RAVINES, CANONS, GORGES. 



17 



way downward, forming narrow cations with almost perpendicular walls 

 several thousand feet deep, so that in many parts we have the singular 

 phenomenon of a whole river-system running almost hidden far below 

 the surface of the country, and rendering the country entirely impass- 

 able in certain directions (see Frontispiece). Nor is the erosion con- 



Fig. 10. — Canon of the Colorado and its Tributaries tf'rom a Drawing by Newberry). 



fined to canons ; for the rain-erosion has been so thorough and general 

 that much of the upper portion of the plateau has been wholly carried 

 away, leaving only isolated turrets (buttes) or isolated level tables with 

 cliff-like walls (mesas) to indicate their original height. All these facts 

 are well shown in Fig. 10. The explanation of these deep and narrow 

 cafions is probably to be found in the predominance of stream-erosion 

 over general disintegration and rain-erosion, which is characteristic of 

 an arid climate (Gilbert). 



Chief among these canons is the Grand Canon of the Colorado, 300 

 miles long and 3,000 to 6,200 feet deep, forming the grandest natural 

 geological section known. Into this the tributaries enter by side-ca- 

 nons of nearly equal depth, and often of extreme narrowness. Fig. 11 

 represents the natural proportions of such a canon. 



Time. — These remarkable canons have evidently been cut wholly 

 by the streams which now occupy them, and which are still continuing 

 the work. The work, probably commenced in the early Tertiary with 



