52 



CAPE COD GEOLOGY 



south of Scituate, disturbed beds of orange-colored sand occur above blue clay- 

 that probably belongs to the Gardiners formation. 



The Jacob sand, which was laid down during the retreat of the Wisconsin 

 ice front, lies between the Gardiners clay below and the Herod glacial gravel 

 above and probably marks the retirement of the sea after its maximum sub- 

 mergence of the land in Gardiners time. The formation contains no fossils, unless 

 the marine beds at Sankaty Head, on Nantucket, belong to it. These beds, how- 

 ever, for reasons given, are here referred to a much lower horizon in the Pleisto- 

 cene. 



The Jacob sand has not been certainly identified on the mainland north 

 and west of Scituate, but certain beds of fine sand in Boston Harbor encountered 

 in boring may belong at this horizon. If the beds of glacial brick clay of the 

 Mystic valley are extensions of the Gardiners clay, the Jacob sand in that region 

 has been removed by erosion. 



Assuming that the altitude of the area at Highland Light, on Cape Cod, has 

 not been changed with reference to the bed rock of the mainland, the altitude 

 of the nearly horizontal bed of sand in that area that probably represents the 

 Jacob sand may indicate its altitude in the New England Islands prior to the 

 deformation of the beds. The formation at Highland Light ranges from about 

 120 to 140 feet above present sea level, an altitude which, allowing for a rise of 

 the sea floor in Jacob time, would carry the formation well on to the lowland 

 of the eastern counties of Massachusetts south of Boston. The shore line in Jacob 

 time may, therefore, have lain on the present lowland rather than off the present 

 coast. 



MANHASSET STAGE OF GLACIATION 

 Manhasset Formation 



The deposits of the third stage of glaciation are here grouped under the 

 name Manhasset formation. They occupy the geological position to which the 

 "Tisbury group" on Marthas Vineyard had been assigned, but the "Tisbury 

 group" had been defined so as to include certain beds of Gardiners clay and to 

 exclude others. 



Character and subdivisions. — As described by Fuller, the Manhasset forma- 

 tion consists of the following three members, here arranged in stratigraphic order 

 from top to bottom: 



Hempstead gravel member 



Montauk till member 



Herod gravel member 



