THE WORK OF RUNNING WATER. 



61 



fashioned to correspond with the surfaces with which we are famihar. 



It is to be distinctly borne in mind that the foregoing is a hypo- 

 thetical case; it is not probable that such an island ever existed, or ever 

 will; but that does not diminish the value of the illustration, since 

 the principles involved are operating on every land mass, though in 

 less simple relations, 



2. The second case differs from the first in that the sediment washed 

 down from the land is deposited about its borders. This results in 

 the building up of a marginal platform, as shown in Figs. 39-41. As 

 erosion goes on more sediment is washed down and deposited, partly 

 on the narrow marginal shelf which has already been developed, and 

 partly on its outer slope, as shown in the figures. The marginal flat is 

 thus extended beyond the original shores of the island on the one hand, 

 and toward its center on the other. As it develops, its inner portion, 

 and indeed all except its outer edge {ah, Figs. 40 and 41), will be gradu- 

 ally built up above the level of the water. This marginal lowland is 



.-, cS, 



.evel 



Figs. 39-41. — Diagrams to illustrate the effect of rain erosion on an island when all 

 the eroded material is deposited about the shore. The black portions represent 

 deposition The dotted lines represent the original surface The several diagrams 

 represent successive stages in the process. 



developed at a level as low as running water, under the conditions then 

 and there present, can reduce the land. Such a surface may be said 

 to be at grade, since running water neither wears it down nor builds 

 it up. Its angle of slope is a function of (1) the volume of the water 

 running over it, and (2) of the load which the water carries. 



Since the marginal plain of the above illustration extends beyond 

 the original shore of the island, the area of land is increased, though 

 both its average el^V^tion and its mass (above water) are reduced. 

 In case destructive processes did not operate on the marginal graded 



