500 H. E. Gregory — Formation and Distribution of 



Lake Mendota and examples along sea shores might be 

 mentioned. w 



Long shore currents, which cooperate with waves and under- 

 tow, travel in one direction or another in response to wind. 

 The incoherent materials of the coastal belt are accordingly 

 ferried to and fro or deposited in bays or on the lee sides of 

 headlands. If winds of equal strength blow alternately in 

 opposite directions the shore materials may be continually in 

 transport. If winds are of unequal strength, materials make 

 their way in one direction since the weaker currents are unable 

 to return shore debris, particularly that of coarse texture, 

 brought by the stronger current. Materials deposited by cur- 

 rents are thus distributed irregularly but in all cases near the 

 shore line. When waves are present with currents " shingle 

 must go with the wave, sand must go with the strongest cur- 

 rent."* Coarse and fine material may thus move in opposite 

 directions. 



The shape of the profile of shore maturity is significantly 

 modified by the load furnished from the coast ; in fact, the 

 development of the normal compound curve of equilibrium 

 involves the deposition, transport and sorting of the coastal 

 load. In the absence of a renewed supply of sediment the 

 shore profile of maturity would be a steep descent from shore 

 line to the outer limit of effective work of the undertow, con- 

 tinued outward as a gentle slope.f When sediment is supplied 

 deposition will take place at the base of the steep descent lead- 

 ing down from the shore line until a grade suitable for trans- 

 portation of material is established and the debris is carried 

 seaward ending on a slope with convex front. A concavity in 

 the profile is now formed at the junction of the plain of 

 deposition with the plain leading landward over which material 

 is transported. The resultant profile is the curve of equili- 

 brium. With deficient initial slope of subaqueous profile, 

 barriers may be built, thus steepening the profile. In both 

 these ways the shape of the normal curve may be modified. 

 The outer limit of the convex portion, of the curve and hence 

 the width of the shore belt of combined erosion and deposition 

 is determined by the distance to which materials are ferried 

 seaward by the forces acting along a given coastal belt. For 

 the Atlantic coast of the United States Fenneman has shown 

 that the depth to which the whole load can be carried is 50 or 

 60 fathoms ; that is, 40 to 80 miles from shore ; and at 100 

 fathoms or less nothing can be carried except in suspension. 

 It is obvious that the coarse materials may be transported for 

 but a fraction of this depth and distance. 



*Hunt, loc. cit., p. 282. f Fenneman, loc. cit., p. 26. 



