lOO NEW YORK STATE MUSEUM 



Spurs, and the fault east of Freedom Plains, appear to show the 

 effects of glacial plucking. 



The Highland crests were buried by the glacier. Some places 

 along the northern slopes show polishing effects (see plate 6). 

 The excavation of the valleys between the northern spurs of the 

 Highlands was probably materially assisted by the ice. 



Deposits during the advance. Drumlins, or drumlinoid 

 masses of till, are rather numerous in the quadrangle and often are 

 conspicuous features of the topography. They seem to be deposits 

 of the advancing ice sheet which molded them by pressure into their 

 usual elongated domelike shapes. These masses greatly obscure 

 the structural relationships over much of the area. They are the 

 most conspicuous features of the ground moraine. The larger part 

 of the veneer of till, which is very plentiful, probably dates from 

 the advance of the glacier. About 200 feet of boulders and sand, 

 which rest on the bottom of the Hudson gorge, probably are a part 

 of the ground moraine. 



RETREAT OF THE ICE SHEET 

 It is generally held that accompanying and following the retreat 

 of the Wisconsin ice sheet from this region there was a slow sub- 

 sidence of the land. At this time a large body of water filled the 

 old valley of the Hudson within this area. It would appear that 

 the subsidence went on gradually and that during the earlier stages 

 much sand, gravel and sandy clay was deposited on the earlier 

 boulder material that covered the bed of the gorge to a depth of 

 200 feet, and then a thin layer of boulders representing a probable 

 flood of floating ice, and then typical river deposits.^ Finally, it 

 would appear that the subsidence may have brought in estuarine 

 conditions, at which time the Hudson river clays were laid down. 

 These considerations assume an open gorge and postulate the prob- 

 able deposition of the clays entirely across it, their present condition 

 having been brought about by later dissection. It is proper to state 

 that there are exceptions to this idea. Professor Woodworth, from 

 a study of the entire Hudson and Champlain valleys, holds the 

 opinion or belief that, during the deposition of these clays, the 

 Hudson gorge was filled with a long tongue of ice against which 

 were standing bodies of water at a higher level than water could 

 have assumed in the open gorge. He cites many observations to 



1 See J. F. Kemp. Buried Channels beneath the Hudson and its Tribu- 

 taries. • Amer. Jour. Sci. Ser. 4. 1908. 26 :322. 



