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Unlike Valles Marineris on Mars, the Grand Canyon was carved by river action. In the last 10 million years or so, the Colorado River has cut a 1.5-kilometer-deep channel into Earth's crust, slicing through almost 1.5 billion years of geologic history. These oldest rocks, exposed at the bottom of the Grand Canyon, provide geologists with evidence of ancient environments and events. Image courtesy of Dr. Graham Ryder. |