Most of the energy of the river has been expended in downcutting, for
the canyon has apparently been deepened more rapidly than it has been
widened. But as the stream gouged its channel deeper into the bedrock,
an ever-increasing expanse of canyon wall was exposed to other agents of
erosion. Slowly—almost imperceptibly—the walls of the canyon have been
eroded by the processes of weathering and mass-wasting.