EFFECTS OF THE MOVEMENTS OF WATER 25 



courses. In geography you learned of the rich soil of the 

 river-bottoms. This soil is composed of materials carried 

 and distributed by flooded rivers. Rivers in flood some- 

 times destroy the work, and even the lives of men, but all 

 the time they are turning thousands of water-wheels that 

 do work for him, and they make a highway for his boats. 

 Rivers have been of great importance in the history of 

 man. There was a time when nine-tenths of all the people 

 in the world lived on rivers. This was because of the fer- 

 tility of the soil there, and because of the ease of travel 

 by boats. 



Erosion and deposition are two scientific terms that de- 

 scribe the process of shifting materials on the earth's sur- 

 face. Erosion means the "gnawing away" of material. 

 We say that acids erode metals; they seem to gnaw them 

 away. So, also, we say that wind and water erode the 

 land, especially the bare, uncovered parts of the land. 

 The waves gnaw away at the shores, and the streams gnaw 

 away at the hills. And all the material that is eroded is 

 deposited somewhere again. 



Protection by Plants. All this sounds as though the 

 world was a very uncertain place in which to live. It 

 sounds as though the land on which we live might be 

 washed away from beneath our feet. Of course this is not 

 true, but it would be true if it were not for the natural 

 covering and protection of the land that is formed by the 

 plants, especially the grass plants. In places where plants 

 cannot grow erosion may be very rapid. Think of the 

 "shifting sands" of the Sahara Desert, or of the "bad 

 lands" in our own country. In these bad lands, where 

 plants do not protect the surfaces, it is plain to see that 



