CAPE COD GEOLOGY 



45 



Mannetto Formation (Glacial) 



High up in the folded beds in the cliff at Gay Head, on the sides of the hol- 

 low called the Devil's Den, there are exposures of a bed of compact blue clay 

 containing small, angular fragments of rock and rounded pebbles. The bed is 

 unstratified, having the massive character of a bed of till, and has not been well 

 exposed within the last thirty years. In the part of the cliff northeast of the 

 Devil's Den it lies above the horizon of the Dukes boulder bed and of the gravel 

 bed that overlies it. Above the bed of stony blue clay other beds of glacial 

 gravel form the high point back of the "quadrangular fold" of Hitchcock's 

 report. The bed of stony blue clay exposed southwest of the Devil's Den is not 

 at the same horizon. 



At a point on the north shore of Marthas Vineyard near Makoniky, beds of 

 similar stony blue clay crop out under the stratified Gardiners clay. As beds of 

 glacial till containing large boulders occur beneath the Gardiners clay on No 

 Mans Land and at the east end of the Nashaquitsa cliffs, in the district known as 

 Weyquosque, the beds of stony blue clay at Gay Head probably also lie below 

 the horizon of the Gardiners clay. Elsewhere beneath the Gardiners clay there is 

 the bed of coarse glacial gravel that Fuller called the Jameco gravel. The gravel 

 above the stony blue clay at Gay Head may be of early Jameco age, but in the 

 good exposures it is separated from the Jameco by an angular unconformity — ■ 

 that is, it was not deposited when the folding occurred that has given to the section 

 at Gay Head its puzzling structure. The stony blue clay (till) and the overlying 

 bed of gravel are infolded in the section at Gay Head and are, therefore, both 

 referred to the Mannetto glacial stage. 



Folding at Gat Head 



The principal evidence of an advance of the ice at this stage in the forma- 

 tion of the Pleistocene deposits on Marthas Vineyard is found in the folding and 

 overturning of all the preceding deposits. On Gay Head the displacement was an 

 overthrust toward the southwest, which produced isoclinal folds and a conse- 

 quent northeasterly dip of the beds. The stony blue clays already described indi- 

 cate the presence of either land ice or floating ice prior to the folding of the glacial 

 sands and gravels and of an unknown thickness of Tertiary deposits and under- 

 lying Cretaceous clay. An exactly comparable section may be seen in Clay Head, 

 on Block Island. It seems probable that a great glacier advanced southward on 

 to the deposits of the Coastal Plain at this time. This glacial advance appears 

 to be that which Fuller inferred from the presence of the coarse Jameco gravel, a 



