232 CURTIS AND WOODWORTH 



the moraine is shown by the tilting and dislocation of the strata 

 under the Sankaty Head lighthouse, their truncation by erosion 

 and the unconformable deposition on them of the moraine, a rela- 

 tion first described by Upham, who showed that the beds do not 

 belong to the so-called Champlain epoch as Dana was first led to 

 suppose. Northward, at Squam Head, similar beds occur at high 

 angles with folded clays, indicating that a profound disturbance 

 of the strata over the site of the island took place sometime after 

 the deposition of the older Pleistocene. Opinion is not unani- 

 mous concerning the cause of this and the similar dislocations 

 which affect the islands of this group. According to one view, 

 the dislocations originated in movements taking place in the 

 earth's crust beneath, being simply a more pronounced phase of 

 the disturbance which marks the " fall line " from New York 

 southward at the inner edge of the coastal plain. Another view 

 supposes the strata to have been disturbed by the mechanical 

 action of an ice sheet advancing upon the soft strata of the Atlan- 

 tic coastal plain. Since the action took place long before the 

 deposition of the moraine which constitutes the chief feature of 

 the island, the question need not be debated in a paper dealing 

 primarily with the interpretation of a model of these more recent 

 features. 



The superficial formations of glacial origin on Nantucket 

 appear in three very distinct belts extending east and west across 

 the island and appearing on the dependent island of Tuckernuck. 

 The small, wave-washed isle of Muskeget is probably a modified 

 remnant of one of these belts. These deposits reappear on the 

 easternmost part of Chappaquiddick. These bands may be 

 spoken of as the kame moraine, the fosse, and the frontal plain. 

 North of the hummocky ground, known as the kame moraine, in 

 the eastern part of the island, is a small area of till-covered land. 

 It seems to be the unstratified debris left upon the surface when 

 the ice-sheet melted away, and may be dismissed with this expla- 

 nation. Everywhere bordering the island is a fringe of recent 

 marine deposits, in the making of which the original outline of 

 the island has been much altered. 



