SOIL-FORMATION, ICE 27 



In addition to the material carried in suspension, 

 a large amount is rolled along the bottom of the channel. 



Because of the unequal carrying power of streams 

 of different velocity, the load of debris is sorted into 

 groups of somewhat uniform size. In this way have 

 been formed great areas of clay, silt, sand and gravel 

 found in all farming sections, and which owe their 

 peculiar crop-producing properties most. largely to this 

 sorting action of water. 



9. Ice glaciers. Masses of ice have exerted a 

 tremendous influence in the reduction of rocks to soil 

 material. Their action is chiefly mechanical, but is inti- 

 mately associated, as a rule, with the action of water. 

 The chief agency of ice is in the form of glaciers, which 

 issue from regions of high latitude, or of great elevation, 

 and in times past have pushed down over much larger 

 areas of country than they now occupy. A large part 

 of all of the continents have been overrun by such 

 masses, which, through their great weight and almost 

 resistless movement, ground even the hardest rocks to 

 fine powder and mixed the materials from many sources. 

 Fragments of rock imbedded in the bottom of the ice 

 became its tools to scratch and crush the floor upon 

 which it rested. In this way has come about the scouring 

 and pulverization of rocks, analogous to the action of 

 water. The ice appears to have attained a depth of 

 thousands of feet in some places, and consequently was 

 able to override even mountainous areas, sweeping away 

 and grinding to fragments the smaller eminences and 

 irregularities. Since the access of water was limited, 

 there was little opportunity for pronounced chemical 



