Well how old is the canyon? Certainly we can determine the age of the layers of rock as you drop down, but how do you determine the age of the canyon itself, when did the erosion take place? How can that be geographically determined. One method that seems like it could be a good method is to measure the silt at the delta, where the colorado flows into the ocean, but I don't know how it could be done or what factors you would have to account for. Just to play Devil's advocate if the canyon was eroded millions of years ago over time, then wouldn't the silt at the delta be more dispersed, but if the erosion is more recent and due to a catostraphic event, such as the emptying of Lake Bonneville, than the silt layer would be substantially more. Its something to think about at least.

BTW, I think its silly and pretty stupid of the Bush Administration to bully the parks service into changing whatever information they are allowed to provide. That doesn't disount that people can dispute the age regardless of the "official" age stamped on the canyon by the parks service, nor should the parks service need to pay heed to it. The Bush administration is setting a precedent for crap that could easily backfire when the next politically opposite administration enters power.