 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 <unk> 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 . 
