Rivers and Valleys 



the stream and upon each other. He then sees 

 whence conies the sediments which are so im- 

 portant a feature in the lower portions of the 

 river-system. From any commanding eleva- 

 tion in a mountain district, we may see scores or 

 hundreds of those torrent-beds within one field 

 of view. In periods of heavy rain, the roar 

 arising from the moving stones is often a very 

 striking feature. 



Descending the channel of any of these moun- 

 tain torrents, we find that after a few miles of 

 course, though the brook steadily gains in vol- 

 ume by the contributions of tributary streams, 

 it gradually diminishes the swiftness of its de- 

 scent. At a certain point it ceases to bear on- 

 ward all of the larger stones which come into its 

 possession. There fragments gather upon the 

 banks, forming a rude terrace. Still farther 

 down, where the slope is less considerable, the 

 smaller pebbles are left behind, crowded into the 

 interstices of the larger fragments. The terrace 

 becomes more distinct, vegetation gathers upon 

 it, and the waste of the plants forms a soil which 

 partially levels off the surface. Farther on, we 

 come to the field where the annual overflow of 

 the stream during the spring floods heaps a 

 quantity of the sand and mud upon this founda- 

 tion of coarser material; we then have the be- 

 ginning of the alluvial terrace. At first this 

 alluvial terrace is but a narrow belt on either 

 side of the stream which, swollen by its flood- 

 waters, often breaks new channels through this 

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