170 



CAPE COD GEOLOGY 



The folding in this bed is in places greatest along the northwest shore, where 

 the strike is generally northeastward. The folding occurs where the action of 

 the ice would naturally be strongest. It is not easy to explain why overthrust- 

 ing without close folding took place farther southeast of the strongly folded 

 area. Possibly the ice cover here was thin and allowed the whole bed to be 

 pushed southeastward as a more or less rigid layer. The fact that the Gardiners 

 clay has been overthrust on itself tends to give this theory more weight than 

 the theory that the low, gentle warping of the clay bed is due to the weight of 

 the ice, although such action may have occurred elsewhere, or perhaps even 

 here to some extent, for under sufficient pressure this clay would doubtless 

 have been compressed and otherwise molded. 



Date of the Folding of the Gardiners Clay 

 The folding that the Gardiners clay has undergone was caused by the Mon- 

 tauk ice sheet. It is most pronounced along the northwest coast, where the 

 ice probably actually pushed and shoved both the Gardiners clay and the Jacob 

 sand. The distortion of the clay elsewhere is such as would be caused by a heavy 

 mass of overriding ice. The clay, being plastic, would be squeezed out where the 

 pressure was greatest and would accumulate where the pressure was least. The 

 deformation was not due to Wisconsin ice, for the pre- Wisconsin topography of 

 the Vineyard interval was not perceptibly altered by this final glaciation. 



Conditions during Deposition 

 The conditions under which the Gardiners clay was deposited have already 

 been suggested. The nature of the material alone indicates that it was deposited 

 in water in an area that lay rather far from the shore, for it includes no coarse 

 material. The main evidence of its mode of deposition, however, lies in its fossils. 

 These are marine forms which, according to the senior author, lived when the 

 clay was being deposited, for they were found in the position of growth and 

 showed no signs of transportation. Submergence and the active work of wave 

 and current were therefore necessary to supply and transport the material. It 

 is possible that silt-laden rivers may have supplied some of the fine sediment. 



Distribution 

 The Gardiners clay is found only in the western part of the area. It proba- 

 bly does not underlie the whole area, as do the Cretaceous deposits, and it is 

 doubtless absent wherever the earlier deposits rise as high as 100 feet. Sections 

 at Gay Head and Norton Point indicate that the clay was deposited around 



