14 



SOIL PHYSICS AND MANAGEMENT 



repeated differential expansion and contraction of adjacent unlike 

 minerals due to temperature changes of day and night loosen the 

 crystals, causing the rock to crumble. This plays a more prominent 

 part in the breaking down of coarse- than fine-grained rocks. In this 

 way rocks are weakened and finally reduced to soil material by 

 other agencies. 



(b) Freezing and Thawing. When water passes frorn the 



FIG. 7. Columbia Glacier overriding a forest, Alaska. (Courtesy National Geographic 

 Magazine, Washington, D. C. Copyright.) 



liquid to the solid condition its volume is increased by about nine 

 per cent, and the force exerted is 150 tons per square foot, or over a 

 ton per square inch. Water frequently freezes under conditions such 

 that part of this force is used in enlarging crevices in rocks, break- 

 ing off small fragments or displacing masses to such an extents that 

 when thawing occurs they may roll down the slope. This is espe- 

 cially noticeable during a morning thaw on stony slopes free from 



