Yosemite Valley's sheer walls and flat floor evolved as alpine glaciers lumbered through the canyon of the Merced River. The ice carved through weaker sections of granite, plucking and scouring rock but leaving intact harder portions such as El Capitan and Cathedral Rocks. Glaciers greatly enlarged the canyon that the Merced River had carved through successive uplifts of the Sierra. When the last glacier melted, its terminal moraine, left at its farthest advance into the valley, dammed up the melting water to form ancient Lake Yosemite in the newly carved U-Shaped valley. Eventually sediment filled in the lake, resulting in the creation of today's flat valley floor. Glaciers did not reach the Merced Canyon along Highway 140 outside the park, and its river-cut, V-shaped profile contrasts markedly with Yosemite Valley's sheer walls.