NEW EROSION CYCLE IN GRAND CANYON DISTRICT 749 



the one which is located just southeast of Flagstaff. The perfect 

 maturity of form which characterizes the small branches even 

 to their very heads is well shown in Fig. i. The slightly 

 meandering course and mature side slopes of the main valley are 

 seen in Fig. 2. The alluvial filling is suggestive of a well-graded 

 condition of the stream, especially when taken in connection with 

 the gently sloping sides. It is, however, a significant fact that the 

 alluvium, which was deposited after the valley had acquired its 



Fig. I 



mature form, extends to the head of the valley so that its deposi- 

 tion was independent of the grade. ^ 



The region south of the canyon is, indeed, a network of these 

 mature valleys, and it is to be presumed that the corresponding 

 districts elsewhere are also covered with them. They occur on 

 the Marble Platform, but have not been specifically described 



I The same phenomenon is strikingly illustrated by the heavy alluvial mantles 

 of the large volcanoes of the region which, on San Francisco Mountain, appear to 

 be associated with glacial deposits. These alluvial deposits are now undergoing dis- 

 section, thus indicating a reversal of the conditions under which they were laid down. 



