STUDIES FOR STUDENTS 817 



This theory seems to be a survival of De Candolle's erroneous 

 idea that "intermittent friction" is essential to the production 

 of current ripples, and is practically abandoned by Cornish in his 

 more recently published book on Waves of Sand and Snow. 1 Gil- 

 more described tidal ripples on the Goodwin Sands having a height 

 of "two or three feet." 2 



It should be noted that all of the giant ripples referred to above 

 belong to the asymmetrical type; they are true current ripples. 

 So far as I am aware no giant oscillation ripples have ever been 

 observed along modern shores. It may be doubted whether tidal 

 currents could form symmetrical ripples, notwithstanding Rey- 

 nold's suggestion to the contrary. 3 The flow and ebb of the tide 

 constitute an oscillating current, it is true; but the currents are 

 often of unequal force. Where equally strong, each current per- 

 sists long enough to remodel the ridges formed by the preceding 

 current, giving them an asymmetrical form appropriate to the 

 current operating last. On the other hand, Gilbert has described 

 structures in the Medina sandstone formation of New York which 



i 



he believed to be giant ripples of the symmetrical type, formed by 

 oscillating currents due to wave action. 4 In dimensions these 

 ridges were similar to the average examples of tidal ripples described 

 by Cornish, having a height of from 6 inches to 3 feet, and a dis- 

 tance from crest to crest of from 10 to 30 feet; but their nearly 

 symmetrical form did not suggest a similar origin. Gilbert reached 

 the tentative conclusion that they were formed by waves 60 feet 

 high in deep water of a broad ocean. This conclusion was criti- 

 cized by Fairchild, who advanced convincing arguments in support 

 of the opinion that the forms in question were beach structures, 

 possibly successive beach ridges built on the strand. 5 Branner 



1 Vaughan Cornish, Waves of Sand and Snow (London, 1914), pp. 289-90. 



3 John Gilmore, Storm Warriors, or Lifeboat Work on the Goodwin Sands (London, 

 1874), PP- 108-9. 



3 Osborne Reynolds, "Report of the Committee Appointed to Investigate the 

 Action of Waves and Currents on the Beds and Foreshores of Estuaries by Means of 

 Working Models," Rept. British Assoc. (1889), p. 343- 



"G. K. Gilbert, "Ripple-Marks and Cross-Bedding," Bull. Geol. Soc. Amor., 

 X (1899), 135-40. 



sh. L. Fairchild, "Beach Structure in the Medina Sandstone," Amer. Geologist, 

 XXVIII (1901), 9-14- 



