STUART WILSON / SCIENCE PHOTO LIBRARY STUART WILSON / SCIENCE PHOTO LIBRARY
Glacial Cirque in Rocky Mountain National Park, Colorado. This cirque was the birthplace of many glaciers. Several times during the last two million years, snow hundreds of feet deep accumulated at the top of this valley. The tremendous weight of the compacted snow and ice caused the mass to move slowly downhill, thereby giving birth to a glacier. As glaciers crept down the mountain, they scooped out this bowl-shaped cirque, leaving behind steep headwalls, a sloping valley floor, and a broad U-shaped valley.
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