32 ELEMENTARY AGRICULTURE 



now be picked up are so hard is that only the hard ones 

 could withstand the grinding. The softer ones were 

 ground up and helped to form soil. In the western part 

 of the state of Wisconsin, in eastern Iowa, and in north- 

 ern Illinois is a tract known as the "driftless area, "over 

 which the glacier did not pass. Here the soil may be 

 seen in the actual process of formation. The rock on top 

 gradually "rots" and breaks up. The water washes the 



Drawing, showing how rock 

 gradually breaks up and de- 

 cays from the top downward. 



Drawing, showing glacial 

 drift deposited on top of the 

 solid rock. 



lighter portions down and spreads them out at lower 

 levels. The rain and snow work their way into the 

 cracks of the rocks and, freezing there, break them up 

 into smaller pieces. Even the wind breaks off small 

 pieces and carries them away. Great drifts of sand, 

 like snow, may sometimes be seen piled up by the 

 action of the wind. Plants die and decay, and thus 

 help to build up the soil. Roots of trees sometimes 

 work their way into crevices of the rock and, growing 

 there, split off great pieces. Roots also secrete a kind 

 of acid that helps to dissolve the rock. The gases in 

 the air help in breaking up the rock, thus forming soil. 



