CAPE COD GEOLOGY 



207 



in the cliffs along its coast, that surface should be regarded as a product of the 

 erosion of the area since the latest pre-Wisconsin deposit was laid down. As 

 stated above, the highest beds recognized in the cliffs are the deposits of the 

 Hempstead gravel, which overlies the Montauk till. In this respect the surface 

 corresponds in date with that of Marthas Vineyard during the Vineyard interval. 

 The relation of the peculiar wrinkling of the surface, the pits, and the mounds 

 to the underlying structure is more clearly shown here than on any other island 

 in the group. This peculiar configuration may perhaps be a result of the action 

 of the ice sheet of the earlier Wisconsin advance. What part of the deformation 

 of the surface should be attributed to shoving, as suggested by Shaler in 1888, 

 and what part to the differential settling of the underlying deposits is hard to 

 determine, for some parts of those deposits consist mainly of clay and others 

 mainly of sand or boulders. 



Wisconsin Drift 



The correlation of the upper, highly folded bouldery bed with the Montauk 

 till and of the overlying beds of sand and gravel with the Hempstead member 

 leaves little or no drift to be referred to the Wisconsin ice sheet. A rubbly 

 layer a few inches thick forms the subsoil, and numerous boulders are scattered 

 over the surface, but their origin is not certainly known. (See Plate 28, fig. 2.) 

 It seems as reasonable to suppose that they were left on the surface after the 

 erosion of the contorted beds of boulder clay of the older Pleistocene, as to 

 refer them to the more recent Wisconsin ice sheet. To those who, considering 

 only what may be seen on this island, would hold that all these beds of till are 

 of Wisconsin age, the problem seems very simple; but when we attempt to cor- 

 relate the formations and the nature of the surface with clearly equivalent phe- 

 nomena on a larger area, such as Marthas Vineyard, we see that the details of 

 the topography worked out on the folded beds prior to the deposition of the 

 Wisconsin moraine on that island indicate that the entire series of folded beds 

 belong to periods long anterior to the time of the deposition of the surficial 

 glacial drift of the mainland to the north. 



The front of the Wisconsin ice sheet may have extended many miles beyond 

 No Mans Land. The deposits that have been called the Montauk till and the 

 Moshup till (the Moshup underlying the Gardiners clay and locally replacing 

 the Jameco gravel) likewise are products of ice advances that extended beyond 

 the present island. The broad submarine ridge on which the island stands 

 may be a morainal accumulation piled up by deposition and displacement in 

 more than one advance of the ice to this part of the coastal plain. 



