Since then, the canyon-forming has been cumulative. To the corrosive force
of the river itself have been added other factors. Heat and cold, rain and snow, along
with the varying resistance of the rocks, increase the opportunities for erosion. The
canyon walls crumble; the river acquires a cutting tool, tons of debris; rainfall
running off the high plateau creates feeder streams that carve side canyons. Pushing
slowly backward into the plateau, the side canyons expose new rocks, and the
pattern of erosion continues.