THE WORK OF RUNNING WATER. 



65 



certain meridian of the island be slightly softer than that over the 

 rest of the surface, the run-off, which would at the outset be equal 

 on all sides, would effect more erosion along the line of the less re- 

 sistant material than elsewhere. The result would be a depression 

 along this line, and, once started, the depression would be a cause of 

 its own growth. If the soft material were disposed in any way other 

 than that indicated, the final result would be much the same, for it 

 would quickly give origin to a depression which would lead to the con- 

 centration of the surface-waters, and this is the condition for the de- 

 velopment of a gully, a ravine, and finally a valley. 



In the presence of suflnLcient rainfall, either heterogeneity of slope 

 or of material will therefore occasion the development of valleys. If 

 the lack. of uniformity appears at but 

 a single point there will be but a 

 single valley. If it appears at many 

 points the number of valleys will 

 be large. Since it is incredible that 

 a land mass of perfectly homoge- 

 neous material and of absolutely 

 uniform slopes ever existed, it is 

 believed that every land mass, af- 

 fected for any considerable length 

 of time by rain, has had valleys 

 developed in it. The degree of 

 heterogeneity of material and slope fig. 45.— Diagram/ to illu«crate the 

 is usually so great as to lead to the 

 development of many valleys, even 

 on areas which are not large; but 

 for the sake of emphazing the simpler 

 elements of the complex processes 

 of stream work, the hypothetical case 

 of an island with but a single valley, 



and that without tributaries, may first be studied. Under these 

 conditions two cases may be considered, the one where there is no 

 deposition about the island, and the other where deposition takes 

 place. 



1. If all the material eroded from the surface of such an island, 

 both in and out of the valley, were carried well beyond the borders 

 of the land before being deposited, the edge of the island would re- 



effect of sheet and stream erosion on 

 the outline of an island when no de- 

 position takes place about its bor- 

 ders. The dotted line represents the 

 original outline of the island, the full 

 line its border at a later time. The 

 stream develops a reentrant (bay) in 

 the outline. 



