Visitors to the Grand Canyon always want to know: How old is it? Park rangers are instructed to tell them that the canyon has been carved by the Colorado River for the past 5 million or 6 million years. The National Park Service’s Web site, under “frequently asked questions,” notes that the rocks exposed by the canyon are up to 2 billion years old, and then adds: “The Canyon itself — an erosional feature — has formed only in the past 5 or 6 million years. Geologically speaking, Grand Canyon is very young.”

That might need revision. The canyon is more like 17 million years old, according to a new study published online yesterday by the journal Science.

And the Colorado River may not be the only river involved in its formation. The new study contends that a smaller river cut the older, western part of the canyon. Gradually the canyon formed from west to east on a westward-flowing river. Then something happened about 5 million or 6 million years ago — what, exactly, is unclear — to accelerate dramatically the rate of the canyon-carving.

“The canyon is older than we think,” a University of New Mexico geologist and the lead author on the Science paper, Victor Polyak, said. “And there’s a two-step process, I guess you can say.” Not so fast, according to a geomorphologist at Utah State University who has spent his career studying the Grand Canyon, Joel Pederson. The estimated age of 5 million to 6 million years is based on abundant evidence amassed by scientists over many decades, he said. The 17-million-year age is impossible, he said, because there’s no evidence of a large quantity of sediment flowing out of a canyon prior to 6 million years ago.

“They clearly have not taken the time to be rigorous and actually understand the regional geography,” Mr. Pederson said.

Mr. Polyak’s research paired new lab techniques with intrepid field work. Researchers had to climb canyon walls to reach ancient caves containing crucial evidence of the canyon’s history. The scientists examined mammillaries, also known as cave clouds, which are rounded rock structures that tend to form underwater near the top of a water table. In the canyon, these rocks also contain abundant amounts of uranium. In recent years scientists have improved techniques for dating rocks based on the predictable decay of uranium into lead.

Mr. Polyak and geologist Carol Hill suggested the research project to geochemist Yemane Asmerom, Polyak’s boss at the University of New Mexico: Why not use the new lab techniques to measure the ages of the mamillaries? That ought to tell the story, Polyak reasoned, of how the river gradually cut through the plateau and lowered the water table.

Mr. Asmerom was skeptical. Scientists have long struggled to figure out the age of the 277-mile-long canyon, which is 18 miles across at its widest and reaches depths of 6,000 feet. Evidence of how and when the huge incision into Earth’s crust took place tends to get eroded away.

“Forget it. That was my reaction,” Mr. Asmerom said. But he was persuaded to join the effort. Not least among the challenges was simply reaching the caves, some of which are hundreds of feet high on a canyon wall. Mr. Polyak and his team rafted the Colorado River, hiked side canyons, and used ropes to ascend the canyon walls.

The mammillaries offered strong evidence of where the water table had been in the past. But Mr. Pederson, the critic of the study, said Mr. Polyak went too far in assuming that the water table tracks the canyon-carving. Mr. Pederson notes that sometimes a spring will gush from the side of a canyon wall thousands of feet above the river.

Mr. Pederson also raises what might be called the where’s-the-dirt question.

“In order to cut a large canyon in the place of today’s Grand Canyon,” Mr. Pederson said, “you have to remove that mass and put that detritus somewhere.”

But there’s no sign of that detritus, at least not from more than 6 million years ago, Mr. Pederson said.

Mr. Polyak acknowledges that his study raises some difficult questions.

“Where’s this material go? Where is it? I think it’s a good question, it’s legitimate,” he said.