RECORDS LEFT BY RIVERS 



the Colorado and its tributaries in Western America. 

 The Grand Canon of the Colorado is three hundred miles 

 long, and in some places more than six thousand feet deep. 

 The country traversed by it is a network of deep ravines, 

 at the bottom of which flow the streams that have dug 

 themselves down from the top of the Colorado table- 

 land. 



Now suppose that the river has dug itself in as far as it 

 can go. There must be a limit, and the limit is reached 

 when the slope of the bed has been made so slight that 

 the current can only go on languidly. In that case it 

 cannot sweep along stones, or shingle, or even coarse 

 gravel ; and then the river so far from deepening its 

 channel begins to raise it by allowing more of the trans- 

 ported sediment to settle down. If a fast stream meets a 

 slower one deposition of material will take place ; and the 

 same thing will occur when the rivers meet a lake or a 

 sea. Whatever checks the swiftness of a current weakens 

 its carrying power and causes it to drop some of its 

 sediment to the bottom. Therefore accumulations of 

 sediment occur at the foot of torrent slopes along the 

 lower and more level ground. These deposits we call 

 alluvium, and sometimes when the mountain torrent ends 

 abruptly in the plain they may stand up in cones of silt. 

 They are sometimes called alluvium cones or fans. Quit- 

 ting the steep descents, and reinforced by tributaries on 

 either side, the stream ceases to be a torrent and becomes 

 a river. It goes fast enough at first to carry still coarse 

 gravel ; but the big angular blocks of rock have been 

 dropped, and the stones it now leaves in its bed are 



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