THE WORK OF RUNNING WATER. 75 



its valley, and not the valley its stream. Many post-glacial valleys, on 

 the other hand, antedated their permanent streams, as in the cases 

 first described. 



If the gradient of a slope on which valleys are to develop is notably 

 unequal, though without basins, the development of valleys may follow 

 somewhat different lines. If on emergence the seaward part of a new 

 land area assumes the form of a plain, bordered landward by a steeper 

 slope (Fig. 56), the most notable early growth of the valleys would be on 



Fig. 56. — Diagram showing the phases of valley development described in the text. 



the latter. The run-off would develop gullies on the steep slope, but on 

 reaching the plain below the velocity of the water would be checked, 

 and it would drop much of the detritus washed down from above. This 

 deposition would build up (aggrade) the surface, and much or even all 

 the water might sink into and seep through the debris thus deposited, 

 and disappear altogether from the surface, as at h, Fig. 56. This would 

 be most likely to occur where the debris is abundant and coarse, and the 

 precipitation slight. If the water disappears at the base of the mountain 

 (see Fig. 2, PL III), the early growth of the valley may be confined to 



