184 



CAPE COD GEOLOGY 



Hempstead; if it is unconformable it is the Wisconsin. A long period of erosion 

 followed the deposition of the Hempstead, and where the Wisconsin till rests 

 upon the Montauk it may be difficult to tell where the Montauk ends and the 

 Wisconsin begins. 



Structure.— The Montauk till is gently warped and folded, but many of 

 its variations from horizontality are no doubt due to irregularities in the surface 

 on which it was deposited. It was not laid down horizontally, as were the water- 

 laid deposits. Such deformation as it has undergone must be due to the action 

 of the Wisconsin ice sheet, which was relatively weak on Marthas Vineyard, 

 owing to the interlobate axis there, and could not have caused the disturbances 

 found in the beds below the Montauk in other regions. The surface in the 

 Vineyard interval was overridden by the Wisconsin ice but was not greatly 

 disturbed. 



Conditions during deposition. — The conditions that prevailed during the 

 Montauk stage of the Manhasset were those that are associated with a vigorous 

 continental ice sheet. The area that now forms Marthas Vineyard was covered 

 by a thick glacier, which extended at least to the southern limit of the island 

 and probably beyond. The extent of the deposits of Montauk time indicates 

 a much more vigorous or longer action of the ice than that in Wisconsin time. 

 The great thickness of the ice is indicated by the distortion of the older beds, 

 which may be attributed to the downward pressure of overlying ice without 

 horizontal pressure. The region must have remained buried under the ice for 

 a long time, for the till laid down is thick and there is nowhere any evidence 

 that it accumulated rapidly. Toward the end of this stage there naturally 

 must have been a stage when the ice was stagnant and melting — a stage at the 

 beginning of Hempstead time. After this great ice sheet had disappeared the 

 region must have presented a picture that surpassed in desolation and barren- 

 ness anything now in existence. 



Distribution.— The Montauk till probably underlies the greater part of 

 Marthas Vineyard, but it is exposed only in its western part, except in two 

 areas. These areas are in the northeastern part of the island, near Edgartown, 

 and on Chappaquiddick Island. The first exposure indicated is in a gravel pit 

 at the side of the State road about a mile from Edgartown, where a lower tough 

 bed of till lies beneath bedded gravel, which is in turn overlain by sandy Wis- 

 consin till. The second exposure is on the east side of Sampson Hill, on Chap- 

 paquiddick Island, where a small gravel pit exposes the tough till below the 

 Wisconsin, In addition to these exposures there are a few places in the north- 



