CAPE COD GEOLOGY 



175 



in the direction and strength of the currents, or the advent in the region of 

 the finer material from the outwash of an advancing ice sheet, or to any com- 

 bination of the three. The sequence of the deposits is suggestive. The Jacob 

 sand overlies clay and is overlain by the lower coarser gravel, and that by the 

 Montauk till of the Manhasset formation, a sequence suggesting that the material 

 was laid down by an advancing ice sheet that deposited it more rapidly as it 

 came nearer to this area. 



Structure 



The Jacob sand has suffered the same deformation as the Gardiners clay 

 and the deformation took place under the same conditions except, that as the 

 sand lay above the clay, it was subject to greater erosion by the overriding ice 

 sheet. This fact explains why the Jacob sand is absent in places above the 

 Gardiners clay, which is thus left directly in contact with one of the members 

 of the Manhasset formation. As the Jacob sand is fine grained and is plastic 

 when wet, and as it overlies a bed whose upper surface was very unctuous and 

 slippery, it acted exactly as part of the underlying clay. At a few places where 

 it was more directly exposed to the ice, the sand has been more deformed 

 than the clay. 



Conditions during Deposition 

 The conditions that prevailed during the deposition of the Jacob sand were 

 similar to those that prevailed during the deposition of the Gardiners clay, 

 except that deposition was more rapid, probably because of the advance of the 

 ice sheet, and that the material laid down was slightly coarser. In addition, 

 there must have been a slight progressive elevation of the area, so that in the 

 period during which the deposit was laid down the water was somewhat shal- 

 lower than it was in the preceding period. In the main, the period of deposi- 

 tion of the Jacob sand was a time in which the greater part of the area was 

 under water. 



Distribution and Exposures 



The Jacob sand is everywhere underlain by the Gardiners clay, and its 

 distribution is therefore practically the same as that of the Gardiners clay. 

 At some places, however, the Gardiners clay is not overlain by the Jacob sand, 

 which had been removed by erosion, largely by the Montauk ice. Thus, 

 although the distribution of the Jacob sand is nearly the same as that of the 

 Gardiners clay, its area is somewhat smaller. 



The Jacob sand is therefore exposed in the same places as the Gardiners 



