<P> Numerous vertical joint planes controlled where and how fast erosion took place . Most of these long, linear and very deep cracks trend northeast or northwest and form parallel, often regularly spaced sets . They were created by uplift - associated pressure release and by the unloading of overlying rock via erosion . The great majority of Yosemite Valley's widening, for example, was due to joint - controlled rockfall . In fact, only 10% of its widening and 12% of its excavation are thought to be the result of glaciation . Large, relatively unjointed volumes of granite form domes such as Half Dome and monoliths like the 3,604 ft (1,098 m) high El Capitan . Closely spaced joints lead to the creation of columns, pillers, and pinnacles such as Washington Column, Cathedral Spires, and Split Pinnacle . </P> <P> Starting about 2 to 3 million years ago a series of glaciations further modified the area by accelerating mass wasting through ice - wedging, glacial plucking, scouring / abrasion and the release of pressure after the retreat of each glaciation . Severe glaciations formed very large glaciers that tended to strip and transport top soil and talus piles far down glacial valleys, while less - severe glaciations deposited a great deal of glacial till further up in the valleys . </P> <P> At least 4 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 . The Sherwin may have lasted almost 300 thousand years and ended about 1 million years ago . A Sherwin - age glacier was almost surely responsible for the major excavation and shaping of Yosemite Valley and other canyons in the area . </P> <P> The Tahoe, Tenaya, and Tioga stages were part of the Wisconsinan glaciation . The Tahoe glacial stage is thought to have reached its maximum extent around 70,000 to 130,000 years ago; little is known about the more recent Tenaya . Evidence also suggests that the most recent local glacial stage, the Tioga, started about 28,000 cal (calibrated Radiocarbon dating #Measurements and scales) years ago, reached its maximum extent 20,000 to 25,000 cal yr ago, and ended by ~ 15,000 cal yr ago . Glaciers reformed in the highest cirques during a minor late - glacial readvance, the Recess Peak event, between about 14,200 and 13,100 yr ago . </P>

What type of rock is the oldest type of rock at yosemite