Lava Falls
Fortunately, a series of basaltic flows that flowed into the Grand Canyon provide a chance to use radiometric and other techniques to give us some idea of the erosional history of the canyon. Of course these age numbers continue to be refined as new techniques are developed.
A fresh look at Western Grand Canyon Lava Dams: Dating the Rocks
Wikipedia article
At many National Parks, the Visitor Center book shops are run by private companies that work with the Park Service for selection of materials. One such company is Eastern National.