CAPE COD GEOLOGY 



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mound-and-kettle surface north of this bluff, or "ice contact," is separated from 

 the outwash plain by a depression or fosse that varies in width from half a mile 

 to more than a mile. The mounds and kettles of the so-called morainal hills 

 were once regarded as accumulations made inside and under the ice while the 

 sand plain was being laid down outside of the ice, but it now appears that most 

 of the drift in the mounds and hollows consist of stratified beds of gravel, sand, 

 and clay laid down long before the advent of the Wisconsin ice sheet. Where 

 the relation of form and structure can be discerned the contorted beds and the 

 mounds and hollows conform to such an extent as to indicate that their form 

 and structure are alike the results of deformation by the overriding Wisconsin 

 ice sheet or by earlier ice sheets, a deformation subsequent to the formation of 

 the surface that bevels the tilted and eroded beds of the island. The Wisconsin 

 drift or till in the so-called morainal belt is very thin, composed only of scat- 

 tered boulders and here and there a rubbly layer laid down on older stratified 

 material. The so-called moraine is not a true moraine in the sense of a deposit 

 made at the edge of the ice sheet. 



On Nantucket the ridges and hollows of this submarginal push moraine or 

 pseudo-moraine are typically displayed east of Nantucket Harbor, in the Shaw- 

 kemo Hills, Altar Rock Hills, and Folger Hill. They are arranged in roughly 

 parallel lines that sweep around with a somewhat curvilinear trend similar to the 

 lobation of moraines. East of the deep crease known as Madequecham Valley, 

 which extends south-southwest from Polpis Harbor to Madequecham Pond, 

 the ridges trend somewhat north of west in the higher part of the area. Between 

 Folger Hill and Sesachacha Pond the ridges and depressions trend between north- 

 west and west of north. On the west side of Madequecham Valley the strong 

 ridges trend northeastward, and although this trend is not shown by some ridges 

 it reappears in strong ridges as far west as the east shore of Nantucket Harbor. 

 Yet within this northeastward-trending group of ridges there are kettle holes 

 whose long axes trend north of west. 



The change in the direction of the lines of ridges and hollows on opposite 

 sides of Madequecham Valley suggests that the origin of this valley is in some 

 way associated with the origin of the hollows between the ridges, and that al- 

 though that valley was occupied by a subglacial stream, as Shaler first pointed 

 out, its position was determined by the deformation or dislocation, through 

 ice thrust, of the underlying gravel and sand. It may be possible also that along 

 this line there was a significant break in the nature of the deposits or in the 

 masses of ice that pushed them forward. 



