NANTUCKET, A MORAINAL ISLAND 233 



The significant features in the glacial formations are assembled 

 in the accompanying diagrammatic cross section. 



It is most convenient to consider the frontal plain first in 

 describing the above named features of the island. This plain 

 begins rather abruptly on the north as a terrace overlooking a 

 more or less depressed region. The height of the plain along 



a 



Fig. 4. — -Cross section (diagrammatic) of the Island of Nantucket, showing the 

 relation of the kame moraine (A), the fosse {B), and the frontal plain (C). D is the 

 present beach. The dotted line represents the supposed profile of the ice sheet when 

 the frontal plain was building. 



this summit line is, where greatest, about 60 feet above the sea 

 level. The slope of the terrace to the fosse on the north is well 

 marked, but not so steep as that of the typical moraine terrace 

 of Gilbert, 1 or so sharply cuspate as the ice-ward edges of the 

 sand plains described by Davis 2 . Yet this slope taken in con- 

 nection with the fact that the plain inclines southward with well 

 defined drainage creases, and that the materials are coarse at the 

 crest line and grade into finer gravels and sands southward, 

 affords good evidence that the plain was built against the front 

 of the ice-sheet by excurrent streams. Viewed in this light, the 

 terraced head of the plain indicates the east and west line along 

 which the ice front stood in its southernmost extension. 



This ice-contact slope is most distinct in the eastern part of 

 the island, where it turns to the southeastward, as if the ice sheet 

 extended seaward in this direction, covering at least the area 

 now forming the Nantucket shoals. In the vicinity of the town 

 of Nantucket, the hillock of pre-Pleistocene clays already men- 

 tioned has given rise to the type of sand plain which is dominant 

 on Martha's Vineyard, one in which for the greater part of that 

 island, the top of the sand plain was not built up to the base of 

 the ice front where that rested on elevated ground. On the 



'Lake Bonneville Monograph I, U. S. Geol. Surv., 1890, pp. 81-83. 



2 Bull. Geol. Soc. Am. Vol. I, 1890, p. 195. 



