502 II E. Gregory — Formation and Distribution of 



aqueous platform and a barrier beach thrown up. After recov- 

 ering their form, waves of translation urge material coastward 

 from the barrier, and the barrier itself aids in preventing the 

 return of coarse sediments. The maximum width of gravel is, 

 therefore, found in youth when barriers of no great thickness 

 are farthest off shore, the gravel being confined to the zone 

 between the breaker line and the shore line. As the cycle 

 advances and the barrier progresses landward the zone of 

 gravel is narrowed, and the pebbles are continuously ground 

 to sand. During adolescence, when the barrier is pressed close 

 to shore and currents and tides and oblique waves express 

 their activity in bay bars, spits, tombolos, cuspate forelands 

 and winged beheadlands, gravel in decreasing amounts is kept 

 progressively nearer the shore. At maturity gravel may be 

 absent except at points of origin, for effective shore work in 

 maturity requires that gravel be limited to such amounts as 

 can be ground up by waves and ferried away by currents. 



If the above analysis holds for the normal coastal belt it fol- 

 lows that youthful forms on an over-flat profile, if preserved in 

 the sedimentary record, would be represented by a belt of sedi- 

 ment which may include conglomerate, a few miles at most in 

 width and a few tens of feet thick, rather sharply limited sea- 

 ward and interbedded landward with bands of various materials 

 brought to the coast by streams. If adolescent features were 

 preserved along the ancient shore, the bulk of the coastal 

 gravels would be represented by a band, a few hundred (or at 

 most a few thousand) feet in width and a few tens of feet 

 thick, of crossbedded conglomerates and sandstone unevenly 

 distributed and showing great variation in texture vertically, 

 transversely, and longitudinally. In exceptional cases where 

 strong off-shore bottom currents are initiated by excess of 

 water piled in bays, gravel may be carried to depths beyond 

 the reach of waves and thus preserved. These exceptional 

 accumulations are doubtless small in extent and thickness. If 

 mature features were preserved in the sedimentary record, a 

 narrow band of conglomerate with water-worn pebbles on one 

 side and unassorted debris on the other may mark the plane of 

 unconformity. 



It is believed that all flat subaqueous profiles favor landward 

 transportation of coarse sediments. Observations of a group 

 of pebbles on the Long Island Shore and of another on Lake 

 Whitney, near New Haven, showed that in the course of a 

 year about 40 per cent of the fragments had moved landward 

 and none seaward. These experiments were abandoned because 

 of the impractibility of evaluating the effect of ice work. On 

 a reservoir at Tuba, Arizona, of 26 pebbles 1/4 to 1 inch in 

 diameter, in a position unaffected by ice, 19 were found to 



