CAPE COD GEOLOGY 



249 



of the known section in the Sankaty sand of Nantucket are here placed in the 

 Pleistocene. The formations recognized in the Cape Cod district are shown 

 below in descending order. 



Wisconsin glacial deposits: 



Falmouth substage: Falmouth moraine and associated outwash plain. 

 Nantucket substage: Deposits of gravel and till and kames; ice-block holes on south 

 side of Cape Cod. 

 Vineyard interglacial stage : Represented by erosion phenomena. 

 Manhasset formation: 



Hempstead gravel member (?) : Beds of sand about Pleasant Bay and Chatham, 

 which rest on dark clay that may represent Montauk till; also beds of gravel 

 overlying boulder clay (Montauk?) in Barnstable and areas farther east. 

 Montauk till member (?): Boulder clay near and east of West Barnstable; deposits 



forming Great Hill; and pebbly till at Nauset head. 

 Herod gravel member (?) : Beds of gravel and sand that lie beneath morainal out- 

 wash deposits near Chatham; gravel overlying boulder clay in Barnstable. 

 Jacob (?) sand: Fine yellowish sand 40 feet thick, above blue clay (Gardiners?) at 

 Highland Light; fine yellowish sand about a mile east of Truro station, resting on 

 brownish clay (Gardiners?); sandy beds above clays at West Barnstable; sandy beds 

 overlying blue clay (Gardiners?) near Chatham. 

 Gardiners (?) clay: Beds of marine clay, at Highland Light, in Truro; similar beds 

 along the shore of Pleasant Bay at West Barnstable; about 1 mile east of Truro station; 

 and about Chatham. 

 Jameco (?) gravel: Coarse glacial gravel at Highland Light and in bluffs south of it, 



lying beneath blue clay (Gardiners?). 

 Sankaty sand: Fossiliferous marine sand in Province town well and in well at Orleans. 



These deposits will now be described in detail and the evidence for their 

 correlation will be presented. In this account it is convenient to begin with the 

 lowest deposits — ■ the oldest. 



PRE-WISCONSIN DEPOSITS 



SANKATY SAND 

 Deposits that are believed to be an extension northward along the coast 

 of the fossiliferous marine sand found in the cliffs at Sankaty Head, on the 

 east coast of Nantucket, were encountered in a well drilled in 1905 on the site 

 of the old pumping works of the city water board at the eastern limit of Province- 

 town. This well reached a depth of 420 or 450 feet, where the drill struck a rock 

 (syenite) that the drillers believed to be a boulder. Samples of this rock were 

 obtained by Mr. J. H. Blake when the well was drilled, and are now in the 

 geological laboratory at Harvard University. Several samples obtained from 

 shallow wells in the same vicinity consisted of waterworn small glacial pebbles 

 and broken shells. 



