EROSION AND SEDIMENTATION 309 



the formation of deposits of gravel, sand, and silt. In times 

 of overflow of the channel banks this deposit is large, but we 

 must remember that erosion is also large at such times. When 

 streams overflow, the velocity of the water is usually decreased 

 and much of the load of soil is dropped. In this way large 

 areas along the Mississippi River are annually replenished by 

 the richest soil (fig. 149). Great dangers attend such overflow, 

 however, and effort is made to prevent it by building strong 

 dikes in the attempt to hold the river within the channel. 



At the mouth of such great rivers as the Mississippi and 

 the Yangtse the accumulation of sediment lias built up great 

 deltas. Much of the tremendous quantity of soil thus carried 

 into the sea goes far from shore and doubtless sooner or later 

 settles to the bottom to contribute its part to filling the 

 ocean basins. Since this sediment has been eroded from the 

 highland areas, it is evident that its removal from one place 

 and deposition in another contributes slowly but surely to 

 the processes of leveling the earth's surface. 



In China part of the people near the mouth of the Yangtse 

 River flood their fields at a time when the river is heavily 

 laden with silt, in order to replenish their soil. After the silt 

 is deposited, the water is drained away. Untold wealth in 

 soil is constantly being lost to the human race through 

 erosion, and the problem of maintaining the fertility of the 

 soil is annually made more difficult thereby. 



312. Erosion by wind. We have but to recall the dust 

 clouds of windy days to see clearly that the air has a soil- 

 carrying power, and if we will keep a moistened surface, as 

 that of a glass or a towel, in the dust-laden air for a few 

 minutes, we shall be able to get some notion of the quantity 

 of this dust. Snow soon becomes colored by dust carried 

 by the wind from places where the soil is exposed. The air 

 is never entirely free from dust. This air-borne dust is 

 deposited everywhere and in some places has accumulated in 



immense banks known as loess. 

 a 



