THE WORK OF STREAMS 109 



and their tributaries join them at right angles, because all except 

 the master streams are subsequent rivers (p. 102). In such regions, 

 the larger streams cut rapidly in the weaker rocks and often behead 

 the streams that flow across the hard beds. After a stream has been 

 captured its new grade will be steeper than before, and it is likely to 

 cut a trench in its old valley, leaving the remnants of the latter 

 as terraces. In regions of horizontal rocks stream capture is also 

 common. If one of two streams heading toward the same point has 

 a straighter and steeper course, or a greater volume of water, or a 

 load of sediment sufficient for rapid cutting but not so great as to 

 cause deposition, it may cut back more rapidly than the other and 

 in time capture the headwaters of the latter. 



The Erosion Cycle 



The terms youth, maturity, and old age are used to express 

 the characteristics of valleys, and are helpful since they are as descrip- 

 tive of them as the same terms applied to human beings. " They 

 have reference not so much to the length of their history in years as 

 to the amount of work which streams have accomplished in compari- 

 son with what they have before them." 



Youth. — Young valleys are V-shaped, with steep sides, and are 

 occupied by rapid streams unless the land is low. Since they have 

 had but a short life, 



rapids and waterfalls ™ 



are often numerous; 

 the divides are wide 

 and ill-drained, as the 

 frequent occurrence of 

 marshes and lakes usu- #r 

 ally indicates. The ■ 



Grand Canyon of the _ _, , ,. 



p, . . . Fig. 94. — Block diagram showing a region in the 



Colorado, the Steep youthful stage of its erosion cycle. Sea level is repre- 



gorge of the Niagara sented by the bottom of the diagram. 



River, and all narrow, 



steep-sided, or V-shaped valleys are in youth. A region is said to be 



youthful (Fig. 94) when sufficient time has not elapsed for streams 



thoroughly to dissect and drain it; in other words, the streams have the 



larger part of their task before them. The Red River valley of North 



Dakota and Minnesota is such a region, since it has not long been 



