18 GENESIS AND DEVELOPMENT 



rent. Tidal sand ripples, first described by Reynolds*-) and later by 

 Cornish' 2 ') occur in estuaries and also on some shores where the sand 

 is exposed to waves as well as currents. Cornish is of the opinion that 

 they do not require for their formation any cooperation between flood 

 and ebb currents. The size and form of these ripples is constantly 

 changing with the variations in the tide. Cornish describes this in the 

 following words: 



"At neap tides the sands were nearly smooth, and as the tides in- 

 creased the tidal sand ripples appeared, short and relatively steep. The 

 amplitude increased steadily, the average wave-length also increased, 

 apparently by elimination of some of the ridges. When the highest 

 spring tide was passed the amplitude rapidly diminished, the wave- 

 length remaining nearly, but not quite constant, and the mean sand 

 level remaining practically unchanged." 



Tidal sand ripples sometimes attain a considerable size, Cornish 

 giving the wave length of from 1 to 6.7 m. I have often noticed a finer 

 rippling of the proper tidal ripples, and in two instances, on the east- 

 ern coast of Australia, I observed the tidal sand ripples crossed by 

 another set of large ripples. These were formed by a sudden change of 

 the direction of the tide current through the overflow of a neighboring 

 stream. Both these sets of wave formations were then beautifully 

 rippled in the usual way by little current marks, facing almost trans- 

 versely the second set of larger ripples. Cornish attributes the forma- 

 tion of current marks to the pulsation of the fluid rather than to the 

 current itself. 



In the formation of eolian sand ripples it is the heterogeneity of the 

 material which is of the greatest importance. The sorting action of 

 wind is remarkable, and it is evident at the first glance on a group of 

 ripples that the heavier grains always constitute the crest, the lighter 

 the trough. A moderate range of sizes of grains seems therefore most 

 favorable to the formation of ripples. 



Darwin^) remarked the uniformity of pattern in the ripples formed 

 by wind, which uniformity, as a rule, is absent from .ripples made in 



1) Reports of committee appointed to investigate the action of waves 

 and currents on the beds and foreshores of estuaries by means of working 

 models. — British Association for Advancement of Science, Reports '89, 

 '90, '91. 



2) The formation of wave surfaces in sand. — Scot. Geogr. Mag. 17: 

 1—11. 1901. 



3) G. H. Darwin: On ripple mark. — Proceedings of Royal Society, 

 London, vol. 36. 1883. 



