CAPE COD GEOLOGY 



261 



THE VINEYARD INTERGLACIAL STAGE 



EROSION PHENOMENA 



Before the Wisconsin ice advanced to Nantucket and Marthas Vineyard the 

 outer line of islands and, presumably, the region north of them, were long sub- 

 jected to normal fluvial erosion. On Cape Cod there are traces of a pre- Wis- 

 consin topography on the plain in its south-eastern part and in the plains of 

 Wellfleet and Truro. These plains lay in or near the interlobate axis of the 

 ice sheet, where, as there was no erosion by ice and the glacial deposits were 

 small, the older features have been but slightly masked by the Wisconsin 

 deposits. The dissection of the beds of gravel and sand in the area about 

 Chatham, in Harwich, and in Orleans evidently preceded the advance of the 

 Wisconsin ice to Nantucket, because the ice-block holes occupied by the lakelets 

 there differ from those farther west, about Falmouth, for they lie in channels 

 cut in the folded beds of clay and sand of the pre-Wisconsin drift here referred 

 to as probably the Montauk till of the Manhasset formation. The peculiar 

 winding courses of these depressions are not like those excavated by moving ice, 

 and they were evidently not the channels of subglacial streams, because they 

 were occupied by stagnant ice left over from the Nantucket stage of the Wiscon- 

 sin glaciation. Thus the unevenness of the plains of the Chatham district (except 

 the outwash terrace, which was formed during the last advance of the ice of the 

 Cape Cod bay lobe) dates back to an interval of erosion between the deposition 

 of the Manhasset formation and the Wisconsin ice advance. Similar features may 

 be seen in the highland plains of Wellfleet, where the deep channels or "hollows" 

 are flanked by moraines and kettle holes, which show that these channels were 

 in existence when the last ice lay over that region. Moreover, large boulders here 

 and there on the surface, such as those seen near Highland Light, show that an 

 ice sheet lay here after the topography had been outlined. These plains or 

 terraces thus fall into two groups — the high-level Wellfleet plain and the 

 low-level Chatham plain. 



THE WELLFLEET PLAIN 



The Wellfleet plain, which forms the highlands of the forearm of the Cape, 

 is much dissected and is in places masked by moraines and kettle holes. Its 

 highest points reach elevations of 140 to 150 feet north and south of the val- 

 ley of Pamet River. Toward the west it declines to elevations of about 100 

 feet. It overlooks from Truro northward the lower plain of North Truro, which 

 has an elevation of 60 to 80 feet and a surface partly masked and partly carved 



