So they looked at calcite deposits that form in caves at the edge of a dropping water table, called speleothems.
Polyak and colleagues knew speleothems form where a water table is dropping: They've seen similar formations in caves throughout deserts of the U.S. Southwest.
The Grand Canyon speleothems contain minute traces of lead and uranium, and ratios of these elements can pin the formations to a specific place and time.
Study of carbonate deposits at or near the water table led the researchers to conclude that the canyon is oldest at its western end and that it opened up steadily to the east through erosion.
Grand Canyon geologists have long focused on geologic clues, such as traces of volcanic activity, to estimate the age of the canyon. The new, cave deposit-based approach is hard to swallow for some scientists.
One of them is Ivo Lucchitta, a Flagstaff, Arizona, geologist.
Lucchitta said he is "outraged" about the new research.
He said he will join other long-time Grand Canyon geologists in blasting Science for what he says is inadequate vetting of the new study.
There is "a host of published information by many researchers having to do with the issue presented by these authors, and this information is squarely at odds with what these folks propose," he said. "But [Science] completely ignore[s] it."
"If the authors really want to make a contribution, they have to show us why the interpretations that have issued previously are wrong, and why their interpretations and data are sounder," Lucchitta said.
A Grand Discovery?
But Wayne Ranney—a Flagstaff geologist and author of the book Carving Grand Canyon—said the authors may have found the "smoking gun" on when and how the Grand Canyon was formed.
Ranney said he will make room for the new idea in the next printing of his book.
"I'd be awfully surprised if this work doesn't generate a lot of traction with canyon geologists, since it fits well with many emerging ideas in the last few years," he said.
But the authors say they've only started to use this isotopic technology to describe the canyon's history.
Hundreds of deposits probably exist throughout the canyon, they say.
Those deposits may offer the potential to reconstruct the canyon's history "with a resolution perhaps high enough to explain complexities of the canyon's faults, folds, and volcanic and tectonic activity," said study co-author Carol Hill.
Though Hill and her team may tackle some of that work, much of it may be left to future generations of scientists, she said.
"We're kind of just showing the way, because this has never been done before."
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