12 



CAPE COD GEOLOGY 



forms a north-south belt across the state next to the upland. It has a fairly uni- 

 form width of 6 to 15 miles, and its hilltops range between 540 and 650 feet in 

 altitude. Far from this group, but rising to its level, are the Blue Hills of Quincy 

 and Milton, 500 to 640 feet, and Moose Hill, in Sharon, 560 feet. 



The next group forms a very irregular belt of hills between 320 and 380 feet 

 above the sea. These are to be seen mainly around the margins of the river val- 

 leys and in the two hill belts northeast of Wrentham and Weston. Next below 

 them is a group of hills, between 220 and 260 feet above the sea, which are scat- 

 tered over much of the state east of the two higher groups and almost reach the 

 sea in Lynn. On the hills of this group in Lynn and Waltham there is scarcely 

 any glacial drift. . . . The summits of the lowest group range between 110 and 

 160 feet, and their areas form an irregular network along the coast and up the 

 river valleys. 



Across this terraced surface the Buzzard's Bay lobe of the ice sheet swept 

 to Marthas Vineyard in its first advance and to the Elizabeth islands in its second 

 stage. The Cape Cod Bay lobe and the South Channel lobe passed southward 

 outside of the site of Boston Harbor, where the ancient rock surface lies below 

 the platforms above described. A remarkable feature of the sea floor in that 

 region is a depression 99 fathoms deep, which shows that at a distance not more 

 than about 35 miles east of Salem the sea floor drops to a depth of more than 

 800 feet below the platform (220 to 260 feet) which approaches the sea at Lynn. 

 Thus a strong topographic break outlines the eastern rock-bound coast of the 

 state, the interpretation of which is bound up with that of the platforms of the 

 eastern lowland, the date of whose origin is not clearly known. Both of these 

 areas formed the inner margin of the glacially altered coastal plain on the south. 

 Cape Cod and Nantucket belong mainly to the submarine tract, and the islands 

 to the west belong to the eastern lowland. 



The eastern lowland cannot be a down-warped part of the surface of the 

 upland, for the south coast, whose intersection with the sea is formed by the 

 mid-Cretaceous erosion plain, has no corresponding offset to the north along the 

 line of flexure which would have been thus introduced. The lowland appears to 

 have been well opened out by Miocene time, for offshore deposits of this age occur 

 at Gay Head and on the borders of the lowland at Marshfield, and the Miocene 

 deposits contain no debris that would have been laid down on the sea floor by the 

 erosion of the surface from the higher to the present level. The surface therefore 

 appears to have been produced between the end of the Cretaceous period and 

 the local beginning of Miocene deposition. An Oligocene date appears most 

 probable. 



