EROSION AND SEDIMENTATION 



219 



says : " It would take nearly 900 trains of 50 cars each, each 

 car carrying 25 tons, to carry an equal amount of sand and 

 mud to the Gulf. All the rivers of the earth are perhaps 

 carrying to the sea 40 times as much as the Mississippi." 



238. Vegetation and erosion. When visiting the banks of 

 a stream which has recently overflowed, we usually find silt 

 or mud deposited upon the plants along the banks. The more 



FIG. 108. Erosion by waves 

 The trees help to hold the soil against erosion by the lake 



dense the vegetation, the larger the quantity of silt deposited. 

 In many overflowed regions deep layers of rich soil have been 

 built up through periodic deposits made in this way. Where 

 the water of the stream is flowing rapidly, it can carry its 

 load of silt, but when its rate of flow is reduced, it drops 

 part of its load. 



Not only does vegetation exert a strong influence upon the 

 deposit of silt, but it also helps to prevent erosion (fig. 108). 

 The parts of plants above the soil retard the flow of water, 



