60 WORK OF RUNNING WATER 



The Development of Valleys 



The growth of gullies, i. If the slope of the surface is not uni- 

 form the effect is very different. If there is, for example, a slight 

 depression near the base of the slope (Fig. 42), more of the descend- 

 ing water flows through it than over other parts of the surface. 

 The greater volume of water in the depression gives it greater 

 Velocity; greater velocity causes greater erosion, and greater erosion 

 deepens the depression. The immediate result is a gutty or wash 

 (Fig. 40). The gully, once started, tends to concentrate drainage 

 in itself still more, and it is thereby enlarged. The water which 

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

 lengthens it by causing its upper end to advance up the slope; and 

 all which flows through it deepens it. The enlarged gully will 

 gather more water to itself, and, as before, increased volume means 

 increased velocity and increased erosion. As the gully grows, 

 therefore, its increased size becomes the occasion of still further 

 growth, and the gully is transformed into a ravine, which is no 

 more than an enlarged gully. But growth does not stop with the 

 ravine. Water from every shower gathers in it, and growth con- 

 tinues until it becomes 

 a valley. 



It was assumed in 

 the preceding para- 

 Fig. 42. Diagram showing a slight meridional graph that the single 

 depression in the surface of an otherwise even-sloped depression in the slone 

 island. . , . 



was meridional (r ig. 



42) and low on the slope; but almost any 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 

 marked by a single ridge instead of a depression, the effect on valley 

 development would have been much the same, for a ridge, like a 

 depression, would cause the concentration of the run-off along cer- 

 tain lines, and therefore lead to the development of valleys. 



Under the conditions represented in Fig. 42 the lengthening of 

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

 head of the valley working farther and farther back into the land. 

 This method of lengthening is known as head erosion. But the 

 lengthening of the valley is not always wholly by head erosion. 

 The gully begins normally where concentration of run-off begins, 



