176 ROCHESTER ACADEMY OF SCIENCE. 



of rapid sedimentation. The "open work" gravels, the coarse angu- 

 lar materials near the point of origin, point to rapid growth. Further 

 the nature of the deposits indicate a more rapid flow of water in 



some parts than in other parts of the stream's course. 



Theories of Origin. 



The manner of formation of eskers has long been a matter of 

 dispute. That streams associated with the melting glacial ice were 

 responsible for the origin of the typical esker is beyond question, but 

 just what that association was has been the "bone of contention." 

 There have been two opposing ideas advanced, one maintains that 

 eskers originated in streams beneath the ice, the subglacial view, 

 the other idea maintains that eskers were deposited in streams on 

 the surface of the ice, the superglacial view. Students of glacial 

 geology have been advocates of the one or the other idea. Recently 

 there has been another view advanced that has gained a few adher- 

 ents. This holds that eskers were made at the edge of the ice or in 

 reentrants back from the edge. Again some have entertained the 

 idea of esker formation in ice-walled, earth-bottomed canyons open 

 to the sky. Further, some eskers have undoubtedly been formed in 

 ways in which the active agency of water has been lacking. The 

 large question hinges upon what was the predominant method. These 

 various theories w'ill next be considered. 



Subglacial Hypothesis. Statement. This view, advanced at an 

 early date, and follow^ed to-day by the majority of glacialists, at- 

 tempts to explain the origin of eskers by the activity of subglacial 

 streams. These streams flowed in tunnels beneath the ice mass. 

 The water was under considerable pressure or "head," due to the 

 crowding of the ice, if it possessed movement, against the stream, 

 and due to the height of the tributary waters of the stream at their 

 point of origin on top of the rapidly melting ice miles back from the 

 place of deposition of the esker ridge. The water, derived by sur- 

 face ablation, flowed along the glacier surface until it plunged into a 

 crevasse or moulin where its course became englacial for a distance 

 or subglacial. These subglacial waters, closely pent, followed a 

 crevasse or a series of crevasses, or some other line of least resist- 

 ance. The course once established w^as maintained, even if the ice 



