NANTUCKET, A MORAINAL ISLAND 235 



accumulating in that situation by Chamberlin in the Greenland 

 glaciers. Applying the observations made by Chamberlin upon 

 the shearing of the upper ice over the lower and the involution 

 of drift which thus comes about, to the case of the Nantucket 

 type of terminal moraine, we may fairly suppose that when the 

 moving ice sheet became blocked against the head of its grow- 

 ing sand plain, the upper ice began to shear over the lower, 

 blocked prism of ice lying behind the sand plain. This shear- 

 ing movement affected the lower part of the ice sheet for a long 

 distance back from the actual front. At a distance of from one 



I.C E S H E E T __ 



SAJTD PLAIN 



Fig. 5. — Diagram showing supposed mode of accumulation of Kame moraine. 

 D, Prism of dead ice blocked by sand plain barrier. L, Live ice dragging up drift 

 into K M, the position of the Kame moraine. 5 S, Principal plain of shearing. 



to two miles back from the front the bottom ice began to glide 

 over the prism of dead ice lying back of the sand plain. (See 

 Fig. 5.) As a result of this action the subglacial till dragged 

 along on the bottom northward of this belt was gathered in the 

 shear zone with moving ice above and dead ice below. Most of 

 the till accumulated within a belt about a mile wide, leaving a 

 strip in the case of Nantucket from a mile to half a mile wide 

 between this accumulation and the head of the sand plain in 

 which the debris was small in amount as compared with that 

 deposited in the sand plain on one side and in the moraine on 

 the other. On the melting out of the ice sheet, this outer part 

 of the stagnant prism of ice, which was relatively free from drift, 

 would give rise to the depression which separates the sand plain 

 from the moraine. The melting out of the inner thin portion of 

 the wedge of dead ice with its charge of till would result in the 

 hummocky topography which gives the moraine the striking 

 resemblance to a belt of kames. In the case of the water-worn 

 gravels and sands which accumulate in this belt, it is to be sup- 

 posed that in the shearing movement of the upper ice over the 



