ESKERS IN THE VICINITY OF ROCHESTER, NEW YORK. 195 



(10) Numerous large boulders on esker surfaces and in the 

 upper part of esker ridges are difficult to explain under this hypo- 

 thesis (125). 



(11) Eskers are often observed to lie in shallow troughs 

 excavated in the till apparently by the same streams in which the 

 eskers were formed (61, 62). 



(12) Chadwick has noted several eskers in the vicinity of 

 Ogdensburg, N. Y. If these eskers are superglacial in origin it is 

 difficult to see how they were preserved with the vigorous waves and 

 currents of the glacial lake Iroquois laving the ice edge, and the 

 waters advancing into the superglacial channels with the recession 

 of the ice front. In his study of the glacial features in the Thousand 

 Islands district Professor Fairchild has reinforced this argument 

 (34A;34B. p. 149). 



(13) Eskers exhibit a tendency to pass through gaps in cross- 

 ing divides, a feature not easily accounted for under this hypothesis, 

 since they should show little relation to these smaller features of the 

 underlying topography. 



(14) If eskers are superglacial in origin they are not likely 

 to exhibit accordant relations with delta and outwash surfaces, as 

 they usually do. 



(15) No large lakes are known to form on ice sheets, com- 

 parable to such lakes as those in which esker "plains" are supposed 

 to have been formed. 



(16) This theory of esker origin is inconsistent with the 

 "open work" structure observed so commonly in esker gravels (27), 

 for where the stream became graded and the current sluggish the 

 spaces within the gravels would have been filled by the slowly 

 percolating waters. 



(17) If eskers had been let down from a superglacial position 

 to their present attitude, the bedding would have been much more 

 greatly disturbed than the present sections indicate (27). 



(18) Also if eskers had been let down from a superglacial 

 position the material should slip outward more or less, the down- 

 throw should always be toward the margins. Eskers exhibit dis- 

 placements with downthrow toward the crest line, as well as disloca- 

 tions with downthrow in the opposite direction. 



