LAND WATERS STREAMS 



137 



the development of canyons, and many of the young valleys in 

 the western part of the United States where these conditions pre- 

 vail, belong to this class. 



While all canyons are valleys, most valleys are not canyons. 

 In popular usage, the rule seems to be that if a valley is sufficiently 

 deep, narrow, and steep-sided to be distinctly striking, it is called 

 a canyon in regions where that term is in use. Whether a valley 

 is deep, narrow, and steep-sided enough to be striking, clearly 

 depends on the observer. The Colorado Canyon (Figs. 101 and 

 102) is the greatest canyon known, but it is rarely more than a mile 

 deep, and where its depth approaches this figure it is often 8, 10, or 



Fig. 103. Diagram showing the proportions of a valley the width of which 



is eight times the depth. These are approximately the proportions of 

 the Colorado Canyon. 



..v.... : .. ;: 



Fig. 104. Cross-section of the Colorado Canyon. (After Gilbert and 



Brigham.) 



even 12 miles wide from rim to rim. Its width at bottom is little 

 more than the width of the stream; that is, a few hundred feet. 

 Its cross-profile throughout much of its course is therefore not in 

 keeping with the .conventional idea of a canyon. With a depth 

 of one mile and a width of eight, the slope, if uniform, would have 

 an angle of less than 15. Such a valley is represented in Fig. 103. 

 As a matter of fact the slopes of a canyon are not commonly uni- 

 form, but more like those of Fig. 104. The step-like slopes are 

 due to inequalities of hardness. It is perhaps needless to say that 

 to an observer on the rim of the canyon, the slopes seem several 

 times as steep as those shown in the diagrams. 



Like all valleys which are narrow relative to their depth, the 

 Colorado Canyon, great as it is, is a young valley, for it represents 

 but a small part of the work which the stream must do to bring 

 its drainage basin to base-level. 



