TEMPERATURE OF THE HYDROSPHERE 199 



As the temperature of ice sinks below zero, especially at night, 

 further contraction takes place, and thus cracks are formed in the 

 ice sheet, which may extend for hundreds if not thousands of 

 meters in length and cross each other at various angles. Water 

 rising in these fissures freezes, and so prevents the closing of the 

 old fissures on the expansion of the ice during the day. A powerful 

 lateral pressure is thus inaugurated which, if the whole lake is 

 frozen, will cause the ice to move up on the shore, carrying ma- 

 terials with it and building a shore wall of ice-shoved boulders, 

 while at the same time it may scratch the underlying rock layers 

 and produce the effect of glaciation. Such boulder walls and ridges 

 may also be built by the ice floes resulting from the breaking up 

 of the ice in spring. These floes are driven onto the shelving shores 

 by wind and carry stones up with them. This action is pronounced 

 in northern lakes like those of Labrador. Tyrrell (40:64 B.) has 

 described such ridges around Lake Winnipegasie, and Russell men- 

 tions their occurrence on other Canadian and northern United 

 States lakes, where they are found "40 or 50 feet from the water's 

 edge, are 20 feet high, and broad enough to furnish convenient 

 roadways." (Russell-31 :^2.) The same lateral pressure through 

 expansion of the ice causes the buckling of the ice masses in the 

 center of the lake. Only in very shallow water bodies will the ice 

 extend to the bottom. Owing to the greater density of water at 

 4° C, this will sink to and remain at the bottom, and the slow con- 

 ductivity of both water and ice will prevent the reduction of the 

 bottom temperature during the cold season. It is in this way that 

 organisms can survive under the frozen surface of a lake. 



Normal and Excessive Temperatures of Streams and of 

 Ground Water. The temperature of streams in a given area varies 

 according to the season, but also with the volume of water and with 

 its source, the length of the stream, the character of its bottom and 

 banks, etc., so that different streams within the same region and at 

 the same time may have different temperatures. Thus streams re- 

 sulting from melting glaciers will always be cold, although, as they 

 proceed in their course, the waters may be warmed to a certain ex- 

 tent by contact with the warm air. H much sediment is carried 

 in suspension by a stream, its temperature will be proportionally 

 higher. A stream flowing through a lake will, after leaving the 

 lake, have in general the temperature of the surface waters of that 

 body, which alone are carried out. Underground eflluents in like 

 manner have the temperature of the layers of water in which they 

 originate. 



Spring waters vary less in temperature, for their sources are 



