64 GEOLOGY. 



from the sides widens it; that which enters at its head lengthens it 

 by causing its upper end to recede; and all which flows through it, 

 so long as its bottom is above base-level, deepens it. The enlarged 

 gully will gather more water to itself, and, as before, increased volume 

 means increased velocity, and increased velocity increased erosion. 

 As the gully grows, therefore, its increased size becomes the occasion 

 of still further enlargement. 



Continued growth transforms the gully into a ravine, though be- 

 tween a gully and a ravine there is no distinct line of demarkation. 

 But growth does not stop with ravine-hood. Water from every shower 



Fig. 44. — Diagram illustrating the development of a gully, staiting from the condi- 

 tions shown in Fig. 43. 



gathers in the ravine, and, flowing through it, increases its length, width, 

 and depth, until it reaches such proportions that the term ravine is 

 laid aside, as childhood names are, and the depression becomes a valley. 



It was assumed in the preceding paragraphs that the single de- 

 pression in the slope was meridional and low on the slope, but almost 

 ^ny sort of depression in almost any position would bring about a 

 similar result, since it would lead to concentration of the run-off. Had 

 the original surface been interrupted by ridges instead of depressions, 

 the effect on valley development would have been much the same, 

 for a ridge, like a depression, would, in almost any position, occasion 

 the concentration of the run-off, and so the development of valleys. 

 Under the conditions represented in Fig. 44 the lengthening of the 

 drainage depression is effected chiefly at its upper end, the head of 

 the valley working its way farther and farther back into the land. 

 This method of elongation is known as head erosion. But the lengthen- 

 ing of the valley is not always wholly by head erosion. The gully 

 normally begins where concentration of run-off begins, and if this 

 were not at sea-level, the gully might be lengthening at both ends at 

 the same time. This would have been the case, for example, had the 

 original depression of Fig. 43 been half-way up the slope of the island. 



If while the slopes of the island were absolutely uniform its sur- 

 face material failed of homogeneity, the result would be much the 

 same as if the slopes were unequal. If the material Ivini^ along a 



