The winding course of the Colorado River through northern Arizona's mile-deep Grand Canyon is well marked on the map by a vivid contrast in pattern and color. A narrow ribbon of red differentiates the ancient granites and tilted metamorphic rocks along the river banks from the younger, overlying horizontal Paleozoic sedimentary rocks in the canyon walls and at the rim (blue). Abundant water from melting Pleistocene glaciers probably is responsible for the most recent episode of erosion that has deepened the canyon.