74 



PHYSIOGRAPHY 



it is eight to ten miles wide at the top. With a depth of one mile 

 and a width of eight, the slope, if uniform, would have an angle of 

 less than 15. The cross-section of such a valley is shown in Fig. 

 69. But the slopes of the canyon are step-like (Fig. 70), a form 



Fig. 69. Diagram showing the proportions of a valley, the width of which 

 is eight times the depth. These are approximately 



the Colorado Canyon. 



the proportions of 



Fig. 70. Cross-section of the Colorado Canyon. 



which is the result of the unequal hardness of the rock of the canyon 

 walls. The harder strata are the cliff-makers. 



The Yellowstone River also has a great canyon about 1,000 feet 

 deep (Fig. 46 and Fig. 2, PL XVIII, p. 65). It is narrower in 

 proportion to its depth than the canyon of the Colorado. The 

 Snake (Fig. 49) and the .Columbia rivers have wonderful canyons 

 in some parts of their courses, and so has the Arkansas River where 

 it crosses the Rocky Mountains. The canyons of many smaller 

 and less well-known rivers are almost equally striking. 



A narrow valley means that the processes which have made it 

 deep have outrun the processes which make it wide. Valleys are 

 deepened rapidly when their gradients are high and their streams 

 strong. They are widened slowly (1) when the climate is arid ; so 

 that there is little slope wash, (2) when the stream is so swift that 

 it does not meander, and (3) when the material of the sides is such 

 that it will stand with steep slopes. Solid rock, for example, will 

 stand with steeper slopes than loose sand. We conclude, therefore, 

 that (1) great altitude, (2) arid climate, (3) swift streams, and (4) 

 rock which will stand in steep slopes favor the development of can- 

 yons. In other words, young valleys in plateaus and mountains 

 are likely to be canyons. A strong stream in a dry region is possible 

 when the stream is supplied with abundant water from a humid 



