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GRAND CANYON'S multicolored layers of rock provide the best record of the Earth's formation of anywhere in the world. Each stratum of rock reveals a different period in the Earth's geological history beginning with the earliest, the Precambrian Era, which covers geological time up to 570 million years ago. Almost two billion years of history have been recorded in the canyon, although the most dramatic changes took place relatively recently, five to six million years ago, when the Colorado River began to carve its path through the canyon walls.
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The sloping nature of the Kaibab Plateau has led to increased erosion in some parts of the canyon.
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RECORD OF LIFE
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The fossils found in each layer tell the story of the development of life on Earth. The oldest layer, the Vishnu Schist, was formed in the Proterozoic era, when the first bacteria and algae were just emerging. Later layers were created by billions of small marine creatures whose hard shells eventually built up into thick layers of limestone.
The Asymmetrical Canyon
The North Rim of Grand Canyon is more eroded than the South Rim.
The entire Kaibab Plateau slopes to the south, so rain falling at the North Rim flows toward the canyon and over the rim, creating deep side canyons and a wide space between the rim and the river.
The Colorado River
About 5 million years ago the Colorado River changed its course. It is thought that it was encompassed by another, smaller river that flowed through the Kaibab Plateau. Theforce of the combined waters carved out the deep Grand Canyon.
HOW THE CANYON WAS FORMED
While the Colorado River accounts for the canyon's depth, its width and formations are the work of even greater forces. Wind rushing through the canyon erodes the limestone and sandstone a few grains at a time. Rain pouring over the canyon rim cuts deep side canyons through the softer rock. Perhaps the greatest canyon-building force is ice. Water from rain and snowmelt works into cracks in the rock. When frozen, it expands, forcing the rock away from the canyon walls. The layers vary in hardness. Soft layers erode quickly into sloped faces. Harder rock resists erosion, leaving sheer vertical faces.
The Geology of Grand Canyon
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