ICE-SHEET EROSION IN NEW YORK 67 



argument from topography could safely apply only to the latest inva- 

 sion, and if the invasions were simply cumulative in their effects, then 

 all the facts and reasoning as given on pages 57-64 would apply in the 

 same way as to only one invasion. It is safe to discuss the history of 

 the region as involving only the Wisconsin glacial epoch, for no evidences 

 of any earlier and more forceful or extended sheet have been found. If 

 the Wisconsin sheet failed to seriously erode it is unreasonable to appeal 

 to earlier invasions, that were certainly smaller and weaker. 



Effects of ice advance. — When the oncoming ice sheet transgressed 

 our district, it might have had the same directions of flow and the same 

 marginal form as during its retreat, w r hich supposition is the most favor- 

 able to erosion. The transportational and erosional effects were doubt- 

 less the same in principle and general character, but there was certainly 

 a difference in effect or degree. The ice found all the land surface, valley 

 and hill, mantled in a thick sheet of residuum, which had to be cleared 

 away before bed-rock could be affected. In addition to the local geest, 

 the ice was already burdened with its enormous load of subglacial drift 

 that it had gathered on its way ; for it is not thought that any local 

 glaciers could have affected this locality in advance of the continental 

 sheet. With all its bottom load of debris, producing stagnation in the 

 lower layers and acting as a buffer for the underlying rock, erosion must 

 have been practically if not entirely nil. The advancing valley lobes 

 were doubtless faced by lakes, as during their retreat. The upgrade 

 which the ice had to climb is unknown, but it was certainly an uphill 

 advance. The dropping of its basal drift at any line, as the drumlin- 

 belt moraine, for example, would give no increased erosional power to the 

 onmoving ice. 



All considerations lead to the confident conclusion that the ice sheet 

 during its advance did no erosional work. Its effect was to transport 

 and distribute the debris which it had been forced to carry. 



Conditions during maximum extent of the ice sheet. — We may sup- 

 pose that as the valleys become filled with ice during the slow advance 

 of the great ice body, and the latter rolled its front on to the higher 

 ground southward, the ice was less and less diverted by the topographic 

 features until finally the general movement, at least of the upper layers, 

 was toward the southwest, and obliquely across the valleys of the Finger 

 lakes, specially those east of Keuka. This conclusion regarding the 

 direction of flow is derived from the trend of the great terminal and the 

 recessional moraines, which were normal to the ice movement. 



All our knowledge of the behavior of glaciers leads to the conclusion 

 that the deeply buried ice in the valleys of the north-facing slope must 



