The Yosemite Valley : Product of many Geological Forces

That Yosemite V all(prenominal)ey has a labyrinthine geologic history also stub be discerned from the rocks that can be seen there today. Looking at the geologic while within the valley starting with about 136 million geezerhood ago, the evidence shows an early Cretaceous intrusive kind 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 minor episodes occurred between these two phases (1:303).

The dispersion of the different rocks' ages suggests that "major episodes of plutonism were often separated by bad intervals of time" (1:303). During the early Cretaceous period, the Jurassic and older aqueous rocks were intruded by a sequence of magmas called, in the case of Yosemite Valley, the westbound peeping Suite. These intrusions atomic number 18 the most-widespread and most-recognized features in the valley, as with the El Capitan Granite.

The pillowcase of the El Capitan Granite is a massive cliff with a patrician irregular shape in its center called the "Map of sexual union America." The granite itself is extremely sheer, with few through-going joints, and the discoloration of the so-called map is caused by "a complex of diorite dike intrusions that invaded the plutons of the earlier Western Intrusive Suite" (1:303). The very thin white lines that can be seen crisscrossing the granite's face as well as the map atomic number 18 later pegmatite dikes, or sheet identical intrusions.

3. Lambert, D.; The Diagram Group. The field guide to geology. New York: Facts on point; 1988.

A batholith is formed from the slow cooling of resolve magma several miles below the earth's surface. As layers of overlying rock are removed, the pressure is eased on the rock below, which then expands. blowup causes the rock to form curved joints that promote "'sheeting' (pulling off) of rock shells from the sexual mass--the likely origin of the huge domes in . . . Yosemite Valley" (3:103).

For scientists such as geologists, the rocks of the Yosemite Valley can be read like the exquisite illuminated manuscripts of the Middle Ages, whose beauty also holds knowledge for those who know how to read them.

Three processes besides the earlier-mentioned glaciation birth contributed to the corporal shaping of the valley: weathering, stream erosion, and mass wasting. The different types of rocks themselves have also played a key role in how the valley has been formed.

exfoliated rather than glacially scoured. Most of the towering cliffs that circumvent Yosemite Valley are the result of the El Portal glaciation and non the Wisconsin glaciation, which sparely was no thicker than about 1500 feet (450 meters), although it was responsible for the polished surfaces of the valley's bouldery rocks (5:55).

By the Tertiary period, volcanism was pickings place to the north of Yosemite Valley, with a series of uplifts resulting from the faulting and tilting of the Sierra Nevada block. Glacial deposits, alluvium, landslides, rockslides, and talus can all be found in Quaternary period layers in the Holocene epoch. At the same time, erosion and deposition by glacial, fluvial, and mass-wasting processes were ever-changing the deposition of these materials. By the Pleistocene epoch, glacial tills and moraines are evident; some early deposits were buried or destroyed by later ice advances. Severe erosion also was taking place during this time, sculpting giant staircases, hangin
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