SOIL BUILDERS 11 



wash away. These are only a few examples of 

 plants that are particularly valuable for this pur- 

 pose. All plants are soil binders to some extent, 

 as well as soil makers ; they not only enrich it with 

 their herbage, but also hold it with their roots and, 

 if they lie close to the ground, with their herbage 

 also. In hilly sections some plants may be used 

 to great advantage in checking erosion. This 

 problem is discussed in Chapter XL 



HOW ICE HA.S MADE SOIL 



Once the northern part of North America was 

 covered with a great sheet of ice, reaching as far 

 south as Cape Cod, northern Pennsylvania, Ohio 

 and westward to the Rockies. Geologists tell us 

 that this immense glacier must have been several 

 hundreds, and in some places several thousands of 

 feet thick. It slowly bore down from the north, 

 moving only a few inches to a few feet an hour, 

 scraping the surface of the earth and carrying great 

 quantities of rocks, stones and soil to the southward. 

 According to some authorities certain parts of the 



glacier must have exerted a pressure of at least two 

 undred thousand pounds per square inch upon 

 parts of the surface over which it passed. The 

 bottom of the ice sheet became studded with huge 

 boulders, which acted like teeth, tearing and grind- 

 ing the rocks over which the ponderous mass 

 passed. Some of these boulders, scratched and 

 worn, may be still seen in the hillside pastures of 

 New England and other parts of the glaciated 

 region. Some exposed ledges of rock still show the 

 deep grooves that were cut m them by these boulder 

 teeth. 



When the ice melted a mass of soil material was 



