THE ORIGIN OF SOILS 



27 



always pulverising them and promoting chemical decom- 

 position of the minerals of which they are composed. 



Denudation. The debris resulting from weathering may 

 remain in situ, but is often carried away by running water 

 or, in dry climates, by wind. 



Wind-blown sands sometimes accumulate in enormous 

 quantities, and have been known to cover up tracts of 

 fertile country to a depth of several feet. 



Running water is by far the most important agent .of 



FIG. 2. Section showing the Conversion of Rock 

 into Soil. 



denudation. The material washed down by rain is carried 

 by streamlets to the rivers and so to the sea if not deposited 

 on the way. The power of water to carry solid particles 

 in suspension increases with the speed of the current. 

 When the speed diminishes the larger particles are 

 deposited, and, if it becomes slow enough, even the finest 

 will ultimately settle out. The course of rivers is often 

 diverted by the accumulation of detritus at various points, 

 and in time new land is formed, as may be seen in many 

 valleys. In a similar manner deltas are formed at the 

 mouths of rivers. The current is retarded where it meets 



