GEOLOGICAL IMPORTANCE OF SEDIMENTATION 525 



limits as that belt of shore exposed between the highest and lowest 

 monthly tides, forms but a relatively narrow transitional zone. 

 Furthermore, it was shown that the chances for preserval of the 

 true littoral deposits are but slight, for if the land is upraised, they 

 are the most superficial deposits and the first to suffer erosion. If 

 the land is slowly sinking, the margin of the sea moving across the 

 land planes away the deposits made in advance of it to the limits 

 of wave action, and this is ordinarily greater than the depth of the 

 littoral deposits. Under these conditions the unlithified deposits 

 formed in advance of the transgressing sea will only be preserved 

 where protected in some manner from the work of the waves which 

 follow. 



The most favorable places for the development and also for the 

 preserval of a broad littoral zone were found to be the frontal 

 portions of the larger river deltas. In such places it has been seen 

 that a slow subsidence frequently takes place pari passu with the 

 accumulation of river-borne sediment, but the sea does not advance 

 inland, and the littoral deposits are therefore not destroyed, but 

 finally become buried to depths where they are beyond the reach of 

 surface agencies and hence indefinitely preserved. But even in this 

 case the littoral deposits form but a transitional zone between much 

 more extensive and equally well-preserved marine deposits on the 

 one hand and continental flood-plain deposits on the other. 



It was furthermore shown that a considerable portion of the 

 sediment of rivers was deposited not beneath the sea, but upon 

 deltas facing and encroaching upon waters of the oceans or their 

 outlying seas. This subaerial deposit of river waste upon deltas 

 not only stands an excellent chance of preserval and incorporation 

 into the geological record, but is ordinarily more important in area 

 and volume than that of the littoral zone which borders it on the 

 seaward side, and becomes most important where the deltas are 

 broadly developed in shallow seas, since the deposits upon the delta 

 surface hold under such conditions, a greater ratio to those deposited 

 in advance of the delta. Even disregarding other important forms 

 of continental deposits, the magnitude of subaerial deltas and the 

 relatively small quantity of the littoral deposits which will be pre- 

 served indicate that a much larger place should be given to fluvi- 



