Yosemite Documentary: Review

Yosemite Documentary

                The main focus of this documentary was to discuss the environmental conditions that must have occurred to give Yosemite Valley its unique shape and grandeur. This discussion started out as a debate between two thinkers, John Muir and Josiah Whitney. John Muir, founder of the Sierra Club believed that Yosemite’s peaks and box like valley was a result of large glaciers carving into the rock. Josiah Whitney believed that this box like appearance could have only been formed by incredible seismic activity in which the valley pulled apart and sank, carving out the sheer cliffs as it fell. Yosemite Valley is made up primarily of Yosemite granite, one of the hardest rocks in the world. Yosemite Valley was once covered in sandstone, a sedimentary rock. Evidence of granite between layers of sandstone indicate that the granite was once molten. The molten granite was forced up through the mountain through cracks and as it cooled it formed these bands between the sandstone.

 Yosemite granite is comprised of abnormally large crystals giving it its extreme strength and durability. Large crystals form as a result of the molten granite taking a long time to cool. The mountains of Yosemite formed miles under the surface millions of years ago when deposits of molten granite became trapped under the surface. These deposits stretched for hundreds of miles. Traditionally, when granite forms it is laden with cracks, but in the case of Yosemite granite, repeated injections of hot magma into the granite deposits continually reheated the solid granite, fusing the cracks and forcing the granite to take a far longer time to cool. This extended cooling duration is what lead to the abnormal crystal size and this the incredibly strong and flawless rock. At some point in its history, Yosemite suffered a violent geographic shift. A massive section of the earth’s crust underneath the Valleys east side dislodged, reducing the amount of pressure on the east side. This shift in force cause the east side of the mountain range to snap and rise sharply. With this rapid increase in incline, the flow of the Merced River increased dramatically, turning it into a raging torrent. This increase in force gave the river the force it needed to cut into the durable Yosemite granite.

 Evidence of signature scratch marks on the rocks and glacial moraine found deep within the valley is evidence that a glacier once passed through the valley. The glacier carved through the mountains and pulled rock along the surface, depositing this debris at the glaciers mouth. After the glacier receded, the Merced River was blocked by the glacial moraine. The Merced River began to full up Yosemite Lake. With stronger flow, the Merced River was bringing with it large amounts of sediment down from the mountains and depositing it into Yosemite Lake. Over thousands of years this sediment settled forming a flat layer of silt at the bottom of Lake Yosemite. After the Lake dried up, the flat valley floor of Yosemite Valley was left exposed. Further evidence proving Muir’s hypothesis that a glacier carved out the valley was proven when researchers discovered the U-shaped feature of rock which had been covered by the flat layer of silt.

The dome shaped peaks unique to Yosemite were formed during the mountains infancy when the hard Yosemite granite deposits were first exposed to the outside conditions. As the top layer of the rock was exposed, pressure was still exerted on it from all other sides causing the granite to squeeze and fracture at the top, forming layers like an onion. These layers are separated by cracks in the rock which are weak points where the stubborn Yosemite granite is vulnerable to the elements. Over time sections of these layers collapse and fall away in rockslides, leaving a polished, dome like mountain top. The most violent rock slide in Yosemite occurred on July 10, 1996 in which roughly 80,000 tons of rock dislodged and came crashing down to the valley floor, bulldozing its way through the valleys most popular trail. The incident claimed the life of one young man and left a young woman paralyzed. These rock slides are still occurring today and now play a signature role in changing Yosemite’ landscape.