CAPE COD GEOLOGY 



263 



as to prevent the deposition of the sand and gravel that was washed out from 

 the moraine in the Falmouth stage of the Wisconsin glaciation. The streams 

 in this tract flowed in long, straight channels that ran between west of south 

 and southwest. Here kame-like elevations 50 to 70 feet high are common, and 

 some that stand farther north reach a height of 80 feet or more. Much of the 

 glacial material here must be assigned to the Nantucket substage, when it was 

 laid down around ice blocks that were afterward replaced by lakelets. Such de- 

 posits, however, are at many places thin or nearly absent, as shown by the older 

 clays in cuts about Harwich. 



Other features of the Vineyard erosion interval on the Cape are the channels 

 and embayments cut in the Chatham plain, which were graded to a base level 

 of erosion that lay below the present sea level. The relative position of this base 

 level when the channels were cut cannot even be inferred from the depth of 

 the channels that remain, for they occupy positions that must have lain well 

 back from the mouths of the eroding streams. Shaler 1 recognized the pre- 

 Wisconsin topography of parts of Cape Cod and drew a hypothetical map 

 showing the old lines of drainage. The base-level to which this drainage system 

 is related lies indefinitely below the present level of the sea. 



NORTH TRURO PLAIN 

 (See Plate 25, fig. 2). 



The well-defined plain in the northwest part of Truro, including Pilgrim 

 Heights, at the north end of the glacial part of Cape Cod, stands less than 

 80 feet above sea level along the western edge of the higher Wellfleet plain 

 from Highland Light southward nearly to Pamet River. The escarpment be- 

 tween the two plains is a distinct topographic feature despite considerable dis- 

 section. The surface of this plain is creased by shallow channels of late Wisconsin 

 streams, as noted by Grabau, but the material found above sea level consists of 

 gravel, sand, and clay, which may be traced into the lower part of the beds that 

 form the mass of the higher Wellfleet plain. The North Truro plain is therefore 

 a bench or terrace cut back into the Wellfleet plain from the west prior to the 

 last action of the Wisconsin ice or of the streams. There are practically no 

 boulders on the surface, and except for the coastal sections one would infer 

 from the surface features that the plain is wholly of late Wisconsin age. It seems 

 necessary to correlate this terrace plain with the terraces that stand at heights 

 of 60 and 80 feet in the southern part of the Cape. 



1 N. S. Shaler, Report on the geology of Cape Cod, p. 516, fig. 86. 



