xin Action of Water on the Land 247 



structive. Water in this case ceases to carve and com- 

 mences to model the surface of the land. The alluvial 

 deposits are composed of the finest sands, and finally of 

 mud, which assist to raise the level of a wide area as the 

 river wanders over the plain. The alluvial plains of the 

 Mississippi cover 50,000 square miles, a space equal to all 

 England. Remains of dead animals and plants swept away 

 by the river in time of flood become embedded and buried 

 in the alluvial deposits on the margin of rivers or in the 

 mud and sand carried into lakes and seas, where they 

 either decay away or are preserved by various processes. 

 The work of a river has been compared to that of a mill 

 which "grinds slowly, but grinds exceeding small," rough 

 angular blocks being supplied in the torrential hopper, and 

 the most finely powdered material poured into the great 

 sack of the ocean. 



323. River Windings. When a swift -flowing river 

 laden with sediment is checked by any obstacle the sedi- 

 ment is deposited, and a sandbank or mudbank is formed. 

 When an obstruction of this kind is formed on the left bank 

 of a river at A (Fig. 51) the current of the river is deflected 

 from the straight line and strikes against the right bank, 

 rapidly undermining it at the 

 point, while the velocity of the 

 stream is checked opposite on 

 the left side, which becomes 

 built up by the deposit of sedi- 

 ment. The current is reflected 

 back to the left side at C, and || t ; \ ratting in on right 



7 . bank.. Ihe arrows show the 



SO the process goes On, Until the direction of the stream. 



straight river forms a series of 



winding loops as shown by the dotted line. The same 

 effect is produced by the unequal hardness of parts of the 

 bank, the softer being worn away and the harder left as 

 obstacles deflecting the current. The windings once begun 

 are perpetuated by the set they give to the current always 

 against the concave side, which is made more concave, while 

 the deposit of sediment adds to the convexity of the convex 

 side. The narrow neck of land between two concave curves 



