A Canyon-Size Age Difference

Not least among the challenges was simply reaching the caves, some of which are hundreds of feet high on canyon walls. Polyak and his team rafted the Colorado River, hiked side canyons and used climbing ropes. Expert climbers led the way. One cave was 400 feet up. "I always get nervous," Polyak said.

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

Pederson also raises what might be called the where's-the-dirt question. "To cut a large canyon in the place of today's Grand Canyon," Pederson said, "you have to remove that mass and put that detritus somewhere."

But there's no sign of such canyon detritus in sediments older than 6 million years, Pederson said.

Hill, however, argued that there are multiple locations where material from the canyon may have been deposited. And keep in mind, she said, the older canyon wasn't that big.

"We're not talking about the canyon that's there now. We're talking about a small canyon that's slowly and steadily starting to down-cut," she said. "It's not the Colorado River; it's what we might call the proto-Colorado River."

So then: How did the proto-Colorado River turn into the much more dramatic Colorado River, which, in turn, transformed the not-so-big canyon into the Grand Canyon?

"That's the $64,000 question," Hill said.