Flaviatile and Marine Gravels. 503 



have moved coastward 1 to 5 inches, 5 had not moved, 2 had 

 gone farther into the lake, during the lapse of four years. 

 Hunt observed that shingle is traveling toward the head of the 

 shallow Lyme Bay.* 



Initial over-steep profile. — If an over-steep shore profile is 

 presented to waves at the beginning of a physiographic cycle, 

 the conditions are favorable for the accumulation of thick but 

 narrow bands of gravel and also of thin bands of greater extent. 

 In early youth of the cycle the wave-cut nip and the wave- 

 built barrier, if present, are near the coast. If the initial steep- 

 ness is such as to preclude the building of a barrier the breaker 

 line may be immediately at shore line. Longshore drift may be 

 negligible at the beginning of the cycle and the materials worn 

 from the upper portion of the cliff may be deposited directly 

 at cliff base. The thickness of the gravel deposited under 

 such circumstances is limited only by the depth of the water. 

 An examination of coast charts indicates that the bases of few 

 if any sea cliffs are submerged in more than 50 feet of water. 

 If, in order to include doubtful examples, this figure be 

 increased to 100 feet the outside limit of thickness of gravel 

 deposited during a single cycle is believed to be indicated. As 

 the cycle advances, a rock platform is developed at first by 

 building and cutting, later chiefly by cutting. Before the pro- 

 file of maturity is established a platform must be built to a 

 point where grade adjusted to the waves and debris of a par- 

 ticular coast is established. It is conceivable that in certain 

 cases all gravel may be worn to sand before being carried 

 beyond the edge of the wave-cut bench. Becession of the cliff 

 involves the continuous attrition of the materials on the plat- 

 form, as well as the abrasion of the rock platform itself ; 

 otherwise the attack of the waves on the coast is greatly 

 decreased in vigor. Since the newly-cut platform is likely to 

 be of gentler slope than the outer portion of the profile, the 

 coarse debris tends to remain on the shelf rather than to be 

 transported seaward. During adolescence the shore line mi- 

 grates landward, the coastward portion of the profile is already 

 too gentle for the most effective seaward transportation of debris, 

 but the conditions are favorable for the attrition of pebbles 

 and their landward transportation. When maturity is attained 

 and the profile becomes adjusted to the waves and currents, 

 a thin sheet of gravel may be deposited over the upper concave 

 portion of the slope, but with a net tendency coastward. The 

 gravels deposited in youth may be buried by later, finer deposits 

 as the shore line advances toward the land. 



Evidence from maps. — As a check on the above conclusions 



that gravel is deposited within a few miles of the shore line 



*Loc. cit., p. 283. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIX, No. 233.— May, 1915. 

 33 



