EFFECT OF DESICCATION 



913 



fine-textured elastics deposited under subaerial conditions, decisive evi- 

 dence as to whether they originated near the seashore or were of fluvio- 



lacustrine origin. 



ElPPLE-MAEK 



The familiar phenomena of ripple-mark merits consideration in any 

 study of the criteria of marine elastics. Any appreciation of the signifi- 

 cance of ripple-mark must begin with a clear discrimination of the two 

 common types of rip|)le-mark and of the agencies which produce them. 

 These are the symmetrical and asymmetrical types. They are produced 

 respectively by wave and current action. The distinctive characteristics 



Figure 8. — Ripple-mark produced hy Current Action '^ 



This locality is the estuary of Avon River, Nova Scotia 



of the two types are shown in figure 7, a to c. Because of their diverse 

 modes of origin these two types become absolute criteria for differen- 

 tiating current-laid from wave-assorted sands. Current action, owing to 

 the tides, is a daily feature of nearly all coastal marine waters, and leaves 

 its characteristic type of ripple-mark (figure 8), while it is a compara- 

 tively rare feature in lacustrine waters. Wave action, of course, is com 

 mon to both. But the symmetric wave ripple-mark, when produced on 

 the sea-bottom, is apt to be obliterated by the daily tidal currents, which 

 thus have a far better chance of preservation near the seashore than has 

 wave ripple-mark. Further, wave ripple-mark (figure 9) can be formed 

 only at limited depths, while current ripple-mark has no depth limit. It 

 follows, therefore, that an enormously larger area of sea-bottom is subject 



