t58 .1. B.WOODWORTH — GLACIAL PLEISTOCENE IN (JAY BEAD CLIFFS 



Vineyard and Block island and, as I believe, as well on Long island 

 took place later, but yet a very long time before tbe last ice advance. 



Correlation of Deposits with Columbia Formation 



That tbe boulder bed at Gay Head, together with tbe overlying gravels 

 and sands, represents tbe lower part of McGee's Columbia formation 

 follows by the definition of that term as regards the stratigraphic posi- 

 tion and litbological character of the materials at Gay Head. The con- 

 clusion that tbe boulder bed represents the deposits of the first advance 

 upon the Atlantic coast plain is also strictly in line with the understood 

 origin of the Columbia. In the easternmost of these New England 

 islands the Columbia is fossiliferous and is separated from later deposits, 

 also of Columbia age, by the Gay Head diastrophe and unconformity 

 dependent thereon. This lower Columbia was termed the Sankaty in 



my paper of 1896. 



* 



Interpretation of Columbia Epoch 



It has been shown by McGee that the Columbia was a time of sub- 

 mergence to a depth of at least 400 feet south of New York. -There is 

 nothing to disprove this submergence during Sankaty time in the New 

 England islands or on the neighboring mainland. On the contrary, a 

 depression to this depth would account for many bodies of glacial clays 

 which occupy embay ments along the shore, as in Boston basin, and 

 which were deposited anterior to the last ice advance in this region. 



After several years of repeated visits to the Marthas Vineyard sections 

 and a visit to the island of Moen, in the Baltic, the writer is unable 

 to find any direct evidence in either of these fields proving conclusively 

 that the upturning of the beds has been brought about by glacial thrust. 

 It is perfectly clear that the folding and displacements took place after 

 the first invasion of ice in both fields and before the last ; hence clearly 

 within the glacial epoch. Without committing oneself to the verity of 

 the hypothesis of glacial thrust, some conclusions may be drawn from 

 Gay Head as to the mode of action of the supposed ice-thrusts. When 

 we compare the folding accomplished just before the deposition of the 

 boulder bed with that which took place later, we note that the two are 

 as one to ten in volume of beds displaced. On the supposition of dis- 

 placement by ice-thrust, the relative exemption of the coast plain from 

 disturbance during this first advance may be explained by tbe sub- 

 mergence of tbe region, so that the ice rested lightly on the surface, 



