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What is a glacier?
A glacier is a moving mass of ice that survives year to year, formed by the compacting of snow, and then into granular ice and set in motion outward and downward by the force of gravity and the stress of its accumulated mass. Glaciers are usually found in high altitudes and latitudes.
Glaciers are of four chief types. Valley, or mountain, glaciers are tongues of moving ice sent out by mountain snowfields following valleys originally formed by streams. In the Alps there are more than 1,200 valley glaciers. Piedmont glaciers, which occur only in high latitudes, are formed by the spreading of valley glaciers where they emerge from their valleys or by the confluence of several valley glaciers. Small ice sheets known as ice caps are flattened, somewhat dome-shaped glaciers spreading out horizontally in all directions and cover mountains and valleys. Continental glaciers are huge ice sheets whose margins may break off to form icebergs. The only existing continental glaciers are the ice sheets of Greenland and Antarctica, but during the icebergs, they were far more widespread. Glaciers may be classified as warm or cold depending on whether their temperatures are above or below -10°C (14°F).
Glaciers alter topography, and their work includes erosion, transportation, and deposition. Mountain glaciers carve out amphitheaterlike vertical-walled valley heads, or cirques, at their sources. They transform V-shaped valleys into U-shaped valleys by grinding away the projecting bases of slopes and cliffs and leveling the floors of the valleys; in this process tributary valleys are frequently left “hanging,” with their outlets high above the new valley floor. When the tributary valleys contain streams, waterfalls and cascades are formed, such as Bridal Veil Falls of Yosemite National Park. Elevations over which glaciers pass usually are left with gen...
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