VERTICAL FARMING 5 



Glaciation. During the glacial age vast sheets of ice, carry- 

 ing with them boulders and everything else that was movable, 

 passed over much of the United States and Canada, and ground 



BOULDERS WORN SMOOTH BY GLACIAL ACTION 



up the rocks into soil. Large areas of the richest soils were 

 formed in this way. Smaller movements of ice occurred in 

 the valleys of the Northwest and resulted in the formation of 

 some wonderful soil areas there. These movements of ice 

 leveled down the rough surfaces of mountain ranges and 

 scoured out wide valleys. 



Variations of Heat and Cold. The variations of tem- 

 perature from day to night and from summer to winter have 

 also been busy in grinding out soil meal. When it is warm 

 the rocks expand slowly, and contract under lower temper- 

 atures. The different minerals in the rocks expand and con- 

 tract unequally, causing cracking which flakes off the outside 

 of the rock and permits of its being attacked much more easily 

 by other agents of destruction, or perhaps, better said, of 

 creation. The effect of these changes of temperature are more 

 noticed in some of the higher and drier regions where the hot 

 evening gives way quickly to cold night. It has been reported 

 that this action is at times so violent as to split large pebbles in 

 half so quickly that a noticeable report, like the bursting .of a 

 percussion cap, is made. 



