RIPPLE-MARK AND CURRENT-MARK 289 



the depth or turbidity of the water. The shortest 

 of these which I have measured had an average 

 wave-length of 4 feet. Most of my observations 

 of these larger sand-waves have been on sand- 

 banks which dry out at low water in tidal estuaries. 

 Their situation was generally such as to show that 

 they were produced either by the current of the 

 flood tide or by that of the ebb, and their nn- 

 sylnmetrical form supports this conclusion. In 

 deep-water channels up and down which the tides 

 flow and ebb the alternating currents form', we 

 are told, large sand-waves of symmetrical form » ; 

 but I have not had an opportunity of examining 

 these submerged ridges. 



The question I should like to answer somewhat 

 more definitely than I am able is whether the 

 smaller current -mark and the larger current -formed 

 sand-wave grow at the same time and place, as 

 do ripples and waves formed by the action of wind 

 in dry sand. When one examines the larger sand- 

 waves on the dried-out sandbanks of tidal estuaries 

 one sees them rippled over with the current-mark, 

 and this led me to think that pulsations of the 



' See Osborne Reynolds in Reports of the Committee of the 

 British Association appointed to investigate "the Action of 

 Waves and Currents on the Beds and Foreshores of Estuaries 

 by means of Working Models," 1889-91. 



