60 



CAPE COD GEOLOGY 



of the island and the corrugated moraine on its north side have been reduced 

 by marine erosion to narrow coastal belts and the fosse in the middle of the 

 island forms a large part of its surface. 



The ice front, curving northwestward, next appears on the island of Chappa- 

 quiddick, a southeastern appendage to Marthas Vineyard, where, however, the 

 distinctions between the plain, the fosse, and the submarginal moraine are 

 not clearly defined. 



On Marthas Vineyard the frontal features of the ice sheet reappear with 

 typical ice contact slope along the east coast, modified by the overrunning of the 

 ice upon its frontal outwash plain, a feature which Dr. Wigglesworth has found 

 along the entire iceward border of the outwash plain. West of Vineyard Haven 

 the ice either pushed through gaps in the pre-Wisconsin hills or rode over these 

 hills, here and there depositing heaps of boulders, as in North Tisbury. The 

 strong ridges on the western side of the island are only in small part due to the 

 accumulation of frontal or retreatal moraine deposits. The Wisconsin drift is 

 everywhere a thin veneer mantling older Pleistocene deposits or the Cretaceous 

 deposits of the upland. 



As the base of the ice sheet lay mainly above the level of the outwash plain, 

 ice contact slopes are rare except at the south ends of gaps in the hills. The sur- 

 face of the upland in the western part of Marthas Vineyard differs little from that 

 of the upland many miles back of the terminal moraine except for large boulders 

 and occasional morainal mounds of doubtful nature, and the ice front appears to 

 have passed seaward of the western part of the island, lying south of Gay Head, 

 as well as south of the islands west of Gay Head, nearly to Montauk Point. 



From Vineyard Haven eastward to Tom Never's Head, on Nantucket, the 

 ice front formed a lobe whose axis of maximum flowage appears to have passed 

 southeastward, presumably along the line of the glacial striae on Cape Ann, where 

 the stronger lines show a movement S. 10° E. From Tom Never's Head the ice 

 front turned southeastward to form another more marked lobe over Nantucket 

 shoals. The axis of this lobe was probably determined by the South Channel, a 

 deep depression separating Nantucket shoals from George's Banks. From 

 Vineyard Haven westward to Block Island a broad lobe of the ice sheet pushed 

 out beyond the present site of the islands, probably with its axis of maximum flow 

 along the low ground of Narragansett Bay. The topography of the insular tracts 

 would have tended to introduce irregularities in the ice front at its maximum 

 advance, so that the ice probably threw out small lobes in the depressions between 

 the islands, such as that occupied by Menemsha Pond, which acted somewhat 



