914 



E. M. KINDLE CHARACTERISTICS OP MARINE CLASTICS 



to ripple-mark of the current-made type than to the wave type of ripple- 

 mark. It is clear from this that asymmetric ripple-mark (figure 8) is 

 the type commonly preserved in marine depths, wh'ile the symmetric 

 type has a comparatively small opportunity for preservation in marine 

 elastics. In lacustrine deposits, on the other hand, the opposite is true. 

 The almost daily activity of waves and the comparative rarity of current 

 action gives rise to the great dominance in such deposits of symmetrical 

 wave-mark. Even in fluvial deposits sedimentation takes place for the 

 most part in times of great lateral expansion of the stream, when its con- 



PiGURE 9. — Plaster Cast of Ripple-marJc formed hy Wave Action 

 The photograph is of the hottom of Lake Ontario near Wellington 



ditions, as regards lack of Jiotable current, are essentially lacustrine. 

 Under these conditions the great bulk of fluvial deposits would, if ripple- 

 marked, show the symmetric type of ripple-mark (figure 9). 



From these considerations it appears that the asymmetric type of ripple- 

 mark, when it greatly preponderates over the symmetric type in any set 

 of beds, may be regarded as pointing strongly toward their marine origin. 



To this use of ripple-mark in discriminating between marine and con- 

 tinental elastics, the objection may be raised that river deposits found 

 within the limits of the channel will be characterized by the current type 

 of ripple-mark which is not distinguishable from that formed by tidal 

 currents. Tliis objection, however, overlooks the important fact that the 

 direction of the river current is invariable, while the tidal current re- 



