EVALl ATJON OK STUATKinA I'l 1 ICl CRITERIA 429 



The most striking elfoct of' waves is in tlie pj'odiiction of i'i[)ple-niark-^ 

 as distinguished from cnrrent-mark. The size of the ripples is some 

 function of the wave lengtli, hut the relation is not a simple one, and it 

 is not possible at present to determine from the ripples the depth of water 

 in which they were made. It is known, however, that ripple-marks may 

 occur in depths of several hundred feet of water,-^ and it may be pro- 

 duced by broad smooth currents of water affecting the bottom below wave 

 base, and which by their evenness and breadth of movement may prevent 

 the lack of symmetry which is especially characteristic of current-mark 

 as contrasted to ripple-mark. The regularity of ripple-mark produced 

 by wind action illustrates the possibility of currents simulating the 

 effects of waves. Eiver currents, however, tend to prevent regularity of 

 bedding and symmetry of ripple-mark. It appears, therefore, that typical 

 water-made ripi)le-mark associated with regularity of bedding in sand- 

 stones is especially associated with the subaqueous plain of deltas and 

 the bottoms of shallow seas. It is developed, however, to a limited extent 

 also over the subaerial portions of deltas, where shallow waters unaffected 

 by strong currents have stood for a time before being drained away. 

 Consequently it is not the presence of ripple-mark, but its dominance 

 and association with other features which suggests offshore d'eposition. 



The question may now be raised as to the types of cross-bedded struc- 

 ture which marine action will impose on a sand formation. Experience 

 shows that many marine sandstones show cross-bedding on a moderate 

 scale, and even limestones are known to exhibit in some instances the 

 same structure. Kindle has recently called special attention to this 

 feature.^* Gilbert, furthermore, has studied the type of cross-bedding 

 which may result from the superposition of successive beds of ripple- 

 marked sand, and has described the giant ripple-marks of the Medina 

 sandstone in western N^ew York.-^ (Jilbert puts foi'th, merely as a sug- 

 gestion, the hypothesis that these giant ripples may imply correspond- 

 ingly enormous waves in the Medina Sea. The explanation should, how- 

 ever, be sought apparently in some other direction, since ocean waves of 

 the present time are not observed to construct true rii)ple-mark on this 

 scale, and the general evidence of the shallowness of the epicontinental 

 seas, and especially of those with sandy bottoms, would seem to preclude 



--For a paper which gives the bibliography in addition to later observalions. see Cor- 

 nish, Vaughan. On the formation of sand-dunes. The (leographieal Journal, vol. ix, 

 1897, p. 278. 



23 Sir A. Geikie : Text-book of Geology, 4th edition, 1903, p. 562. 



2* American .Journal of Science, September. I'.Hl. pp. 22.5-227. 



-•■• Ripple-marks and cross-bedding. R\ill. (Jeol. Soc. America, vol. 10, 1800, pp. 135-140, 



