ESKERS IN THE VICINITY OF ROCHESTER, NEW YORK. 205 



Russell also observes that the three principal streams along the 

 eastern margin of the Alalaspina glacier in 1891 issued from beneath 

 the ice as subglacial streams. Each flows for some distance between 

 walls of ice and is actively aggrading its channel. One, the Osar, has 

 a ridge of gravel running parallel with it which was deposited on the 

 ice during a former stage when the water flowed about 100 feet 

 higher. No other instance of esker formation was directly observed 

 (80). 



"The formation of osars seems fully explained by the subglacial 

 drainage of the IVIalaspina ice sheet." Streams flowing into the tun- 

 nels on the north side of the glacier are carrying already large quan- 

 tities of sand, gravel and mud, and on emerging from the eastern 

 and southern sides bring out large cjuantities of water-worn mate- 

 rial. A part of the overload is dropped here. These cones obstruct 

 the mouth of the tunnels and thus slackening the flow of water within 

 the tunnel may lead to deposition. The water is conseciuently forced 

 to a higher level in the tunnel, eroding the ice roof as it slowly rises 

 and leading to further deposition on the gravel deposit beneath. 

 When the ice melts, the supporting walls being removed, the gravel 

 will slide down to a position of stability giving the arched, anticlinal 

 structure of eskers. The process would go on in a stagnant ice mass 

 till the waters found new channels (80). 



Russell notes that in the case of the Muir glacier where debris is 

 abundant and of large size, the channel frequently becomes clogged, 

 and the subglacial waters are forced to find a new outlet. Some sub- 

 glacial streams have formed re-entrants, others not, the condition 

 of formation seeming to depend on volume and swiftness of stream 

 and on amount and size of debris on the ice (79). 



General Conclusion Regarding Origin. 

 The testimony of existing glaciers, while probably insufficient 

 to warrant a firm and definite conclusion, yet unquestionably points 

 to the subglacial origin for the typical esker and for the vast major- 

 ity of eskers. A critical study of eskers themselves results in this 

 same conclusion. Objections may be urged against this theory, but 

 they are fewer in number, less vital, and more satisfactorily an- 

 swered than is the case with any other theory so far formulated. It 



