5'- 



The Earth and Its Inhabitants Change unit x 



Fig. 467 Rivers of ice, knowii as glaciers, wear 

 away the land, breakiiig tip bard rock. Note 

 the broken rock below the glacier, (geological 



SURVEY OF CANADA) 



Fig. 468 As the temperature changes the ex- 

 pansion and contraction of the surface layer 

 causes the rock to scale. This is weathering. 



(gilbert — U. S. GEOLOGICAL SURVEY) 



Stream, the small rocks wear away even 

 the hardest rock bv their rubbing and 

 pounding-. Wearing away of soil and rock 

 is called erosion. You have seen it on a 

 small scale after every heavy rain. Even 

 strong winds may erode the soil; and 

 animals, burrowing in the ground, by 

 loosening the particles of the soil, help 

 erosion by wind and water. 



Rocks are broken up in other ways 

 too. Changes in temperature cause them 

 to crack and crumble. This breaking up 

 of rocks in air is called weathcrh?g. 

 Sometimes a seed falling into a tiny 

 crack grows and splits up the rock. Or 

 acids produced by the roots of plants 

 and by organisms of decay dissolve some 



rocks, such as limestone. In connection 

 with this try Exercises i and 2. 



In general, erosion is most rapid on 

 steep mountain slopes. Thus mountains 

 tend to flatten out gradually, throughout 

 the ages. It has been calculated that ero- 

 sion reduces the average height of the 

 United States one foot in every seven 

 thousand to nine thousand years. 



New land is formed. AVhat becomes of 

 the rock and soil that crumbles and is 

 washed or blown awav? Some of it is 

 piled up elsewhere on the land, and as the 

 mountains flatten, the lowlands tend to 

 be built up. But most of it is carried by 

 streams to the oceans or lakes into M'hich 

 they empty. It has been estimated that 



