How old is the Grand Canyon? Old enough to be gazed on by actual dinosaurs, which died out 65 million years ago, or more like 6 million years old, formed about when the earliest human ancestors began walking upright?
This bitter controversy among geologists edged into the open on Thursday when a report published in the journal Science offered new support for the old-canyon hypothesis, which is not the prevailing one. In the report, Rebecca M. Flowers of the University of Colorado and Kenneth A. Farley of the California Institute of Technology used an improved dating technique based on the radioactive decay of uranium and thorium atoms into helium atoms in a mineral known as apatite. They said this yielded a thermal record of these rocks under the canyon floor, hot at great depths but cooler the closer they were to the surface.
An analysis of the data, the geologists said, revealed where surface erosion had gouged out canyons and how much time had passed since there was significant natural excavation in the Grand Canyon region. They thus concluded in the report that the western segment of the canyon was carved to within a few hundred meters of modern depths by about 70 million years ago.
The more ancient origin would put much of the canyon in place in the last epoch of the dinosaurs. Publicity for the journal report duly noted that one of nature's wonders, dinosaurs, might well have stood and gawked at another wonder, one of today's most majestic tourist attractions.
This was only one of the immediate objections to the findings raised by geologists favoring the young-canyon school of thought. They complained that the research results had been hyped. One critic, Karl E. Karlstrom of the University of New Mexico, noted that the early-canyon model had been proposed before and was "now in what I think will be a short-lived revival."
If the interpretation of the findings proves to be correct, it contradicts the prevailing hypothesis that the entire canyon was formed as recently as five million to six million years ago, advocated by many of the notable authorities on Grand Canyon geology. These dates were drawn from the examination of pebbles and other sediments from upstream reaches of the Colorado River system that washed up at the western exit of the canyon.
Dr. Flowers said that when she started this research seven years ago, she had not expected to find the canyon's presumed age to be so ancient. But the first set of experiments with the radioactive helium technique in 2008 was followed up with a new round of tests and more sophisticated levels of analysis.
In their paper, Dr. Flowers and Dr. Farley wrote that their findings implied a dichotomy in the late eastern and early western canyon origins. This history, they said, "supports a model in which much of Grand Canyon incision was accomplished by an ancient Cretaceous river that flowed eastward from western highlands," not from northeast to west, as today's Colorado River does. This was followed by a "reversal of the river's course as topography rose in the east and collapsed in the west," in consequence of the upthrusting Rocky Mountains.
Dr. Flowers said in an interview that the findings supported the opposing ancient-origin hypothesis advanced in recent years by Brian P. Wernicke, a Caltech geologist, who had proposed such a chain of events. It is still not clear when the eastern and western canyons merged into the canyon as it is seen today.
She also foresaw "a fair amount of controversy" over the research results. That turned out to be an understatement, even before the official publication date.
Dr. Karlstrom of the University of New Mexico is a leader among geologists who have devoted much of their careers to Grand Canyon studies. When reporters called this week, he was prepared with four pages of criticism of the new research. He pointed out that at a meeting two years ago of the most active Grand Canyon researchers, "a near consensus view was expressed" in support of the young-canyon hypothesis.
As a rule of thumb, he defined the Grand Canyon as "the canyon you see from the rim today." How fragments of paleocanyons and paleorivers contributed to the Grand Canyon's origin is not established, he said.
Dr. Karlstrom was not entirely negative in his assessment of the research. He praised the thermochronology method the researchers used, saying it "offers one of the few ways we may be able to reconstruct past landscapes in rocks that have long since been eroded away." The Flowers-Farley team, he added, "is pushing welcome new advances" in this dating technology.
"Less welcome to me," he continued, "is their attempt to push the interpretation of their new data to their limits without consideration of the whole range of other geologic data sets."
Correction: November 29, 2012, Thursday
This article has been revised to reflect the following correction: An earlier version of this article misstated the direction in which the Colorado River flows. It flows from northeast to west, not from west to northeast.
This article originally appeared in The New York Times.