Shaping a Canyon EarthCache
This cache has been locked, but it is available for viewing.
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Cache is along the South Rim Road inside Black Canyon of the Gunnison National Park. Entry fee may apply. From Hwy 50 take the East Portal Entrance road. Lots of great information available at the Visitor's Center.
Approximately 2 million years ago the Gunnison river cut through the relatively soft volcanic deposits of the Gunnison Uplift and began to erode the hard Precambrian rock that now forms the walls of Black Canyon of the Gunnison. Many forces join in the erosion of the canyon. Some of the forces of erosion that have shaped the canyon are wind and water erosion, chemical erosion such as the acids secreted by lichens which weaken the rock, and mechanical erosion such as the roots of plant life growing in and widening the cracks in the rock. The chief force of erosion is the force of water from the Gunnison River and the sediment that it carries. The river bed is one of the steepest in the United States. The Gunnison River bed descends an average of 95 feet per mile throughout the 48 miles of the canyon. At the steepest point the river descends approximately 480 feet in one two-mile stretch. Before the construction of dams upstream, the force of the river could reach as much as 12000 cubic feet per second, nearly 30 times the maximum force that is reached today. This was enough force to move very large rocks down the river. Still, the canyon widened only approximately one inch per hundred years...approximately the width of a human hair every year. Many of the forces of erosion are subtle. For instance, the South Rim has eroded farther than the North Rim. This is due to the fact that the South Rim faces North and receives less sun than the North Rim. The South Rim dries more slowly than the North and a moist slope erodes more rapidly than a dry one. As the rocks of the canyon break away and fall into the canyon, they are carried away by the force of the river.
To claim this cache you must answer the following question which can be obtained from the interpretive sign at the coordinates:
Some of the rocks that fall into the canyon are too large to be carried away by the river. Some of them fall into the river and create rapids. Others form steep piles of debris along the canyon walls. What is the scientific name of these steep piles of debris along the canyon walls?
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