26 ELEMENTARY SCIENCE 



erosion is a sort of gnawing process; the sharp hills look 

 as though their sides had been gnawed. On railroad em- 

 bankments it is very important to start the growth of 

 tough grass, whose roots hold the soil together, and keep 

 it from washing away. If the plants were gone it would 

 not be long before the whole surface of the land would 



FIG. 3. Gully cut t 



begin to look something like the bad lands of South Da- 

 kota. As it is, our protected soil is not washed away 

 much except where streams gnaw away at their banks, 

 working under the soil and trees, or where gullies are 

 formed, or where there are steep slopes. 



A Picture of the Wearing Down of the Land. Now, in 

 mind's eye, you can begin to see the way in which water 

 wears down the land. You can see the streams, swollen 



