SOILS 93 



the wheat to make it into food for man, so it became neces- 

 sary for nature's forces to grind the rocks in order to pre- 

 pare the food in them for plants. In other words, the first 

 process in soil formation is the pulverization of rocks. 



Several methods have been used to bring about the break- 

 ing up of the rocks. Among these is change of tempera- 

 ture, or heat and cold. The expansion and contraction of 

 the rock particles due to heat and cold and the water con- 

 tent result in the breaking up of these rocks into small 

 pieces. Another factor is the action of running water. 

 Water running over stones gradually wears off the surface. 

 Rapidly moving water carries pieces of stone along with it, 

 and these rubbing together and over the surface of the 

 stream bed help to break up the stones and make soil. 

 There are also chemical influences of air and water in aiding 

 these physical forces. Another agency which helps to 

 grind rocks is moving ice in the form of glaciers. There 

 is an accepted theory that at one time all the northern part 

 of our country was covered with a thick sheet of ice. This 

 immense glacier pushed its way down from Canada, carrying 

 with it large quantities of rock, grinding them against each 

 other until they were reduced to fine soil material. When the 

 ice melted later, the rock particles were distributed and 

 became the great glacier deposits upon which the soil was 

 formed. 



Soil produced by the mere grinding of rocks alone is not 

 sufficient for crop production, however. The plant-food in 

 this soil must be made available, that is soluble for plant 

 roots to absorb. Water is important in bringing about this 



