276 



CAPE COD GEOLOGY 



an island which has been reduced to sea level during the present stand of the 

 sea, when the cliff on the west side of the North Truro plain was cut. (Plate 25, 

 fig. 2.) The plain probably did not extend west of the line of beach between 

 Race Point and Wood End, because the sea floor there suddenly drops to depths 

 of 24 to 28 fathoms. It is not known whether the original plain was cut back 

 into the older Pleistocene deposits at Highland Light by glacial streams that 

 discharged southward across that part of the Cape in advance of the Wisconsin 

 ice sheet or whether it was formed at sea level by wave action during the latest 

 part of the Vineyard interglacial stage. 



Most of the creases on the North Truro plain drain westward. Fine examples 

 of these creases may be seen at the North Truro railroad station, where they 

 come down from the clayey ground at the western base of the hill on which the 

 Highland light stands. A deep crease springs out from the air, so to speak, at the 

 crest of the cliff south of the lighthouse and follows closely the line of the escarp- 

 ment between the upper and the lower plain. Other short creases, which have 

 curving courses, lead down the northeastern side of the plain, showing that the 

 streams flowed in that direction, unless we may suppose that these creases were 

 formed by the run-off from the plain, an unlikely supposition, because of the 

 freedom with which rainwater soaks into the sand and gravel north of the clayey 

 tracts west of the lighthouse. The bottoms of these creases, like the bottoms 

 of those of the Eastham plain, were as low as the present sea level, and the 

 streams that occupied them probably found their way between the ice and the 

 gravel banks and joined those of the Chatham drainage system, which appear 

 to have been contemporaneous. The structure of the beds in the North Truro 

 plain and the extension of these beds into those at Highland Light, notwith- 

 standing the community of level of the two plains, does not seem to warrant 

 the assumption that a glacial lake was held in elsewhere by the glacier. 



The Wellfleet Plain 



There is abundant evidence in kettle holes and in downsunk beds of strati- 

 fied drift in the outer coastal section that at least the superficial parts of the 

 southern and eastern divisions of the Wellfleet plain were shaped by the Wiscon- 

 sin ice and that deposits were laid down from that ice either directly or by the 

 action of water. Kettle-hole topography is typically developed around the lake- 

 lets south of Pamet River. What appears to have taken place is as follows: 



1. Deposition of a foundation of stratified material, including the beds at 

 Highland Light that suggest the Jameco gravel, the Gardiners clay, the Jacob 

 sand, and possibly the lower part of the Manhasset formation. 



