16 SOILS: PROPERTIES AND MANAGEMENT 



the adobe of the Southwest owe their origin, at least 

 partially, to the carrying power of wind at a time when 

 aridity existed over all this area. 



12. Ice. — When in large bodies, as in glaciers, ice 

 exerts a tremendous grinding power. Glacial ice, by its 

 mobility and motility, adapts itself to all topography, 

 and as it moves slowly forward it grinds and scours and 

 abrades even the hardest rocks. The great masses of 

 pebbles and rocks which are picked up and imbedded 

 by glaciers, especially in their lower surfaces, increase 

 their cutting power many fold. The effect of glaciers 

 is of particular interest because of the fact that all of 

 the northern part of the United States was covered at 

 one time with a great ice sheet, and our northern soils 

 are due either directly or mdirectly to the advances and 

 retreats of this ice sheet. Formed in northern latitudes 

 due to a change in climatic conditions, the ice sheet 

 slowly covered many thousands of square miles of terri- 

 tory, and as the ice was usually several thousand feet 

 thick, hills, and often mountains, were overridden. Their 

 tremendous weight made the grinding action almost 

 irresistible. In the retreat, or melting back, of the ice, 

 a mantle of this groimd-up and well-mixed material was 

 deposited as soil; while the streams flowing from its 

 front, or into glacial lakes, were furnished with heavy 

 sediments for distribution in other regions. 



13. Heat and cold. — The changes in temperature of 

 the air, and the soils and rocks, tend vastly to augment 

 the effect of the denuding agents. Constant expansion 

 and contraction is productive of weakness and ultimate 

 physical breakdown. Heat is conducted slowly through 

 rocks, thus leading to differential heating and unequal 

 expansion or contraction. Rocks, as already noted, are 



