THE WORK OF STREAMS 



165 



canons. The canon of the Colorado River (Figs. 164 and 

 165) is the largest in the world. The Colorado has been 

 able to cut a very deep valley because the surface of the 

 plateau in which it is formed is high above base level. The 

 valley is still narrow because (1) the climate is arid and a 

 number of the agents which widen valleys (p. 140) have 

 accordingly worked slowly, (2) most of the rocks of the 



FIG. 165. Portion of the Grand Canon of the Colorado River, Arizona. 

 (Walcott, U.S. Geol. Surv.) 



canon walls are capable of standing in steep faces, and 

 (3) under these circumstances, the valley has not existed 

 long enough to have been made wide. (How can the river 

 be of large volume when the climate of the region is arid?) 

 Vast as the Colorado Canon and its tributary canons are 

 (Plate VII), the region is nevertheless in a youthful stage of 

 erosion, for very little of the work of reducing it to base level 

 has been accomplished. As ages pass, the canon will be 

 worn slowly deeper and wider by the water and weather, 

 and the side canons will become larger and more numer- 

 ous, until finally the great plateau will be reduced to a 

 nearly level plain but little above sea level. (Wh&t will be 



