Geologists Phil Pearthree and Jon Spencer from AZGS, and Jim Faulds and Kyle House from the Nevada Bureau of Mines are challenging a recent report that claimed the Grand Canyon started forming 17 million years ago. [right, National Park Service]
Tomorrow's issue of Science carries a technical comment that rebuts the interpretation of Victor Polyak and associates published last March, that used cave deposits to track water-level declines and attributed them to downcutting of the Canyon. The Polyak article received widespread attention in the geologic and general news media.
Our team determined instead that "water-table declines at these sites were more likely related to local base-level changes and Miocene regional extensional tectonics." They conclude that geologic evidence shows the Colorado River arrived in the western Grand Canyon region about 5-6 Ma.
A second critique of the Polyak interpretation is authored by Joel Pedersen and his colleagues, and raises complementary concerns.
The rebuttal by Polyak et al, comes down to them arguing that they see no absolute data that negates their interpretation, so therefore, they must be right. Not the most compelling argument, but read the whole debate yourself.