The Yosemite Valley is the product of many geological forces. One such is glacial scouring, which left the spectacular glaciated granite terrain that can be seen today. Valley glaciers have been described as "tongues of ice in mountain ranges [that] start in ice-worn rock basins called cirques" (3:154). In these cirques, old snow develops into firn, or nevT, a "mass of ice pellets compacted by the weight of the snow above" (3:154).
As more snow falls on the firn, it spills over into the valley below, filling it with ice. As this ice moves across the landscape, it collects rock debris, which polishes and scours the valley walls and floor. This scratching action leaves a definitive calling card in the form of grooves that identify glacially scoured land. The glacier also changes the shape of the valley from its original V-shaped cross section to the U-shape of the glacial trough (3:156).
Glacial erosion shears off spurs that project into the valley, making the valley walls even steeper and transforming tributary streams into "free-leaping" waterfalls such as Yosemite's Bridalveil Fall, which is 620 feet (189 meters) high (1:308). Most of the highest waterfalls in the United States are in Yosemite National Park (1:306), with Bridalveil Fall among the highest, along with Yosemite Falls, 2,435 feet (740 meters); Ribbon Fall, 1,612 feet (491 meters); Staircase Falls, 1,300 feet (396 meters); Nevada Fall, 594 feet (181 meters); Illilouette Falls, 370 feet (113 meters); and Vernal Fall, 317 feet (97 meters).
That Yosemite Valley has a complex geologic history also can be discerned from the rocks that can be seen there today. Looking at the geologic sequence within the valley starting with about 136 million years ago, the evidence shows an early Cretaceous intrusive phase that lasted about 10 million years. The main late Cretaceous phase occurred from about 90 to 80 million years ago, and in the valley it is thought that m...