380 



FIRST YEAR SCIENCE 



That the ice was here seems to be sure, but exactly 

 when or why is unknown. This period when the ice was 

 of great extent is called the Glacial Period. Probably dur- 

 ing the earth's history there have been several of these 



periods, but to the last 

 is due the great changes 

 wrought upon the pres- 

 ent surface of the 

 country and upon its 

 plants and animals. 



171. Glacial Lakes. 

 In northern countries 

 are found ponds and 

 lakes filling the irregu- 

 lar depressions in the 

 deposit left by the re- 

 treating ice. Lakes of 

 another kind are also 

 sometimes formed in 

 glaciated regions. The 

 advancing or retreating 

 ice may happen to make 

 a barrier to the escape 

 of the drainage, and thus may form a lake with an ice dam 

 at one end. The lake will continue to exist only as long 

 as the ice obstructs the drainage. 



The Marjelen Lake in Switzerland is a well-known ex- 

 ample of this. The Aletsch glacier, the greatest of all the 

 Swiss glaciers, obstructs a lateral valley, forming an ice 

 wall about 150 feet in .height, behind which the drainage of 

 the side valley accumulates and forms a lake. Pieces of 

 ice from the glacier fall off into it, forming icebergs which 

 float upon its surface. 



Sometimes a crevasse opens in the ice wall, and then the 



A HUGE PERCHED BOWLDER. 



