INEQUALITIES OF SEDIMENTATION 365 



the relatively new conception of succession as a process which 

 results inevitably from the activities of each plant or animal 

 community destroying by its own activities the possibiHty of indefi- 

 nitely continued existence in the same place. The operation of 

 this and other factors, some of which produce more sudden results, 

 tend inevitably to vary the rate and shift the area of accumula- 

 tion of the organic constituents of sediments. In a comparable 

 manner sedimentation sets its own time hmit on its operation in 

 any given area. Alluviation terminates automatically when the 

 deposits of a valley reach a thickness corresponding to the height 

 of the maximum flood waters of the valley. Sedimentation 

 ceases in a lake when the deposits approximate the lake surface. 

 Although this familiar conception of the self termination of terres- 

 trial sedimentation in fluvial and lacustrine waters has been 

 generally recognized and applied to terrestrial sediments it has not 

 been extended to marine conditions. But its application though 

 less obvious is almost equally justified. In foreland structures 

 the building of coarse deposits of sand, gravel, and shingle is limited 

 vertically by the height to which wave action reaches. Horizontally 

 the direction and strength of currents are the chief limiting factors. 

 When these rather definite limits have been reached by a particular 

 spit its supply of sediments goes elsewhere. 



In areas remote from the shore line wave base and current 

 action are determining factors in sedimentation. Wave base 

 differs for different classes of materials as pointed out by Barrell.^ 

 It is deeper for fine than for coarse materials. Mud deposits can 

 never build to the surface in the open sea because of the dissipating 

 influence of the waves. Sand deposits fed by an abundant supply 

 may, however, reach the surface. Whatever the upper limit of 

 a particular formation may be, it will depend on the interplay of 

 wave and current action, and when reached the further growth 

 will be lateral or else diverted to a new area by a shifting of the flow 

 of materials. Any adequate conception of marine sedimentation 

 must include numerous great river-like currents some with inter- 

 mittent activity, others periodic, while a few doubtless follow 



'Jos. Barrell, "Rhythms and the Movement of Geologic Time," Bull. Geol. 

 Soc. Amer., XXVIII (1917), 778. 



