In a recent study of rocks in the Grand Canyon, researchers found evidence that the canyon is much older than previously thought. Most geologist have set the development of the Grand Canyon at 5 to 6 million years ago, as the Colorado River cut into layer after layer of the Colorado Plateau. The most recent results come from geochemical studies of the retention of helium in apatite in the rocks of the canyon. The results led the scientists to conclude that an ancient river must have carved through the rocks there about 70 million years ago. Other scientists draw a distinction between that more ancient river and canyon and the modern canyon.
For more information on this study, see the article in Science at http://news.sciencemag.org/sciencenow/2012/11/a-grand-old-canyon.html.