STRAND AND UNDERTOW MARKINGS OF UPPER 

 DEVONIAN TIME AS INDICATIONS OF THE PRE- 

 VAILING CLIMATE 



BY JOHN M. CLARKE 



The most coHsplcuous features on the surfaces of the sandy 

 strata of the Portage group in the Upper Devonian of New York, 

 are markings which have commonly been assigned to the movmg of 

 the waters over the strand. Wherever the beds of this important 

 formational term of the Devonian become preponderantly sandy, 

 these phenomena are observable. They have long been known ; 

 indeed, while they have commanded very little attention in the lit- 

 erature, and have been generally passed over as almost self-explana- 

 tory, the best outstanding descriptions of them are those given by 

 the first official geologist reporting upon the region where these 

 developments are most manifest. Reference is made to the descrip- 

 tions of them given by James Hall, in the Geology of the Fourth 

 District of New York, 1843, pages 232-37. These and the very 

 striking illustrations he presented of them, have passed current as 

 final, and even though occasionally his figures have been reproduced, 

 there has been, so far as the writer knows, no published modifica- 

 tion of his explanation of their origin. 



During the many years which have elapsed since the date of that 

 report, and especially since the writer's attention has been particu- 

 larly directed to the phenomena, mechanical and biological, of the 

 Portage formation, there has grown up in the Museum a very exten- 

 sive collection representing the various expressions of these strand 

 phenomena, from the study of which it has become perfectly 

 obvious that we have not had a proper understanding of their, 

 origin. It is thought that the present paper may suggest a truer 

 conception of them. 



Preliminary observations. The phenomena which we have to 

 discuss or to debate, are conformations in strong relief, invariably 

 on the under side of the strata as they lie in normal position. The 

 beds which bear these surface figures are slabs and flags of thin, 

 hard sandstone ; in other words, flagstones in which lamination beds 

 are obvious but cleavage only potentially developed. In every case, 

 also, the relief figures on these slabs in normal position fit into cor- 

 responding grooves and depressions in the underlying and inter- 

 vening sandy shales. Emphasis is here to be laid upon the expression 



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