INEQUALITIES OF SEDIMENTATION 349 



In the light of the abundant evidence it is hardly possible to 

 doubt that currents play a great role in localizing the accumu- 

 lation of sediments. In the words of the late Clement Reid: 



There is a general impression that marine action cannot go on much below 

 low water; but this is altogether a mistake. Tidal scour may go on at any 

 depth, provided the current is confined to a narrow channel, so as to obtain 

 the requisite velocity. If in addition there is a to-and-fro motion such as that 

 caused by the Atlantic swell at depths of at least 50 fathoms, the actual current 

 required to remove even coarse sand need only be very gentle. The oscil- 

 lation in one direction may not reach the critical velocity; in the other this 

 velocity may just be exceeded; the movement, therefore, of the sand grains 

 may always be in one direction especially if the courses taken by the ebb and 

 flood tides do not coincide, or their velocities differ.^ 



The relative importance in sedimentation of the role of wave 

 action which is constantly before our eyes is apt to be over- 

 estimated. Current action which even when most vigorous 

 is seldom visibly in evidence is apt on the other hand to be unduly 

 minimized. 



Shifting of deposits. — The power of currents to transport 

 sediments is nearly everywhere subject to periodic variations 

 resulting from the influence of storms. In some regions there 

 appears also to be a seasonal acceleration and retardation of tidal 

 currents. Branner was told by a pilot on the Brazilian coast 

 ''That the currents inside of the Abrolhos and Parcel das Peredes 

 reefs set strongest to the southwest in May, June, and July; that 

 they run in the same direction at other times of the year, but not 

 so strongly." Such variations in current velocity must result 

 frequently in considerable deposits being laid down and sub- 

 sequently removed by the same current. 



A change in depth of wave base and in the courses of currents 

 resulting from the elevation or depression of a coast line may 

 result in shifting the position of an entire formation. As Vaughan 

 Cornish- states, "A very small disturbance of conditions if long 

 continued can shift the largest shoal. Such a disturbance is the 

 recession of the coast line. It is probable that a shoal off the coasts 



' Reid, Submerged Forests (New York: G. P. Putnam's Sons, 1913), p. 85. 

 ' Cornish, "On Sea Beaches and Sand Banks," Geog. Jour., XI (1898), 641. 



