When glaciers form in mountains, the ice pulling away from the head of the valley plucks away rock into a bowl shape called a cirque. As the glacier flows down the valley, it grinds away obstructions along the sides, creating a smooth, straight valley with a U-shaped cross section. This view looking down from an airliner shows cirques and glacial valleys in the San Juan Mountains of southwestern Colorado. The dark brown rocks on the summits are volcanic rocks. 30 million years ago the San Juan Mountains were a volcanic plateau much like Yellowstone is today.
The sharp peak at right center is almost 13,000 feet high and located at 37o 35' 55" N, 107o 14' 15" W. The small lake below it is nearly 12,000 feet above sea level and is located at 37o 35' 29" N, 107o 14' 08" W.
Created 25 November 2005, Last Update 14 December 2009
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