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The Colorado cuts the Grand Canyon
The Grand Canyon is a gorge in Arizona, some 450 kilometres long and over a kilometre and a half deep, cut by the Colorado River. The canyon is due not to erosion alone but to the Colorado Plateau rising while the river already lay there and cut down just as fast. Exactly when this happened is still disputed: dates range between six and seventy million years, probably because different reaches of the canyon are of different ages and were later joined. The layers also hold one of the world's clearest angular unconformities, the Great Unconformity, where more than a billion years of history is missing entirely.
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