8 



CAPE COD GEOLOGY 



and Cape Cod extend into the sea to points reached by the ice sheets, whose 

 fronts were aligned from east to west. The great submarine banks off the east 

 coast represent similar features, but their surfaces have been highly modified by 

 the action of waves and current. 



In broad outline, the region here considered is formed of two great series of 

 lobate frontal moraines, between the inner and outer lines of which lie Nantucket 

 and Vineyard sounds, and back of the inner line of which, in depressions like those 

 of the sounds in origin, lie Buzzard's and Cape Cod bays. In this festooned dis- 

 tribution of the land the moraines define the outer border of lobes of the ice 

 sheet, formed along paths of the most rapid motion of the ice, the maximum rate 

 of flow of which in each lobe occurred along a line that was directed southward 

 and that lay near the central axis of the lobe. The interlobate moraines, repre- 

 sented by the arm of Cape Cod in Wellfleet and Truro on the one hand and by 

 the Plymouth moraine on the other, were places of minimum rate of flow of the 

 ice, and therefore of heavy deposition. These interlobate lines or axes point west 

 of north, toward obstructions to the flow of the glacier. The direction of the 

 glacial striae on the mainland show that the lobe lying in Buzzard's Bay and in 

 the country somewhat west of it moved along the Merrimac Valley in New 

 Hampshire, and that the lobe lying in Cape Cod Bay moved along a line east of 

 the Mount Washington range, which, together with the Blue Hills of Milton, 

 appears to have been the chief cause of the slack ice motion represented by the 

 Plymouth interlobate moraine. A like relation is traceable in the Wellfleet inter- 

 lobate axis to Mount Bigelow, in Maine. Still farther east another interlobate 

 axis is faintly suggested in George's Bank, which lies beneath the sea nearly in 

 line with Mount Desert and Mount Katahdin, in northern Maine. The outline 

 of the Cape and of the islands shaped by glacial action is thus a geographic reflex 

 of lines of depression and elevation in the country to the north. 



Cape Cod and the outer islands south of it contain few streams, because the 

 porous gravel and sand permit rain water to sink nearly if not quite to sea level. 

 The height of the water table in the plains is shown by the surface of numerous 

 lakes and ponds. An extensive seepage of the water takes place along the coast 

 at points between the levels of high and low tide. 



The streams of the islands are limited to areas where deposits of clay occur, 

 as in the upland, western part of Marthas Vineyard. On the south side of the 

 Cape, in Barnstable, the overflow of lakes forms small brooks, which, however, 

 flow in channels cut by the much larger streams that were fed by the melting ice 

 during the last stages of the glacial epoch. Mashpee River is the largest stream of 

 this class. 



