CAPE COD GEOLOGY 



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gravel. It might, therefore, be expected that the ice would have completely de- 

 stroyed all the preexisting topography, but it seems rather to have passed gently 

 over the surface without materially altering it, except that it deposited a rela- 

 tively small quantity of material in characteristic forms. (Plate 16, figs. 1 and 2). 

 Hence the topography is in two ways exceptional: because the ice passed over 

 only soft rocks and because its action was weak, whereas in other parts of New 

 England it seems to have been strong. 



The southern margin of the Wisconsin ice sheet was made up of a series of 

 lobes. 1 Only two of these lobes will be considered here. What may be called 

 the Buzzards Bay lobe extended over the site of Buzzards Bay reaching as far 

 south as the northwestern part of Marthas Vineyard. Its margin extended from 

 near Vineyard Haven southwestward nearly parallel to the shore and thence 

 to No Mans Land. The Cape Cod Bay lobe covered the site of Cape Cod and 

 what is now Nantucket Sound and reached the northeast shore of Marthas 

 Vineyard. Its margin extended from near Vineyard Haven, where it was in 

 contact with the Buzzards Bay lobe, southeastward toward Edgartown, across 

 Chappaquiddick to Tuckernuck and Nantucket. 



North of the point where the margins of these lobes met there must have 

 been an interlobate line of slack ice. Along this line was deposited an interlobate 

 moraine (in this report called the Plymouth interlobate moraine), which is 

 weakly developed between Vineyard Haven and West Chop and was strongly 

 developed from Falmouth to Plymouth 2 at a later stage. 



Form of the Island 



During the first stage of the Wisconsin ice Marthas Vineyard occupied an 

 interlobate area between the Buzzards Bay lobe and the Cape Cod Bay lobe. 

 Its position at that time mainly determined its triangular form, which was de- 

 veloped in a reentrant angle between these two lobes. The western and north- 

 eastern areas, however, were intraglacial, but the great plain was distinctly 

 extraglacial. The topography of the western area is due to the action of the 

 Buzzards Bay lobe on the preexisting topography. The topography of the 

 northeastern area is due to the action of the Cape Cod Bay lobe. The great plain 

 was formed between these two lobes by the water that flowed from the melting 

 ice, largely that coming from the Cape Cod Bay lobe. 



1 Chamberlin, T. C, Preliminary paper on the terminal moraine of the second glacial epoch, Third 

 Ann. Rept. U. S. Geol. Survey, pp. 291-402, 1883. 



2 Upham, Warren, Terminal moraines of the North American ice sheet, Am. Jour. Sci., 3d ser., 18, 

 p. 204, 1879. (This paper contains the first reference to this interlobate moraine.) 



