NANTUCKET, A MORAINAL ISLAND 231 



ties of its glacial form, despite the low relief of the island, are 

 readily discerned. 



This island, more than any other one of the New England 

 islands, 1 approaches closely the purely morainic type. On Mar- 

 tha's Vineyard, Block Island, and Long Island, the relief is so far 

 influenced by the topography of folded and eroded beds older 

 than the moraine, that the true morainic expression is not fairly 

 seen. It is in the nature of glacial drift to mask to a large extent 

 the older rocks on which it lies. The drift of Nantucket affords 

 no exception to this statement. Beneath this mantle of till, 

 gravel, and sand, whose relief is shown in the model, there is a 

 basement of pre-Pleistocene clays and older Pleistocene beds, 

 which are exhibited in the sea cliffs on the east coast, and again 

 on the north shore. These pre-morainal beds give rise to certain 

 peculiarities in the topography, forms which even the morainal 

 deposits do not entirely conceal. Remove both the moraine and 

 the plain of gravel and sand on the south side of the island, and 

 these older deposits would still stand above the present sea level 

 as a number of small islands, one at Sankaty Head, one at Squam 

 Head, and a larger islet about the size of Tuckernuck, extend- 

 ing westward from the site of the town of Nantucket. Other 

 small islets might remain where the later drift now covers these 

 older rocks. 



The oldest known formation on the island is a bluish clay, 

 probably of Cretaceous age. This clay makes up the ridge south 

 and west of the town of Nantucket. The beds of this series are 

 highly folded, as are also the strata of the same, and even more 

 recent, date in the islands westward to Staten Island. Beneath 

 these beds at an unknown but probably not great depth we should 

 expect to find the extension of the granites and gneisses of 

 southeastern Massachusetts. 



Newer than this oldest clay formation is a series of sands and 

 sandy clays containing a Pleistocene marine fauna, that of the 

 well-known Sankaty Head beds. That these beds are older than 



1 A name proposed for the islands from Nantucket westward off the southern coast 

 of New England, having a common geologic and geographic development. 



