CAMBRIAN AND ORDOVICIAN RIPPLE-MARKS 



705 



Instead of the usual rounded or angular trough between the crests, 

 the ripple ridges rise from interspaces which are almost or quite 

 flat. The gently rounded parallel ridges are separated by flat 

 interspaces generally one and one-half to two times their width. 

 Traces of a diminutive ridge in the middle of the flat-bottomed 

 trough may sometimes be noted. This type of ripple-mark is 

 comparable in its essential features with, though not precisely like, a 

 form of ripple-mark which 

 I have seen developed in 

 Lake Deschenes near 

 Ottawa. A photograph of 

 these sand ripple-marks 

 made from a cast of a 

 mold taken under water at 

 Lake Deschenes is here 

 reproduced in Fig. 3. 

 Comparison of the two 

 figures shows broad, nearly 

 flat troughs in each sepa- 

 rating the comparatively 

 narrow rounded ridges. 

 In the Lake Deschenes 

 specimens the miniature 

 secondary ripple-marks are 

 more clearly developed 

 than in the sandstone 

 ripple-marks. The lake 

 ripple-marks were formed 

 under natural conditions 

 in water 6 inches deep and 



were under observation during and after their development for a 

 period of three days. They are the product of water oscillation 

 resulting from a very gentle on-shore breeze. This breeze and the 

 accompanying wavelets were not strong enough to produce the 

 asymmetrical type of ripple-mark with steep leeward and gentle 

 windward slopes which results from more vigorous movement of 

 the water and leaves a record of the direction of the movement of 



Fig. 2. — Cambrian sandstone slab showing 

 asymmetrical type of ripple-mark. 



