Today's [1/13] Doonesbury, which is based off of this press release (or an email about the press release with three FW:'s), claims that the Grand Canyon Park Service is not allowed to tell the geologic age of the Grand Canyon. A three-minute Internet search finds this National Park Service page which tells the geologic age of the Grand Canyon.

If you look at the press release, you'll see that they have good documentation about the book but are vague as to the "not allowed to tell" claim (Who gave the order? When was it given? Etc.)

UPDATE: The folks who released the press release have disappeared the claim that park rangers aren't allowed to tell the geologic age of the Grand Canyon. Google cache is, for now, here.