GLACIAL ACTION 47 



trated in the workings of large rivers where the 

 pulverized rock is deposited along the river and at its 

 mouth. Large areas of the soil in valleys and river 

 bottoms have been formed in this way, and in most 

 cases these soils are of high fertility. The action of 

 water is not alone confined to forming soils along 

 water courses, but is equally prominent in the forma- 

 tion of soils remote from streams or lakes, as in the 

 case of soils deposited by glaciers. 



51. Glacial Action. At one time in the earth's 

 history, the ice-fields of polar regions covered much 

 larger areas than at present. 20 Changes of climate 

 caused a recession of the ice fields, and resulted in the 

 movement of large bodies of ice, carrying along rocks 

 and frozen soil. The movement and pressure of the 

 ice pulverized the rock and produced soil. This 

 action is well illustrated at the present time where 

 mountains rise above the snow line, and the ice and 

 snow melting at the base are replaced by ice and 

 snow from farther up, moving down the side of the 

 mountain and carrying along crushed stones and soil. 

 When the glacier receded, stranded ice masses were 

 distributed over the land. These melted slowly and 

 by their grinding action hollowed out places which 

 finally became lakes. The numerous lakes at the 

 source of the Mississippi River and in central Min- 

 nesota are supposed to have been formed by glacial 

 action. The terminal of a glacier is called a moraine 

 and is covered with large boulders which have not 

 been disintegrated. The course of a glacier is fre- 

 quently traced by the markings or scratches of the 



