88 WORK OF RUNNING WATER 



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. 70 and 71) 

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

 and where its depth approaches this figure its width at the top is in 

 most places 8, 10, or even 12 miles. 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. 



72. As a matter of fact the 

 rig. 72. Diagram showing the pro- , , 



portions of a valley the width of which is sl P es of a . canyon are not corn- 

 eight times the depth, about the propor- monly uniform, but more like 

 tions of the Colorado Canyon. those of Fig ^ The step _lik e 



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 



vv: . -.. : >> ' :;: '-. : -: : .,;,r.\-,:.;-;:.-;:;-;. 



-^. ; vv -JvM^ 



*^::'^v;r;:^;^ : ^^:^v.vV:^^V^^ 



Fig. 73. Cross-section of the Colorado Canyon. (After Gilbert and Brigham.) 



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

 its drainage basin to base-level. 



While aridity and high altitude are conditions which favor the 

 development of canyons, as shown by the fact that most canyons 

 are in high and dry regions, they are not indispensable. Niagara 

 River has a canyon below its falls, and the surrounding region is 

 neither high nor arid. The narrow part of the valley is so young 

 that side erosion has not yet widened the valley or lowered its angle 

 of slope to such an extent as to destroy its canyon character. This 

 canyon is often called a gorge, a term frequently applied to small 

 valleys of the canyon type. 



