204 PHYSICAL GEOLOGY 



partly of the action of storms in heaping up the ice. Shore ice may 

 hold a load of pebbles, both on its upper surface and near the bottom : 

 the former falling on the ice from the cliffs, as a result of the loosen- 

 ing of the rocks by frost ; the latter being obtained from the beach 

 which is frozen to the bottom of the ice. Therefore so far as the 

 position of the debris is concerned, shore ice resembles glaciers (p. 156). 

 During storms this ice is broken into great rafts or floes, and large 

 masses are driven upon the shores by the force of the wind and waves, 

 while in calmer weather they are moved backward and forward by 

 the tides. The stones embedded in the bottom of the ice grind and 

 crush the rocks over which they are pushed, scratching and polish- 

 ing rocky shores very much as glaciers polish and scratch the rocks 

 over which they move (p. 183). As in the case of glaciers, the rock 

 tools which accomplish this work are themselves ground to powder 

 (p. 159). It is probable that many of the striations on the rocks of 

 the coast of Labrador, and even on coasts as far south as Newfound- 

 land, were produced by floe ice and not by glaciers. The striae made 

 by the former, however, seldom have a uniform direction. 



Ice in Lakes. — Ice has much the same effect in protecting and erod- 

 ing the shores of lakes as in the seas, but the protection which it af- 

 fords is probably relatively greater, because the waves are usually 



less effective. The 

 absence of strong 

 shore lines in some 

 glacial lakes (such as 

 those which formerly 

 existed in New York 



and Massachusetts) 

 Fig. 187. — Diagrams showing the effect of ice shove , . , , , » 



in producing "walled lakes." (After Hobbs, Earth which may have been 

 Features.) in existence for a long 



time, may have been 

 largely due to a protecting fringe of ice which prevented the waves 

 from cutting back the shores. 



If a lake freezes over completely and is repeatedly subjected to 

 considerable changes in temperature, it may, by the expansion of the 

 water in refreezing, produce a strong " push " on the shores. The 

 expansion of the ice which accomplishes this result is produced as 

 follows. Water freezes at 3 2° F., and in so doing expands one ninth 

 in volume, but when the temperature of the ice becomes lower than 

 3 2° it contracts. This causes the ice to pull away from the shores 



