CAPE COt) GEOLOGY 



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included the large boulders that it had brought from the mainland (Plate 17, 

 fig. 2) . The quantity of material in the moraine thus formed was not large, and 

 at some places practically no material was deposited, so that pre-Wisconsin beds 

 are now found at the surface. The larger part of the deformation of these older 

 beds is due to earlier ice advances, and the fact that the pre-Wisconsin surface 

 can still be recognized suggests that the Wisconsin ice overrode that surface 

 without greatly disturbing it. 



THE CAPE COD BAY LOBE 



The topography and the surface deposits on the eastern half of Marthas 

 Vineyard, including Chappaquiddick Island, are products of the action of the 

 Cape Cod Bay lobe. They resemble in many ways those of Nantucket, although 

 their mode of development at some places was not so simple nor so clearly marked. 

 The eastern part of the island differs very strikingly from the western part. 

 Its topography is much less rugged and only a few of its elevations rise more than 

 a hundred feet above sea level. (Plate 17, fig. 1.) Its deposits consist almost 

 entirely of sand and gravel laid down by the Wisconsin ice sheet. The Cretaceous 

 and Tertiary strata are not exposed, and the pre-Wisconsin glacial deposits are 

 exposed only at some places. 



The southern limit of the ice in this area is marked by a well-developed ice- 

 contact slope in the area south of Lagoon Pond. On Chappaquiddick Island this 

 slope is well defined at but two places, and for short distances only. Elsewhere 

 it can be traced only by the distribution of the boulders, for at many places 

 the topography north and south of the contact is much alike. A close study of 

 the surface, however, reveals a difference in the origin of the deposits. A poorly 

 developed ice contact extends along a line that runs northwestward for about 

 a mile and a quarter from a place near the shore at the south end of Poucha 

 Pond, at the southeastern extremity of Chappaquiddick. Boulders are found 

 along this line, the largest of them about 12 feet long. The lowland northeast 

 of this line represents the fosse; the higher land southwest of it is a narrow 

 fringe of outwash plain. This plain slopes toward Katama Bay and is cut by 

 several shallow creases. The ice contact is not clearly evident in the south- 

 central part of Chappaquiddick Island, but it can be traced by a line of boulders 

 that curves westward toward a small marsh south of Snow's Point, where it 

 turns rather sharply to the northwest. The fosse also is lacking for some distance 

 here, but it reappears about half a mile east of the marsh. The outwash plain, 

 which is interrupted only by a few creases, extends as a fringe continuously 

 along the east side of Katama Bay. 



