THE WORK OF RUNNING WATER 75 



region above. The Colorado, Snake, and Arkansas rivers are ex- 

 amples. 



The deeper canyons of the West make travel athwart their 

 courses almost impossible, while their rivers rarely serve the needs 

 of navigation or irrigation. The ancient cliff-dwellers often made 



Fig. 71. Cliff dwellings, southwestern Colorado. 



their homes in the recesses of canyon walls (Fig. 71), probably 

 because these positions could be easily defended against enemies. 



Canyons must finally become valleys of another type, for the 

 stream in the canyon will in time cut to base-level. The valley will 

 then stop growing deeper, but widening will still go on, and the 

 narrow valley will become so wide that it will cease to be a canyon. 



Bad lands. The name bad land is sometimes given to the type 

 of topography shown in Fig. 3, PL XV, p. 60. Such topography 

 is developed in the late youth or early maturity of certain high 

 regions, and is found in various parts of the West, especially in 

 western Nebraska, Wyoming, and the western parts of the Dakotas, 

 where the formations are largely sandstone or shale, alternating 

 with beds of clay. A semi-arid climate, where the rainfall is fitful, 

 seems to be favorable for the development of bad land topography. 



