Grand Canyon Old, Says the News (But Not the Data)

This week's issue of Science has an article claiming that its uranium-lead dates prove the Grand Canyon formed not 6 million years ago, but more like 17 million. Without having access to the new Science paper, I can still consult previous presentations on the subject by the study's authors, Victor Polyak, Carol Hill and Yemane Asmerom. Here's one from the 2004 GSA meeting and here's the team's more detailed results from the 2007 GSA. The dating was done on cave deposits that are now exposed high on the canyon's walls, and they show that the deposits formed at steadily lower elevations as time passed, starting around 16.8 million years ago.

The dates are good, but the case is not truly made. As I mentioned a couple weeks ago, the rocks and sediments of the region match a different scenario, with the canyon not being cut down by a river but dug from underneath by groundwater. The thing is, both scenarios could produce the same set of cavestone ages. That's because those ages represent one thing: the elevation of the water table when the deposits formed. To get from there to the downcutting of a canyon, as Polyak proposes, is a leap unless there is other evidence of an actual canyon.

Of all the coverage in the media, only Larry O'Hanlon's story for Discovery News makes this distinction. And for good measure, he cites Carol Hill's paper in Geomorphology proposing a novel alternative scenario that accepts Polyak's downcutting theory but invokes the groundwater-sapping mechanism farther upstream, near the Kaibab Uplift. One way or another, we have a new ballgame in Grand Canyon studies.