CAPE COD GEOLOGY 



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shark teeth, and, occasionally, a cast of a mollusk. This bed has yielded also 

 a bone of an early type of American camel and the astragalus of a highly devel- 

 oped Pleistocene horse, which were dug out of the bed some years ago by me 

 (Plate 22). The cetacean bones, the teeth of sharks, and the molluscan remains, 

 all in the condition of pebbles swept together by the action of water, have by 

 all previous observers been referred to the Miocene. Some of the chert pebbles 

 carry fossils now known to be of Helderberg (Lower Devonian) age. Until the 

 Pliocene and early Pleistocene fossils were discovered it seemed likely that the 

 bed of conglomerate was a beach or shoal deposit that had been rudely assembled 

 by currents or waves. It now seems more reasonable, however, to regard the 

 bed as a deposit laid down in a shallow stream on the border of the coastal 

 plain, in which the coarser debris, of mineral and organic origin, washed out 

 of neighboring banks from Cretaceous and Miocene beds, perhaps from the 

 greensand bed, accumulated in the open air on the surface of eroded Cretaceous 

 sand and clay. This interpretation accounts for the local occurrence of the 

 bed, which has not been recognized, either as an unbroken deposit or as rolled 

 fragments, outside of Gay Head and the western part of Marthas Vineyard. 

 Both north and south of the exposure mentioned, rolled fragments of this bed 

 occur on the unconformable surface separating the Cretaceous deposits from 

 the older Pleistocene boulder beds and gravel deposits. Some cobbles, a few 

 inches in diameter, have been found at the same geological horizon near Peaked 

 Hill and on the south slope of the island near that hill as far east as a roadside 

 ravine at the east end of the flattish hill on the north side of Chilmark Pond, 

 a hill that affords an excellent view of the south shore. 



The entire absence of pebbles of granite, gneiss, and other feldspar-bearing 

 rocks in this conglomerate indicates its derivation from wash of the underlying 

 Cretaceous and Tertiary deposits at a time when the beds of the Coastal Plain 

 stretched northward over the area of the Sound and Buzzards Bay. This concep- 

 tion presents a picture of the dawn of Pleistocene time, when the probably drier 

 climate of the Pliocene epoch, indicated by the remains of a camel, had given 

 way to the cooler and moister Pleistocene climate, before the first ice sheet had 

 driven the native horse beyond the limits of glaciated New England. 



