The Irish Naturalist. 



December 



would probably stand so high over the level of the land as 

 to block the drainage outlets of the central plain sufficiently 

 to allow of the formation of lakes at suitable levels. The 

 formation of this central plain lake is quite feasible and 

 probabl}^ played a certain small part, but we are without 

 knowledge of the lake terraces which should be formed 

 vmder these conditions, and this proposal of a great lake 

 must be regarded as a rather extravagant geological demand 

 in order to account for the formation of such relatively 

 small surface features as the Eskers. The remnant of the 

 ice sheet lying in the central plain just before and during 

 the formation of the Eskers must have been in a state 

 of extreme decay, and it is probable that the stagnant ice 

 . would be traversed in many directions by tunnels and 

 channels which might become filled with sand and gravel 

 carried by the subglacial streams flowing through it. It is 

 also possible that some of these many channels and tunnels, 

 if empty at the time, might be filled from above when the 

 upper level of the ice had sunk so near the level of the land 

 as to break through the roof of the tunnel. In certain 

 cases, and especially where the slope of the land surface 

 was downhill it is probable that the edge of the local ice 

 sheet ended in a stretch of locally ponded water giving rise 

 to the type of Esker which requires this method of for- 

 mation. 



It is probable that no general method of formation was 

 in operation during the whole period, that purely local 

 and transitory factors produced many of the Eskers, and 

 that it would be wise to remember that Eskers have been 

 formed in other lands under conditions very different from 

 those obtaining in central Ireland. A great literature on 

 the subject has arisen in Sweden, Finland, and North 

 America, and the student will find in Giles' recent paper,' 

 to which my attention was called by Prof. Cole, F.R.S., 

 Director of the Geological Survey of Ireland, an exhaustive 

 series of arguments and an extensive bibliography on this 

 f very fascinating geological problem. 



^ Giles, A. W. " Eskers in the vicinity of Rochester, New York." 

 Proc, Rochester Acad. Sci., vol. v., pp. 161—240, 1918. 



Geological Survey Office, Dublim. 



