ESKERS IN THE VICINITY OF ROCHESTER, NEW YORK. 183 



they had been transported but a short distance, their rotundity in 

 the lower part of esker courses, the rounding of large boulders, 2 to 

 4 feet in diameter, indicating rapid, violent flow under great "head" 

 of a stream acting upon subangular glacial materials. The presence 

 of large boulders in the gravels may be explained as being derived 

 from the basal ice, in which they were occluded, either by lateral 

 melting revealing the boulders, or by being crowded in by ice push 

 or by overriding ice, possibly some fell from the roof as the level of 

 the stream rose in the growing subglacial channel. Large boulders 

 on esker surfaces may be accounted for in these several ways. The 

 presence of till on the surface and distributed through the esker 

 mass may likewise be explained. 



The absence of till from the surface of many eskers may be 

 accounted for by subsequent erosion, especially during the time just 

 following ice retreat when the surfaces of the easily eroded mate- 

 rials were unprotected by vegetation or other covering, or by the 

 subglacial tunnel becoming roofless by surface ablation before the 

 esker-forming stream was diverted, or by the esker becoming so 

 high that it rose above or nearly above the upper limit of the zone 

 of till in the basal part of the ice. Subsequent sliding may have 

 concealed till present upon the fresh esker surface. Also the surface 

 of the ice may have been kept free of debris by its washing toward 

 the ice edge or into mouHns continuously till the ice surface was 

 lowered to the top of the esker leaving little to be deposited on it 

 from the ice surface. Absence of outwash on surface may be ac- 

 counted for in similar ways : Removal by subsequent erosion, never 

 deposited if subglacial stream flowed along crest of esker deposit till 

 the channel was open to the sky, or if the stream was diverted after 

 the esker formation, outwash would not have been likely to have been 

 carried out onto the esker surface, at the edge of the ice. 



The knolls characteristic of the crests of eskers may be ex- 

 plained by local enlargements in the roof of the tunnel in which 

 deposition took place, or by superglacial material falling through 

 holes in the ice onto the partially uncovered ridge, or by being added 

 by superglacial or englacial streams cascading downward from the 

 surface. Also they may be explained by irregularities in the surface 

 on which the esker rests, or by irregular sliding of the esker material 



