LACUSTRINE CLASTICS. DELTAS 607 



Thus when the current is powerful and surcharged with sediment 

 sHghtly coarser material may be carried to the lake bottom than 

 is the case during the period of lessened river activity. A series of 

 annual layers is thus formed which in any given case may serve to 

 measure the length of time required for the formation of the de- 

 posit. This method has been applied by Berkey (8) for the de- 

 termination of the time required for the deposition of finely bedded 

 clays. 



DELTAS. 



Deltas are the terminal deposits of rivers, and, as such, have 

 an intimate association with the continental elastics. It is true, of 

 course, that a part of the delta of the seacoast is of submarine 

 origin, and its discussion therefore falls more essentially under the 

 heading of marine elastics. Still, deltas are peculiar features of 

 relatively limited distribution, and in no way represent normal 

 marine conditions. Indeed, a part of the delta is always typically 

 non-marine, and the place of the delta is therefore intermediate 

 between true continental and marine elastics. Moreover, deltas 

 are common in lakes, these belonging, of course, entirely to the 

 continental division of the hydroclastics. 



A typical delta may be taken as one that is built into a body of 

 standing water, the level of which is essentially a permanent one. 

 Subsequent drainage of the water body may expose the delta as 

 we have seen, but all such changes bring the delta-building process 

 to an end. On the whole, deltas are more abundant on lakes than 

 on the seashore, partly because lakes are not subject to tidal cur- 

 rents and fluctuation of level, but partly also because wave activity 

 on the sea coast is greater. Fluctuation in level due to tides does 

 not necessarily militate against delta building, as is shown by the 

 Indus delta, which is built where the tidal range is 10 feet, while 

 that of the Ganges is built into a sea having a tidal range of 16 

 feet. Where wave action is strong, however, and especially where 

 long shore transport of material is pronounced, delta building is 

 restricted. Deltas are thus the triumph of river deposition over 

 wave and current destruction, and their location and extent will be 

 determined by the relative importance of the opposing processes. 

 As examples of typical existing deltas on the sea coast may be 

 named that of the Nile, on the Mediterranean ; of the Po, on the 

 Adriatic ; the confluent deltas of the Rhine and Meuse ; and that 

 of the Ems, on the North Sea ; the deltas of the Lena and of the 

 Mackenzie, on the open Arctic Ocean ; those of the Ganges, Brah- 



