164 



CAPE COD GEOLOGY 



and especially the glacial striae on one of the boulders, shows that it was formed 

 by the action of ice, and it is therefore regarded as the product of an early ice 

 sheet at a time not far from the beginning of the Pleistocene epoch. To this 

 bed Woodworth has here given the name Dukes boulder bed. 



Mannetto formation. — Immediately above the Dukes boulder bed in the 

 quadrangular fold, directly below the lighthouse, is a bed of stony blue clay 

 till not noted in previous reports and first seen in 1916. This bed is exposed 

 also at another place in these cliffs, south of the quadrangular fold. This material 

 is clearly an ice-laid till, and it differs from the underlying boulder bed in con- 

 taining no boulders and some clay. It was probably formed by a later ice advance 

 than that which deposited the boulder bed, and it is here assigned to the Man- 

 netto stage. 



Overlying the stony blue clay is a bed of gravel somewhat thicker than either 

 of the lower beds. This is the uppermost member that is involved in the quad- 

 rangular fold, and, with the exception of a thin Wisconsin coating, it forms 

 the highest part of the cliffs. Whether this gravel belongs to the Jameco or to 

 the Mannetto stage is not certain, but it is here assigned to the Mannetto be- 

 cause the Jameco deposits succeeded the folding and erosion. 



Evidence of pre-Gar 'diners age of deposits. — The beds exposed in this part 

 of the Gay Head cliffs are all older than the Gardiners clay, a fact shown by the 

 relations between the older beds and the Gardiners clay where the clay is ex- 

 posed at the north end of the cliffs. In the Gay Head section the Cretaceous, 

 Tertiary, and oldest Pleistocene beds are all closely folded and are overlain 

 uncomformably by a thin veneer of Wisconsin drift except at the end. of the 

 section. At the north end of the section the Gardiners clay is exposed, lapping 

 up unconformably on the underlying beds, as if these beds had formed an island 

 at the time the Gardiners was laid down. The Gardiners clay has been slightly 

 folded, but much less than the older beds. Exposures at Nashaquitsa cliffs 

 and elsewhere attest this fact. 



The posl-Mannetto period of folding. 1 — According to earlier reports on 

 Marthas Vineyard most of the dislocation of the beds in the Gay Head cliffs 

 antedated the deposition of what was then known as the "Tisbury beds," on 

 the north shore of the island. As defined, these beds included the Gardiners 

 clay and certain younger deposits. The sections now exposed on Block Island 

 and Marthas Vineyard indicate that most of the folding and overthrusting of 

 the Cretaceous and Tertiary beds preceded the deposition of the Gardiners 



1 Note by J. B. Woodworth. 



