168 



EARTH FEATURES AND THEIR MEANING 



youngest and coarsest upon the top in nearly horizontal position 



(Fig. 174, 3). 



It has been estimated that the surface of the United States 



is now being pared down by erosion at the average rate of an 



inch in 760 years. 

 The derived ma- 

 terial is being 

 deposited in the 

 flood plain and 

 delta regions of its 

 principal rivers. 

 Some 513 million 

 tons of suspended 

 matter is in the 

 United States car- 

 ried to tidewater 

 each year, and 

 about half as much 

 more goes out to 

 sea as dissolved 

 matter. If this 

 material were re- 

 moved from the 



,2. &eda /br/necf of one rime. 



To/jeer beets 



/we -set- beefs 



Bottom 



3 terrtca/ <Secr/o/7 across The teds. 



FIG. 174. Diagrams to show the nature of delta de- 

 posits as exhibited in section. 



Panama Canal cutting, an 85-foot sea-level canal would be ex- 

 cavated in about 73 days. The Mississippi River alone carries 

 annually to the sea 340 million tons of suspended matter, or 

 two thirds of the entire amount removed from the area of the 

 United States as a whole. It is thus little wonder that great 

 deltas have extended their boundaries so rapidly and that the 

 crust is so generally sinking beneath the load. 



