

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Most of the flat lying rocks visible from the rims are Paleozoic in age, recording events that took place on the North American continent hundreds of millions of years ago, long before dinosaurs roamed the earth.

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The youngest of these layers is the Kaibab limestone, the top layer, deposited in shallow warm seas at the end of the Palazzoic period, 65 million years ago. Below the rim, these layers become progressively older, reaching back into the early Palazzoic.

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4,000 feet below the rim, in the walls of the inner gorge, are the oldest rocks of this region, the igneous and metamorphic rocks known as the Vishnu group.

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Very different from the sedimentary rocks above them, these ancient sheets and gneisses are almost 2 billion years old and form the very basement of the North American continent.

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The Grand Canyon, an open book of America's natural history.