Evolution of the Yosemite Valley The area that was to become the Sierra Nevada once lay beneath a sea at the west margin of North America. The rock that was formed on this sea floor from deposited silt. mud, and marine organisms was subsequently lifted above sea level and flexed into a mountain range surmounted by a chain of volcanoes much like today's Cascade Range. Granite that formed from molten rock at the roots of these volcanoes eventually would remain as the core of the Sierra Nevada after the overlying sedimentary and volcanic rock gradually weathered and eroded away.
50 million years ago. The landscape consisted of rolling hills, broad valleys, and meandering streams. The Merced River meandered through a wide trough whose slopes supported hardwood forests.
10 million years ago. A more dissected landscape ensued as the whole range was uplifted and tilted westward. This westward tilt accelerated the Merced River's flow and the river cut deeper into its valley. The climate grew cooler and drier. Forests of coniferous trees, including sequoias, dominated.
3 million years ago. A canyon landscape developed with continued uplift. The raging Merced cut its canyon as much as 3.000 feet deep. Its tributary streams, with smaller drainage basins and volumes of water, cut more slowly. The Ice Ages approach brought a colder climate and thinning forests.
1 million to 250,000 years ago. At least one and perhaps more glacial advances filled Yosemite Valley to its brim. Half Dome projected 900 feet above the ice, but many peaks to the north were engulfed. The Valley was gouged and quarried into a U-shaped trough with steep walls. Many Merced River tributaries were now cascades high above the Valley.
30,000 years ago. During the Tioga glaciation Yosemite Glacier. a smaller ice sheet, advanced into the Valley and terminated near Bridalveil Fall. As thin as it was, it had little erosive power to enlarge the Valley further.
10,000 years ago. The last Valley glacier hat melted and its terminal moraine has dammed the Valley to create a shallow lake, Lake Yosemite. This was only the last of many Lake Yosemites that probably followed each glaciation. The deep excavation created by earlier glaciers, as much as 2,000 feet into bedrock beneath the present floor of Yosemite Valley, was already filled with glacial till and sediments long before the Tioga glaciation. This last advance of ice had insufficient erosive power to reexcavate the Valley to any appreciable depth. Lake Yosemite eventually filled in with silt, leaving today's level Valley floor. The photograph shows Mirror Lake. Today it is filling in through the same natural processes. Glaciers did not directly create today's free-leaping waterfalls, although they helped set the stage. The falls plunge into alcoves in the Valley walls, produced by frost-splitting of rock fragments off the lower parts of the cliffs over which the waterfalls formerly cascaded. Where the cliffs are dry most of the time, frost action is not so effective.
The text and diagrams are from “Evolution of the Valley,” Yosemite: Official National Park Handbook (Washington: Government Printing Office, 1989), pages 66-67. Paintings by H. A. Collins, Sr. (1938)
Copyright © 2004 Dan Anderson. All rights reserved.
Last updated 6 July 2011.
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