In 1869 John Wesley Powell, who was later to head the US. Geo logical Survey, led a pioneering expedition down the Colorado River and through the Grand Canyon(Figure 11.1). Writing about the rock layers that were exposed by the downcutting of the river, Powell said, "The canyons of this region would be Book of Revelations in the rock-leaved Bible of geology. He was undoubtedly impressed with the millions of years of Earth history exposed along the walls of the Grand Canyon and the other canyons of the Colorado Plateau(see chapter opening photo and Figure 11.2)