144 THE ICE AGE IN NORTH AMERICA. 



object-lesson as to the formation of kettle-holes. All the 

 elements for their production are there, in and beneath the 

 accumulated debris. As the heat slowly penetrates the pro- 

 tective covering, especially upon the sides, where it is the 

 thinnest, it melts the ice and thus undermines the earthy 

 material, which, in due time, slides down to the base, and 

 thus gradually leaves a cone of ice in the middle, surrounded 

 at the edges by a continuous ridge of dirt. Eventually the 

 ice all melts away, and a miniature kettle-hole is formed. So 

 far as I know, the application of this principle to the expla- 

 nation of the extended phenomena under consideration was 

 first made by the late Colonel Whittlesey,* of Ohio, in study- 

 ing the Kettle range of Wisconsin. How completely this 

 theory was confirmed by my study of the Muir Glacier, in 

 the summer of 1886, has already been related.f 



To prevent misapprehension, it should here be remarked 

 that we have not intended to affirm that the whole bulk of 

 Martha's Yineyard and Long Island consists of glacial depos- 

 its. The nuclei of those islands certainly existed before the 

 Glacial period, for at Gay Head, on Martha's Vineyard, and 

 in the vicinity of Port Jefferson, on Long Island, and at 

 some other places, there are extensive beds of Tertiary clay 

 underlying the glacial deposits, and rising above the water- 

 level. The glacial deposits simply form a capping of more 

 or less thickness to these older ones. It is still true, however, 

 that the glacial deposits have determined, in the main, the 

 present topographical features. 



In following the moraine across New Jersey, we find that 

 beginning at the sea-level, its base rises to a height of more 

 than a thousand feet, where it crosses the Blue Hills of 

 western New Jersey; and everywhere its surface is character- 

 ized by the knobs and kettle-holes, whose manner of for- 

 mation has just been explained. 



Crossing the Delaware River, these characteristic phe- 



* Report upon "The Drift Formations of the Northwest," in 

 "Smithsonian Contributions to Knowledge" 1866 

 f See above, p. 57 et seq. 



