to surface pressures, resulting in exfoliation (responsible for the rounded shape of the many granite domes in the park) and mass wasting following the numerous fracture joint planes (cracks; especially vertical ones) in the now solidified plutons. Pleistocene glaciers further accelerated this process and the larger ones transported the resulting talus and till from valley floors. 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 high El Capitan. Closely spaced joints lead to the creation of columns, pillars, and pinnacles such as Washington Column, Cathedral Spires, and Split Pinnacle. Glaciations 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. 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. 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) years ago, reached its maximum extent 20,000 to 25,000 cal yr ago, and