98 GEOLOGY. 



a stream may develop a narrow valley in indurated rock where it 

 would not do so in loose gravel, and, other things being equal, it will 

 develop a narrower valley in rock which is horizontally bedded than 

 in rock the beds of which are inclined. Aridity, high altitude, and the 

 proper sort of rock structure therefore favor the development of canyons, 

 and many of the young valleys in the western part of the United States 

 where these conditions prevail, belong to this class. 



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

 distinction between a canyon and a valley which is not a canyon is 

 not sharp. The canyon depends for its distinctive character on the 

 relation of depth, width, and angle of slope to one another; but any 

 definition of the depth, width, and angle of slope necessary to constitute 

 a valley a canyon is arbitrar}^^ 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. 79 and 80) is the greatest canyon known, but it is rarely more 

 than a mile deep, and where its depth approaches this figure it is often 

 eight, ten, or even twelve miles wide from rim to rim. Its width at 

 bottom is httle more than the width of the stream; that is, a few hun- 

 dred 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. 81. 

 As a matter of fact the slopes of a 

 canyon are not commonly uniform. 

 The slopes represented in Fig. 82 Fig. 81.— Diagram showing the rela- 



correspond more nearly than ^^^^^^ ^^ depth and width of a valley, 



,, f T-i- r^-i 1 l^ ± ^ the width of which is eight times the 



those of Fig. 81, to the actual ^^^n-^^ 



slopes of the Colorado Canyon. The 



inequalities of slope are occasioned by the 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 Colo- 

 rado Canyon, great as it is, is a young valley; for it represents but a 



^ In regions where canyons are common, the term is often appUed to all valleys. 



