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Grand Canyon National Park Geology The canyon is primarily composed of sedimentary rock and metamorphic rock. The Inner Gorge, sections of which are visible from the South Rim, contains some of the oldest exposed rock surfaces in the world. The Vishnu Schist, as this rock is called, is all that remains of a once giant mountain range that existed about 2 billion years ago during the late Precambrian era. The vertical seams of dull rose quartz in the walls of the Inner Gorge were formed by molten material from the interior of the earth that penetrated into the base of the mountain range and forced its way into cracks in the rock before cooling and hardening. Gradually, the mountains were worn down by erosion, and the great sedimentary deposits that compose the rest of the canyon were laid down on the eroded roots millions of years later. At various times during the Precambrian era (2 billion to 600 million years ago), wide, shallow seas covered most of northern Arizona. Shell bearing animals had not yet evolved, and the most common rock types laid down during this period were sandstone and shale. A few limestone layers were deposited by algae, which grew abundantly in these seas. About 1 billion years ago, the land in the Grand Canyon region rose above sea level, tilting northeastward as it rose. The layers that are exposed at the top of the Inner Gorge were originally deposited horizontally; but as a result of the uplift, they now lie at a 10° to 12° angle. This era of uplift was followed by an interval of erosion that lasted for perhaps 400 to 500 million years. Erosion was so severe that all traces of several earlier sedimentary deposits were obliterated. These missing deposits are known as unconformities. Other types of unconformities were created when the Grand Canyon was above sea level and no sediments were laid down and, as a result, no rocks were formed. Vast seas again flooded the Grand Canyon region during the Paleozoic era, which began about 600 million years ago. The deposits they laid down as they receded were subjected to some erosion before being inundated by new seas. Unlike the earlier seas, those of the Paleozoic era were populated by a hard shelled animal similar to the modern oyster, clam, and coral. The remains of these animals now form enormous limestone layers in the Muav and Redwall formations. This cyclical pattern of deposition and erosion continued until about 70 million years ago, resulting in an accumulation of sandstone, limestone, and shale layers that amount to more than 3,000 feet. In the Permian period, which began some 270 million years ago, the Grand Canyon region remained arid. Formations such as the Coconino Sandstone were created by desert conditions. Sand, blown by stiff winds, stacked up in shifting dunes covering thousands of square miles. The layers left by these dunes are easy to recognize by their buff color and their distinctive herringbone pattern, which geologists call dune cross bedding. The desert period was followed by one of warm, shallow seas that resulted in the Toroweap and Kaibab layers, the latter being the upper most stratum of the canyon. Fossils of brachiopods and mollusks, as well as fossils of sponges and sharks' teeth, are found in these layers, indicating abundant marine life. Little evidence remains of yet another chapter in the unfolding story of the Grand Canyon. During the Mesozoic era, which began 230 million years ago, additional layers of sediment, 8,000 feet thick in places, were deposited. Wind and rain took their toll, and only two remnants remain: Cedar Mountain, near Desert View and the eastern boundary of the park; and Red Butte, about 15 miles south of Canyon Village. The Cenozoic era, which began some 70 million years ago, was the last great formative period of the canyon's history. During this era, the region remained above sea level. The dominant geological process was a combination of widespread erosion and sustained volcanic activity. The San Francisco Peaks, 70 miles to the southeast, were created, and thousands of feet of rock deposited during the preceding Mesozoic era were stripped away. Later in this period, the Colorado River began to cut down in earnest through the rock layers to the Inner Gorge. The spectacular buttes that stand out in the canyon like lonely sentinels are a byproduct of the action of the river. The buttes, which are made up of once continuous strata that stretched from rim to rim, are identifiable by the small caps or tips that rise from their flat tops. Over the years, various explorers and surveyors have given them exotic names such as Vishnu and Wotans Throne. Extract from "The Sierra Club Guides To The National Parks Desert Southwest". Published by Stewart, Tabori & Chang. Distributed by Random House. 1984. |
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