CAPE COD GEOLOGY 



85 



oscillation of the land in relation to sea level. At the end of the Cretaceous period J 

 the land probably rose and was somewhat eroded before any marine beds were 

 laid down, but the possibility of transgression by the sea in Eocene time is shown 

 by fragments of fossiliferous Eocene material found in the drift on Cape Cod. 

 Oligocene and early Miocene time was a period of uplift. 



At the time of the deposition of the greensand, regarded as an extension 

 of the upper Miocene Chesapeake beds, to the south, the Miocene was probably 

 a period of submergence of the lowland, in which the shore may well have lain 

 against the upland somewhat east of Worcester, or even farther inland. The 

 beds of greensand represent the offshore facies of deposition during this trans- 

 gression of the sea. The Miocene sea teemed with life. Whales, dolphins, sharks, 

 and probably also walruses and seals, roamed along off the shore, and crabs, 

 mollusks, gastropods, and barnacles inhabited the bottom. The temperature of 

 the water in this Miocene sea appears to have been not very different from that 

 of the present sea off the south coast of New England. The only trace of land life 

 is a single molar tooth of a primitive rhinoceros found at Gay Head. 



In early Pliocene time in southeastern Massachusetts the land was uplifted 

 and the sea withdrew. Small deposits of shell-bearing sand then laid down along 

 the south coast have been found on Gay Head and Long Island. The fauna, 

 so far as known, was chiefly molluscan and probably lived in shallow water along 

 the coast. The shell beds at Sankaty Head, which are of upper Pliocene and 

 Pleistocene age according to Dall, have heretofore been regarded as of inter- 

 glacial Pleistocene age, and are so considered in this report. The upper beds 

 of what may be called the Macoma incongrua zone, according to Dr. Wilson, 

 contain species which, according to Dall, are not now living in the North Atlantic 

 but are found in the North Pacific, indicating that a connection once existed 

 across or around North America from Nantucket to the North Pacific Ocean. 

 The presence of these North Pacific types of mollusks indicates a lowering of the 

 temperature of the ocean water of 5 to 10 degrees Centigrade as compared with 

 that of the Miocene sea in the same latitude. 



The Pleistocene deposits of the New England Islands and Cape Cod consist 

 chiefly of beds of boulders, gravel, sand, and glacial clay arranged in a more or 

 less rhythmic series, separated by erosion between stages of glaciation. The 

 earliest of these glacial deposits are exposed only in two areas on Marthas Vine- 

 yard and Block Island. Subsequent to what appears to be a third glacial advance, 



1 Stephenson, L. W., The Cretaceous-Eocene contact in the Atlantic and Gulf coastal plain, U. S. 

 Geol. Survey Prof. Paper 90-J, pp. 155-182, with map from Long Island southward, showing Cretaceous 

 boundary, 1915. 



