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CAPE COD GEOLOGY 



The ground about Hummock Pond, as Dr. J. Howard Wilson has pointed 

 out, consists largely of beds older than those of the plain, clay appearing at or 

 near the surface on the east side of the pond, where also a few boulders may be 

 seen. 



Few boulders are found on the plain. One was seen in 1915 on the line of 

 the old railway, a mile or so from the supposed position of the ice front at the 

 time the plain was formed. An older sheet of till may have furnished this boulder 

 as well as those seen about Hummock Pond. On the other hand, the Wisconsin 

 ice sheet, in its earlier substages, may have advanced beyond the site of the 

 present island and its effects may have been largely masked by the building of 

 the plain. 



The plain must have originally extended a mile or more south of the greater 

 part of the present southern coast line. Tom Never's Head (see Plate 11, fig. 1) 

 is a bluff cut by the sea in the iceward part of the plain where the fosse and the 

 ice contact slope turned southeastward along the ice front, which appears to have 

 extended as far southeast as Nantucket shoals. The moraine probably also con- 

 tinued in this direction. On an ancient map of the shoals, near their southern 

 border, is the inscription "'Bow Bell is great stones," which may be interpreted 

 as evidence of the presence there of glacial boulders. 



THE FOSSE 



The longitudinal valley or fosse that lies between the crest of the outwash 

 plain and the corrugated Nantucket moraine is due to the building up of the plain 

 south of the ice front and to the formation of the moraine north of it. It is, there- 

 fore, a depression held open by the ice that lay along the border of the ice sheet. 

 It is evidently not a drainage channel cut by a stream that flowed from the 

 edge of the ice during its retreat, for these drainage channels, such as that of 

 Long Pond, stretch across the fosse and the plain alike. The bottom of the 

 fosse, although it contains a few boulders, is the smoothest surface on the island, 

 and it is prevailingly swampy (see Plate 11, fig. 2). Gibbs Pond, which stands 

 within the fosse, appears to occupy a shallow depression unconnected with glacial 

 streams. The fosse was apparently not eroded during the Wisconsin stage, and 

 although it contains some Wisconsin drift it may represent the old surface of the 

 island, upon which the outwash plain was built. 



THE NANTUCKET MORAINE 



The general position of the ice front on Nantucket is well defined by the low 

 bluff at the northern margin of the creased outwash plain. The moraine-like 



