GEOLOGICAL IMPORTANCE OF SEDIMENTATION 439 



Suppose that the shore, to begin with, is a smooth, gently indined 

 plane, represented in cross-section by OB or OC. The level of low 

 tide being OD, the volume of water passing O, the lowest part of 

 the littoral, will be represented in the one case by the triangle OAB, 

 in the other by the triangle OAC. But the areas of these triangles 

 are to each other as their bases AB and AC. Hence in this ideal 



Flood hide level. 



Fig. 5. — Diagrammatic relation of the width of the littoral zone to unadjusted 

 slope of land surface. 



case the volume, and consequently the mean velocity, of water flowing 

 past OA will vary directly with the width of the Httoral zone. But 

 Revy has shown that the average velocity is an arithmetical mean 

 between the bottom and surface velocities, and that the swifter the 

 current, the more nearly the bottom velocity approaches the mean,^ 



■Original ujidhh 

 • Resulhanh tuidl-h 



Flood Hide level. 



Fig. 6. — Diagram of littoral zone adjusted to slope, wave work neglected. 



There jore the bottom velocity increases somewhat j aster than the width 

 of the littoral in the ideal case. But the eroding ability of water 

 varies as the square of the bottom velocity and the transporting 

 ability as the sixth power. On an originally flat shore the mid-tide 

 would consequently possess such scouring power for some distance 

 on each side of the original low-tide limit that, as shown in Fig. 6, 



I Hydraulics of Great Rivers (London, 1874), p. 147. 



