ESKERS IX THE VICINITY OF ROCHESTER, NEW YORK. 187 



Objections. Various objections have been urged against this 

 hypothesis of esker origin. They are indicated below : 



(1) The enormous thickness of the ice, measured in terms of 

 hundreds, even thousands, of feet would not permit of tunnels be- 

 neath itself (111). 



(2) Swift streams do not deposit under ordinary conditions, 

 much less deposit sand, gravel and boulders altogether (111). 



(3) Eskers and kames are composed of the same materials. 

 Kames are deposited at the edge of the ice where the issuing waters 

 have lost their "head." If this be true it is reasonable to suppose that 

 eskers should be composed of coarser materials than kames (111). 



(4) Eskers could hardly escape destruction by the ice con- 

 stantly moving over them, especially those 50 to 100 miles long ex- 

 tending far back from the ice border (111). 



(5) This theory does not adequately explain the discontinuity 

 of eskers. They cannot be due to irregular glacial erosion of an 

 originally continuous ridge for the ends of segments are deposi- 

 tional rather than erosional. Further, breaks are common where 

 the trend of the ridge was parallel to the direction of ice movement 

 and glacial erosion at a minimum therefore. Also the absence of 

 stratified materials in the intervals, and the abruptness with which 

 the ridges pick up on either side of a break seem to militate against 

 the idea that these discontinuities are due to lack of confinement of 

 the stream to a definite channel where these breaks occur. It is 

 urged that a stream so separated as not to be able to transport a load 

 could hardly build a ridge 100 feet high and of proportionate breadth 

 just beyond the point where the stream had had no definite channel to 

 which it had been confined, and further what was the source of sup- 

 ply of the materials if the stream had not been definitely organ- 

 ized as a transporting stream above this ridge. If the suggestion by 

 Stone (99) be valid the appearance of the esker ridge below the in- 

 terval must represent a sudden change in the character of the tunnel 

 (from srhall to large), and that large tunnels carrying water of low 

 velocity can exist beneath ice masses 100 miles back from their 

 terminals (111). 



(6) Eskers are not found between the ]\Ialaspina glacier and 

 the shore across which the ice has recentlv receded, although num- 



