xiii Action of Water on the Land 257 



sliding over each other. 2 Since glacier ice is known to 

 consist of grains or lumps (from the size of a pea to that 

 of a melon), each of which is a single crystal, the flowing of 

 glaciers can be readily explained. Part of the accumulated 

 snow on a mountain slope is got rid of by avalanches or 

 snow-slips, which are powerful erosive agents, breaking 

 through everything in their path. 



337. Glaciers, although solid, flow like rivers, the centre 

 and surface moving nearly twice as fast as the sides, which 

 are retarded by friction with the valley. Compared with 

 rivers their motion is very small. The Mer de Glace, the 

 most famous glacier in Switzerland, creeps at the rate of 

 about an inch an hour in the centre during summer, and 

 only half as fast in winter. Some of the great glaciers of 

 Greenland move much faster, advancing from 50 to 60 feet 

 in a day, although 20 feet is a more common rate. The 

 thickness of glaciers in the Alps often exceeds 1000 feet, 

 and their length averages about 5 miles ; the longest is the 

 Aletsch Glacier, which measures 1 5 miles, including the 

 parent snow -field. As a glacier descends along the valley, 

 stones, clay and sand loosened by erosion fall from the slopes, 

 and rest as huge heaps of rubbish, called lateral moraines, 

 along each side of the ice. When two glaciers traverse con- 

 vergent valleys the lateral moraines on one side of each coal- 

 esce to form a medial moraine (see Fig. 54) down the centre 

 of the united ice-flow. In time a great glacier carrying the 

 ice of many tributaries becomes roughened with numerous 

 parallel ridges of rock rubbish along its length. The heat 

 of the Sun in summer continuously melts the ice, except 

 where it is protected by the overlying moraines, which thus 

 stand out prominently on the surface. Isolated blocks of 

 stone similarly protect and remain perched on ice pillars, 

 while the general surface is being lowered. As a glacier 

 forces its way along an irregular valley the ice is severely 

 strained, and cracks or crevasses result, which are narrow 

 and close at first, but gradually widen out in consequence 

 of the centre moving more rapidly than the sides. Huge 

 clefts are thus formed extending through the ice from surface 

 to bottom, and swallowing up masses of moraine rubbish. 



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