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CAPE COD GEOLOGY 



67 



quate cause of them. Merrill seems to have held that the folding took place 

 during the last ice stage, the Wisconsin stage. Shaler and his assistants, in their 

 work on Marthas Vineyard, found that the disturbances at Gay Head long 

 antedated the last or Wisconsin stage of glaciation; and Woodworth reached the 

 same conclusion in regard to the date of the disturbance on Long Island, 1 where 

 dislocations appear to have occurred at different times before the Wisconsin stage. 

 The latest surveys confirm the theory of glacial thrust, showing a succession of 

 dislocations and glacial deposits so correlated as to indicate that at each ice in- 

 vasion there was more or less crowding, crumpling, folding, and disruption of the 

 beds. 



These dislocated deposits are comparable with true moraines, but they are 

 genetically and lithologically distinct from them, for they were moved as a mass 

 and not as detached fragments, such as boulders. The displacement of the beds 

 appears to be the result of crumpling in a zone beneath an overthrust mass of 

 ice that was pushed forward under competent gravitational pressure from the 

 ice sheet. The effect of the ice thrust is wholly structural; it does not involve a 

 change in the nature of the deposits or in their classification. The disturbed beds 

 are therefore not to be grouped with moraines composed of rock debris that has 

 been eroded, transported, and deposited by glaciers or by streams of water flowing 

 within or out of glaciers. They form the "contorted drift" of English writers, 

 the "schichtengestorung" of German geologists. They were laid down beneath 

 a moving ice sheet; they are subglacial deposits, and their structure in the New 

 England Islands is rather typically that of isoclines overturned in the direction 

 of the movement of the ice, with dip toward the ice mass. 



The surface of the contorted drift as it was left by the melting ice sheets 

 is not well shown in the older greater typical masses in the New England Islands, 

 such as those at Gay Head and Clay Head, but it is suggested by the present 

 surface of areas covered by the last ice sheet, such as that on Block Island and No 

 Mans Land. In these two islands the surface above sea level has not been at all 

 changed, either by marine action or by the work of streams, and the form of the 

 subglacial surface is there clearly shown. This surface is of the type known in the 

 United States as "morainal," that is, it consists at some places of groups of 

 mounds and hollows, some of them circular in horizontal section, and, at others, 

 of long ridges or troughs, more or less sinuous in outline. This surface differs, 

 however, from that of a true frontal or terminal moraine in that it is not primarily 



1 Woodworth, J. B., Pleistocene geology of portions of Nassau County and the Borough of Queens, 

 N. Y. State Mus. Bull. 48, p. 632, 1901. 



