138 



PHYSICAL GEOLOGY 



of the rocks, such valleys are called structural valleys. They 

 usually contain stream valleys hi their bottoms. 



FIG. 125. Sketch of a gully. 



The beginning of a 

 valley. Figure 125 

 shows an infant valley 

 or gully, which con- 

 tains running water 

 only during rains. In 

 the future, rain water 

 running down the 

 slope into the head of 



FIG. 126. Gullies near River- 

 side, Cal. (Fairbanks.) 



the gully will make it 

 longer ; water coming over 

 the sides will make it 

 wider ; and the water which 

 flows along the bottom 

 will make it deeper. Thus 

 it may become sufficiently 

 long and wide and deep to 

 be called a ravine, and 

 finally a valley. When its 

 bottom is worn below the 



FIG. 127. A mountain ravine near 

 Marshall, N. C. (U.S. Geol. Sure.) 



