A snow that falls on the walls floors of valleys in the high mountains regions tends to accumulate to great depth, because of the rate of melting, particularly in winter time, is far lower than the rate at which the snow falls. As a result, the earlier snows, compress by later fall, are change into a compact body of ice having a granular structure. Sublimation is a change from solid state into vapor without an intermediate liquid stage. When the depth of the glacier reaches about 30m (about 100ft) the whole mass begins to creep slowly down the valley. This flows continues as long a superabundance of snow falls at the top of the glacier. As the glacier flows down the valley to lower altitude where it is not replenished by snow fall, it melts or wastes away, the meltwater forming the source of streams and lakes. At the base of the glacier is a layer of clear ice that may approach a density of 0.7 to 0.8 and flows like a viscous fluid. The lower glacier is under such great pressure that any cracks or separations occurring in this layer are quickly healed. Upper layers, however, may suffer tensions and strains from moving over underlying obstructions or from differential movement, in which the center of the glacier moves more rapidly than its edges. A the top is a mantle of fleshly fallen snow with a very low density of not more than 0.1. Below this is a layer in which the snowflakes have diminished in size to become granular snow, of which the density may be 0.3 or greater. This is caused either by the influence of moisture and pressure exerted by accumulated snow or by sublimation and recrystallization. These strains produce crevasses that may be many meters deep and are frequently covered by newly fallen snow. A large crevasse known as the bregschrund is usually formed in the shape of a semicircle at the head of a glacier, between the glacier it-self and the headwall of the valley in which it lies.
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