206 J, B. WOODWORTH — UNCONFORMITIES OF MARTHAS VINEYARD. 



tation of the mass by the oxide of iron is widespread. The small cup- 

 like nodules of reddish color which abound in this formation and are 

 sometimes found in the later glacial deposits owe their origin to the 

 changes which have taken place in the Cretaceous pebbles involved in 

 this early Pleistocene formation. The outside of these pebbles being 

 more or less completely consolidated by the oxide of iron, when the beds 

 are disturbed the nodules thus formed escape and the unconsolidated 

 sand or clay of the interior runs out, leaving the hollow pebbles so 

 common on the New England islands. 



The origin of the beds is evidently to be found in some kind of ice 

 action at the opening of the Pleistocence period. The section in the 

 southern part of the Gay Head cliffs shows that the deposition of these 

 boulders was preceded by some folding of the underlying Cretaceous 

 series. In a later episode in the history of this section it will be shown 

 that profound folding of the beds preceded another epoch of accumula- 

 tion of thick deposits, probably of glacial origin. In so far the evidence 

 is favorable to the hypothesis that the disturbances in the New England 

 islands are due to successive advances of an ice-sheet upon the soft 

 strata of the coast plain. Absolute proof that the deformation was ac- 

 complished by the thrust or displacement of the ice, is wanting, however, 

 in that in both cases the dislocations took place before the deposition of 

 the beds which, by their nature, point to a glacial or glaceo- aqueous 

 origin. 



Compound gravels and sands of the Sankaty epoch. — Succeeding the boulder 

 bed, or overlying directly the Neocene and older strata where the boulder 

 bed is wanting, is a group of gravels and sands of a granitic character, 

 the fragments of which are often subangular. The nearest analogues of 

 these gravels are to be found along those parts of the New England 

 shore where waves and currents are handling over and depositing the 

 waste cut from kames and sand-plains of the last glacial epoch. These 

 sands attain a thickness of at least 25 feet on Gay Head, and are the 

 uppermost beds involved in the principal folding over the island of Mar- 

 thas Vineyard. For the reason that beds occupying a similar position 

 with reference to the folding on the island of Marthas Vineyard carry 

 the well known marine fauna at Sankaty head, the name Sankaty group 

 has been proposed for this horizon.* 



In the east portion of the Weyquosque or Nashaquitsa cliffs the same 

 sands occur, carrying abundant water-worn Miocene fossils, including 

 crab nodules, pebbles of cetacean bones, sharks' teeth, fossil sponges (?) 

 (seaman's biscuit), and numerous small unidentified fish teeth. 



* Seventeenth Annual Report qf the U, S. Geological Survey, pt. i, pp. 26, 27. 



