234 CURTIS AND WOODWORTH 



western part of the island and again on Tuckernuck, the ice-con- 

 tact slope can be distinguished, affording a base line of reference 

 from which to work out the relations of the glacial deposits to 

 the ice sheet. 



Accepting the slope at the head of the plain as denoting the 

 position of the ice front, it follows that the fosse and the kame 

 moraine are features originating in the area occupied by the ice. 

 The fosse is simply the unfilled ground between the head of the 

 plain and the belt of accumulations known as the kame moraine. 



The kame moraine is supposed to be contemporaneous with 

 the sand plain ; one was building up by the action of excur- 

 rent streams outside of the ice while the other was accumulating 

 inside the ice by the combined action of ice and water. This 

 idea that the kame moraine is not frontal but submarginal in 

 relation to the ice sheet by which it was built, first suggested, it 

 is believed, by Salisbury for certain portions of the terminal 

 moraine westward on the mainland, is consistent with the inter- 

 pretation which has been placed on the origin of the kames near 

 the heads of sand plains. Both ice-laid and water-laid drift 

 tend to accumulate in the form of knobs and basins in this situ 

 ation. At present, the explanation of the phenomenon can 

 hardly be said to rank as an hypothesis, much less as " demon- 

 strable theory." 



One supposition is that the kame moraine marks the site of 

 an earlier frontal deposit, e.g., a sand plain, subsequently over- 

 ridden by the ice sheet in its advance to the line marked by the 

 head of the frontal plain. Stratified beds of sand and gravel 

 seen under a coating of till in patches of kame moraine, as at 

 Bridgewater, Mass., and the sandy clays under the till of the 

 Nantucket kame moraine, show the possibility of the extra-gla- 

 cial origin of the original deposit. But this explanation does 

 not account for the seeming regularity in the occurrence of the 

 belt of kame moraine at a distance of from half a mile to a mile 

 back of the head of the outwash plain. 



A second supposition makes the kame moraine built up under 

 the lip of the ice sheet in the manner in which debris was seen 



