Pete McBride/National Geographic Creative
Standing on a mesa high above the town of Rifle, Colo., Andres Aslan is having a hard time staying quiet. The lanky geologist from nearby Colorado Mesa University normally speaks in a low-key professorial drone. But here, looking down at a sprawling river valley blazing with autumnal cottonwoods, his enthusiasm cranks up his volume. “This could be it,” says Aslan, gesticulating wildly. “This may end up being the most important site anywhere.”
What’s important about this mesa, called Taughenbaugh, is the gravel under Aslan’s feet. It was laid down 1.75 million years ago by the Colorado River. The modern Colorado wends through the valley beneath. Over those millions of years, the river eroded away all the rock layers that once existed between the high mesa and the valley below.
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