HOW ROCK BECOMES SOIL 



285 



was brought to its present position by glaciers. A study of 

 the maps of Illinois, Iowa, and other states will show that 

 the soil of the so-called corn belt is of glacial origin. 



FIG. 140. The continental glacier 



The map shows the extent of the continental glacier in North America. After 

 the United States Geological Survey 



285. Effect of plants and animals. When the roots of 

 plants grow into crevices of rock, they often exeit sufficient 

 force to split it (fig. 141). Many lowly plants, such as 



