66 



GEOLOGY. 



cede from its original position toward the center, as illustrated by 

 Figs. 37 and 45; but the recession would be most rapid where the 

 valley joins the sea (Fig. 45). At this point therefore a reentrant 

 would be developed (a, Fig. 45), and the island would lose its circular 

 outline. Continued erosion would cause the shore-line to retreat on 

 all sides, but fastest at the lower end of the valley, and the final result 

 would be a base-level differing from that developed under the con- 

 ditions specified on p. 60, in that the last part to be brought low 

 would not be the center of the original island. 



Under the foregoing conditions the profile of that part of the valley 

 which is above sea-level (cb) would be convex, following the analogy 

 of sheet erosion on a hypothetical island of uniform slopes and homo- 

 geneous material with no marginal deposition. Its side slopes, like- 

 wise developed under the influence of running water augmented in 

 volume from top to bottom, would also be convex. 



2. If the sediment washed down from the land is deposited about 



its borders, both the outline of the 

 island and the profile of the valley 

 will be altered. Deposition at the 

 debouchure of the valley follows 

 the same principles as deposition 

 elsewhere; but if all the sediment 

 brought to the sea be deposited 

 at the shore, the seaward extension 

 of the land by deposition would be 

 more rapid opposite the valley 

 than elsewhere, and the island 

 would lose its circular outline, and 

 develop some such form as is 

 shown in Fig. 46. In this case the 

 profile of the upper end of the valley, 

 and the upper parts of its side slopes, 

 as well as the upper parts of the 

 extra-valley slopes of the island, 

 are convex (compare Figs. 39 and 

 40); but the convexity above is 



Fig. 46. — Diagram showing the outline 

 of an island as modified by sheet and 

 stream erosion where eroded material 

 is deposited at the shore. The dotted 

 line represents the original outline; 

 the full line, a later one. The ex- 

 cess of deposition at the end of the 

 valley causes a projection of land into 

 the sea. 



exchanged for concavity below, the 

 change beginning at the point where downward erosion of the descend- 

 ing waters is checked. As a valley lengthens, the larger part of its 



