I92I. 



HiNCH. — The Eskers of Ireland. 



141 



just before the advance of the Irish Sea glacier, irom the 

 north, and they are also of value in distinguishing the 

 glacial drifts in certain of the coastal areas from the 

 Neolithic Raised Beaches, otherwise they must be classed 

 among the erratics brought from the north by the Irish 

 Sea Glacier, and are in no way different from the Ailsa 

 Craig riebeckite, the Antrim flints and chalk, and the 

 Lias fossils found along with them. The are in no sense 

 palaeontological evidence for the proposed submergence, 

 and, in addition, it has to be stated that many of the records 

 of mollusca alleged to have been found in the glacial deposits 

 westward of the line drawn from Clogher Head to Castle- 

 comer must be regarded with a very active scepticism. 

 Every case cited "by Prof. Gregory has been gone into care- 

 fully and each case has been found to belong either to the 

 area traversed by the Irish Sea glacier, or to rest on evidence 

 of a very vague kind. 



A postglacial submergence of the central plain having 

 been excluded as being outside the limit of probability, 

 can any alternative proposal for the necessary stretch or 

 stretches of water be substituted ? The facts which must 

 be remembered in any discussion of the origin of the Eskers 

 are : — that they are the final product of the decaying ice- 

 sheet, that in this stage of decay the original direction of 

 the ice-movement would have ceased to count, that the 

 average height of the Eskers above the surface of the land 

 is only from 40 to 60 feet, and that their structure is such 

 that they may have been deposited in a comparatively 

 short space of time. The causes required to produce them 

 need, therefore, not be of any considerable geological 

 importance. At the stage of the Esker period all the ground 

 in Ireland over 350 feet would be ice-free and the extent 

 of this ice-free ground would continue to extend as the ice 

 sank lower and lower in the plain. The ice-sheet would 

 then be in the final stages of dissolution and in thickness 

 probably under 60 feet, except in Gal way Bay, South 

 Dublin and a few other lowland localities. In these dis- 

 tricts the motionless ice would have melted much more 

 slowly than the ice in contact with the warmer ice-free 

 inlands and at the time of the formation of the Eskers 



