The Sierra Nevada is a young mountain range, probably not more than 10 million years old. Forces in the earth, probably associated with the development of the Great Basin, forced the mountains to rise. During the last 10 million years, at least four ice ages have coated the mountains in a thick mantle of ice. Glaciers form and develop during long periods of cool and wet weather. Glaciers move very slowly through the mountains, 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 landscape of hanging valleys, waterfalls, craggy peaks, alpine lakes and glacial canyons.