 A series of glaciations further modified the region starting about 2 to 3 million years ago and ending sometime around 10 @,@ 000 BP . At least four major glaciations have occurred in the Sierra Nevada , locally called the Sherwin ( also called the pre @-@ Tahoe ) , Tahoe , Tenaya , and Tioga . The Sherwin glaciers were the largest , filling Yosemite and other valleys , while later stages produced much smaller glaciers . A Sherwin @-@ age glacier was almost surely responsible for the major excavation and shaping of Yosemite Valley and other canyons in the area . 
