192 



CAPE COt) GEOLOGY 



kame deposits. It is seen in typical form in exposures along the banks of Lagoon 

 Pond and in East Chop cliffs, although it is rather thicker at these places than 

 elsewhere. In the western area the till is somewhat coarser and is rather widely 

 distributed. It can be seen in almost any of the cliffs along the shore of Vineyard 

 Sound. In the region south of the middle road it is very thin, and was at one 

 time even thought to be absent. The history of late Wisconsin time in this 

 region seems to have been similar to that in the northeastern area. Near the 

 end of the Wisconsin stage the ice advanced temporarily, as is shown by the 

 ice-contact slope north of West Tisbury post office and by several exposures 

 of till along the border of the great plain, as well as by occasional surface boulders. 

 During this advance a thin coating of till was laid down, which in some places, 

 especially on the higher ground, may now be almost removed. 



Nantucket Moraine 



Origin and character. — The deposits that form the Nantucket moraine 

 were laid down along the ice front. They were in part deposited directly by the 

 ice, but most of them show bedding and were therefore laid down in water. 

 The gravel in them is typical coarse granitic glacial gravel, which seldom occurs 

 in horizontal beds because it was "dumped" in masses in front of the ice. The 

 form of the surface of these deposits is peculiar and is one means of identifying 

 them. The numerous large boulders that cover the surface of these morainal 

 deposits are perhaps their most conspicuous feature. These boulders are obvious- 

 ly ice borne and must have come from a great distance, or at least from parts 

 of the mainland where bed rock of the same kind is found in place. Some of the 

 boulders have probably traveled 60 miles from their parent ledges. 



The most conspicuous part of the Nantucket moraine lies between Chap- 

 paquonsett and Nashaquitsa and was formed by the Buzzards Bay lobe of the 

 Wisconsin ice sheet. The materials in this part of the moraine are coarser and 

 the boulders are more numerous and larger than those in the limb of the moraine 

 formed by the Cape Cod Bay lobe, which extends from the head of the inter- 

 lobate angle southeastward along the shore of Nantucket Sound to Edgartown 

 and across Chappaquiddick Island. The material in this part of the moraine 

 usually consists of gravel and sand and till. The source of the material of both 

 parts of the moraine was the mainlaid on the north, but as the two lobes of 

 ice came from different parts of the mainland they brought somewhat different 

 materials. 



The western limb of the moraine rests on the eroded surface of older Pleis- 



