94 



CAPE COD GEOLOGY 



The northeast arm of Nantucket has been at times cut off from it by storms 

 that washed away the beach at the head of the harbor, and the island of Tucker- 

 nuck has sometimes been joined to it by storm-made beaches. Tuckernuck is 

 virtually an extension of the so-called terminal moraine along a line running 

 from Nantucket to Chappaquiddick, an island at the east end of Marthas Vine- 

 yard. Muskeget, which lies west of Tuckernuck, is a low, gravelly islet. South 

 of Muskeget are the Gravelly Islands, small outlying parts of the same dry 

 shoal 



GEOLOGY 



Correlation of Formations 



The geology of Nantucket is shown on Plate 8. The following table shows 

 Dall's and Woodworth's interpretations of the formations recognized on Nan- 

 tucket : 



Pliocene and Pleistocene Formations on Nantucket 



H 



W 

 o 



o 



H 



H 



Arranged with lower part of Sankaty 

 sand as Upper Pliocene (Dall's interpre- 

 tation) 



Arranged with Sankaty sand as equiva- 

 lent of glacial Weyquosque formation on 

 Marthas Vineyard (Woodworth's inter- 

 pretation) 



Wisconsin glacial stage ; Nantucket substage 



Vineyard interglacial stage ; erosion 



Manhasset formation (glacial); Hempstead gravel member 



Montauk gravelly till member 



Jacob sand (transitional; interglacial) 



Gardiners clay (interglacial) at Sachem spring 



(Jameco gravel and associated deposits of till on Marthas Vineyard; not recog- 

 nized above sea level) 



Stony blue clay, passing laterally into stratified gravel and locally contorted drift 

 of probable Manetto age 



m 



w 

 o 

 o 



« 



H 



Oh 

 Oh 



P 



Upper shell bed at Sankaty Head 

 Lower shell bed at Sankaty Head 



Ferruginous gravel (of Desor and Cabot), 

 lying unconformably on light-brown 

 sandy clay 



Fossiliferous Sankaty sand and bed of 

 waterworn marine shells at Squam 

 Head (at horizon of the Weyquosque 

 formation of Marthas Vineyard) 



Ferruginous gravel of Desor and Cabot 

 (Not exposed) 



The geological formations that lie deep beneath the surface of Nantucket 

 are supposed to be extensions of the beds of Upper Cretaceous clay and sand 

 seen on Marthas Vineyard, but such materials have not been seen above sea 



