QUESTIONS 305 



water, especially in the sea, the buoyancy of the water is 

 sufficient to crack the ice, and the end of the glacier floats 

 off as an iceberg. 



There are many evidences that large areas of what are now 

 the most thickly populated regions of North* America and 

 Europe were once covered with thick layers of ice. This 

 mantle of ice after several advances and retreats finally dis- 

 appeared. The period of the last of these several advances 

 of glacial ice to southerly latitudes is called the glacial 

 period. These ancient glaciers have left unmistakable 

 traces, They scoured out depressions in the earth, some of 

 which now form small lakes and ponds. They pulverized 

 the rocks in their course and transported the soil thus formed 

 to latitudes where it now serves agricultural purposes. They 

 changed the direction of flow of many rivers and dammed 

 back great sheets of water into lakes which disappeared 

 when the glaciers melted, leaving flat, fertile plains. 



The very plants themselves and the animal life which they 

 support must die and return by decay to the soil from which 

 they came. Thus even decay must be looked upon as a 

 soil-forming process which is friendly to man. Decay is 

 produced by bacteria and other minute forms of life which 

 must have considerable moisture in order to thrive and 

 accomplish their work. 



Sun, water, ice, bacteria, the movements of the atmosphere, 

 and the slow movements of the earth's crust are constantly 

 working in league with one another to tear down what many 

 of the same agencies have worked steadily to build up. 



QUESTIONS 



What examples of rock weathering have you ever seen ? 

 In what ways has wind acted as a soil builder? 

 In what ways has ice acted as a soil builder? 



