116 GEOLOGY 



is no distinct line of demarkation between the two. But growth 

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

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

 and depth, until it becomes a valley. 



It was assumed in the preceding paragraphs that the single 

 depression in the slope was meridional 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 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 



Fig. 75. Diagram showing a slight meridional depression in the surface o^ 

 an otherwise even-sloped island. 



concentration of the run-off along certain lines, and therefore lead 

 to the development of valleys. 



Under the conditions represented in Fig. 75 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 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, and if this is not at sea-level, the gully may be length- 

 ening at both ends at the same time. This would have been the 

 case, for example, had the original depression of Fig. 75 been half- 

 way up the slope of the island. Valleys developed under the control 

 of surface slope are consequent valleys, and their streams are con- 

 sequent streams. 



2. If the surface material of a slope is of unequal resistance, 

 the water flowing over it may develop irregularities of slope, even 

 if the slope was uniform at the outset. Thus if the material of a 

 certain part of the slope is less resistant than that elsewhere, the 

 run-off will erode most where the material is least resistant. The 



