While geologists debate the details, it is clear that the Sierra Nevada is a young mountain range, probably not more than 10 million years old. Incredible forces in the earth, probably associated with the development of the Great Basin, forced the mountains to grow and climb toward the sky. During the 10 million years at least four periods of glacial advance have coated the mountains in a thick mantle of ice. Glaciers form and develop during long periods of cool and wet weather. Today, a few small glaciers remain in the parks. They are the southern-most glaciers in North America. Glaciers move through the mountains like slow-motion rivers carving deep valleys and craggy peaks. The extensive history of glaciation within the range and the erosion resistant nature of the granitic rocks that make up most of the Sierra Nevada have together created a spectacular landscape of hanging valleys, towering waterfalls, craggy peaks, alpine lakes and gigantic glacial canyons.
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