| Jack Barrett photo June l947- This rock formation on the north wall of Yosemite Valley some distance from Yosemite Falls is part of the "El Capitan granite" about l08 million years old according to THE GEOLOGICAL STORY OF YOSEMITE NATIONAL PARK by Norman King Huber l987 U.S.Geological Survey Bulletin #1595. El Capitan granite intruded into older rocks. It dominates the western half of Yosemite Valley and the monoliths of Three Brothers, Turtleback Dome,Cathedral Rocks and El Capitan itself. El Capitan granite contains phenocrysts-large crystals embedded in finer maerial. These are absent from the younger Taft granite, which forms the brow of el Capitan and the upland toward Fireplace Bluffs.Taft granite is finer textured and lighter in color and occurs at Dewey Point and near the Fissures on the south wall of the Valley.Thew oldest rocks in the park date from the Ordovician about 480 million years ago. The Sierra Nevada mountain range is an enormous pluitonic batholith formed deep in the earth in the Mesozoic era 230 to 70 million years ago. Plate tectonics explains how North American pushed westward, and materials subducted beneath the ocean floor were recycled into new mountains. It is believed that admixture of ocean waters with rock materials lowers the melting point and forms molten materials that may solidify deep in the earth as in the Sierra Nevada or come to the surface as volcanoes like the Cascade Range -Rainier, st. Helens,Mt. Hood,Jefferson, Adams,Mazama(Crater Lake)Thielsen, Three Sisters, Shasta, Lassen.The basalts of the Columbia Plateau are another great feature of the American West. Quartz,potassium feldspar,plagioclase (sodium-calcium) feldspar, biotite,and hornblende are the five dominant minerals of the Yosemite region.On fresh surfaces quartz and feldspars are translucent light gray.Feldspars turn chalky white on weathered surfaces, and thier crystals reflect sunlight from strong planes of cleavage. |
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