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For all the people who have looked at the Grand Canyon - the visitors who have peered over the edge, the tourists in helicopters who've flown overhead, the producers who filmed that special three-part "Brady Bunch" episode - nobody knows how old it is. Some scientists say the Grand Canyon is young, in relative terms, just six million years old, while others argue that it dates back to the days of the dinosaurs, much earlier. NPR's Nell Greenfieldboyce reports that one group now thinks it has the answer.

NELL GREENFIELDBOYCE, BYLINE: At the south rim of the Grand Canyon, there's a walking path called the Trail of Time. It uses the Grand Canyon's awesome scenery and rocks to try to convey the vastness of geologic time. And it tells tourists that the Colorado River carved the Grand Canyon in the last six million years.

KARL KARLSTROM: The trail of time exhibit has what we considered at the time, in 2010, to be the scientific consensus on the age of Grand Canyon, and that's five to six million years old.

GREENFIELDBOYCE: Karl Karlstrom is a geologist at the University of New Mexico who worked on the exhibit. He hiked the canyon as a kid and has studied it professionally for 30 years. He says figuring out when a canyon was carved is not easy because rivers carve canyons through erosion.

KARLSTROM: And erosion takes away material and so geologists are left without the rock record, without the physical evidence of the carving. What you are left with is a landscape, a land form.

GREENFIELDBOYCE: In recent years, scientists have started using sophisticated new techniques that can reconstruct the history of erosion by analyzing the chemistry of one of the minerals that make up canyon rocks. And in 2012, one research team concluded that the Grand Canyon was actually cut about 70 million years ago, in the late Cretaceous, which startled experts like Karlstrom.

KARLSTROM: That whole episode - is it old, is it young - caused my group to rethink what we meant by old and young and what is the evidence.

GREENFIELDBOYCE: Now, in the journal Nature Geoscience, he and some colleagues describe a new creation story for the Grand Canyon. They say it is neither old nor young; rather, it's both. They think that about six million years ago, a river forging a zigzagging path across the Colorado plateau found part of its way through old canyons that already existed which means that although the Grand Canyon as a whole is relatively young, a couple of sections are more ancient.

KARLSTROM: We're making a major leap from thinking of a canyon that has a simple history, it was all carved at once either at 70 or at five to six, to a more sophisticated understanding of how landscapes actually evolve through time.

GREENFIELDBOYCE: Not everyone thinks this will be the last word. Brian Wernicke is a geologist at Caltech who is in the more ancient Grand Canyon camp. He doesn't buy all the arguments in this new paper. But to him, the important thing is that it shows a real shift in people's thinking.

BRIAN WERNICKE: In the mid-2000s there was this monolithic view that oh, yeah, the Grand Canyon is very young and it was cut six million years ago by the Colorado River which was born six million years ago. OK? That's not the discussion right now. We've all learned that it's a lot more complicated than that.

GREENFIELDBOYCE: He expects that over the next year, there will be more data and more debate. Karl Karlstrom says, for a scientist, all this is exciting.

It's spectacular. You know, if you're willing to change your mind, based on evidence, it's great fun.

He says now, when he takes river trips through the canyon, he sees it with new eyes. Nell Greenfieldboyce, NPR News.

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