74 



SOILS AND THEIR USES 



ing of water; the work of air; the action of the sun and heat; the 

 action of growing roots; the agency of plants; the work of earth- 

 worms, insects and burrowing animals. The action of the ancient 

 ice sheets called glaciers had a mighty influence in grinding the 

 rock of the northern part of this continent. No other single in- 

 fluence has been so great. 



Soil Movements. The ancient glaciers not only ground up 

 rocks but moved them from place to place. Much soil was carried 

 by the moving ice. Soils became mixed. The surface contour 

 was greatly changed. High places were ground off and moved 



FIQ. 60. When eroding fields are neglected the soil is carried away to the streams below 

 a barren waste is the result. (Fights of the Farmer.) 



away. Valleys were filled and at melting-points moraines were 

 formed. These were sometimes in the form of hills, long ridges, 

 or occasionally served as dams across valleys. Water was im- 

 pounded in the form of lakes. The long chains of lakes of Minne- 

 sota, New York, Canada and elsewhere were thus formed. 



Running water has ever been a mighty force in the movement 

 of soil (Fig. 60) . Very heavy rain may form streams which carry 

 with them large quantities of clay and silt. In regions where 

 the natural streams are muddy much movement of soil has taken 

 place. In regions where the streams are clear less soil is being 

 carried. 



