ICEBERGS. 701 



a very important means of erosion. They swell by the summer melt- 

 ing, and become violent, plunging torrents, and thus produce great and 

 rapid work while the glacier is slowly creeping along. The larger part 

 of the erosion of valleys along the courses of glaciers has been per- 

 formed by these streams. The snow and ice of Alpine valleys often 

 cause, indirectly, violent erosion and transportation of material, by 

 damming up streams. In no other way can barriers be thrown so 

 readily across profound valleys; and the deluges caused by the accu- 

 mulated waters, when they break loose, are often very destructive. 

 The Alps are full of examples. Again, the valleys are sometimes 

 dammed up by great moraines, making lakes ; and such lakes some- 

 times break through their barriers, and flood the valley below with 

 tearing waters. 



A glacier sometimes acts as a dam against the sides of a valley, 

 and forces a stream to flow far above its natural level. On the Aletsch 

 glacier, a lake which was thus made, and had existed for a number of 

 years, was recently discharged beneath the ice. By such a method 

 of damming all the effects of river action, such as erosion and alluvial 

 and flood deposits, may be made in a glacial region far above the level 

 of natural drainage. 



4. Icebergs. 



A glacier on a sea-coast often stretches out its icy foot into the 

 ocean ; and, when this part is finally broken off, by the movement of 

 the sea, or otherwise, it becomes an iceberg. Greenland is the great 

 region of icebergs, no less than of glaciers. They carry away the 

 stones and earth with which the glacier was covered during its land- 

 progress, and transport them often to distant regions, whither they are 

 borne by the polar oceanic currents. 



Dr. Kane describes the great pack of icebergs that occupies the 

 centre of Baffin's Bay, and mentions that some were 300 feet high, 

 and large numbers over 200 feet. There were 280 icebergs of the 

 first magnitude (the most of them over 250 feet) in sight at one time. 



In the Antarctic, the long ice barrier observed by Captain Wilkes 

 had a height above the sea of 150 to 200 feet ; and some of the bergs 

 were 300 feet high. 



As the specific gravity of ice is 0-918 (at 32° F.), the proportion in 

 weight of the mass out of water is about one twelfth. 



The icebergs of the Atlantic melt mostly about the Banks of New- 

 foundland, or between the meridians of 44° and 52°. They have been 

 observed in this occean as far south as 36° 10'. 



Icebergs are (1) a means of transporting stones and earth from one 

 region to another (see p. 534). (2) When grounded on rocks, they 



