The first and largest glacier was the Sherwin or Pre-Tahoe glacier, which eroded the upper Merced watershed to an extent close to its present form. Three stages followed during the Wisconsinian glaciation; these were the Tahoe, Tenaya and Tioga stages, of which the Tioga was the smallest. The Tioga glacier left at the mouth of Yosemite Valley a rocky moraine. This moraine was actually one of several moraines deposited by the four glaciations, which include Medial Moraine and Bridalveil Moraine. After the Tioga Glacier retreated this moraine formed a lake that flooded nearly the entire valley. Gradual sedimentation filled Lake Yosemite, creating a broad and flat valley floor. Sediments of glacial origin continued to travel down the Merced River following then, helping to form the flat floor of the Central Valley.