April, 1915. T/ie Irish Naturalist. 65 : 



I 

 \ 



THE POST-GLACIAL LEVELS OF LOUGH NEAGH. 1 



BY W. B. WRIGHT, B.A., F.G.S. 



i 



The very suggestive note by Mr. Stelfox in the January ! 



issue of the Irish Naturalist (pp. 8-9, supra), should excite \ 



considerable interest in the post -Glacial levels of our larger 1 

 lakes. A study of the 25-foot or Early Neolithic beach 



around the coast of Ireland leaves no doubt that a great \ 



part of the country has been subjected to Post-glacial \ 



tilting or warping. This beach, which is only a few feet \ 



above present high-water mark in Donegal Bay and j 



southern Wicklow, rises steadily as it is traced round the j 



coasts in the direction of County Antrim, where it attains j 



an altitude of 18 or 20 feet above high -water mark. The gra- j 



dient is in reality very slight, the rise northward being on the \ 



average about an inch to the mile. It appears to be some- | 



what steeper in the north, and in the neighbourhood of \ 



Lough Neagh it might be taken at about two inches to the ; 



mile, this being a liberal estimate. On this basis the eleva- ! 



tion of the outlet would only be about two feet greater than \ 



that of the south end of the lake, so that one would only ' 



expect this small amount of post -Neolithic submergence. ■ 



This is quite a measurable quantity, and could no doubt , 



be detected by careful observation, but it is clearly no ex- j 



planation of the 30 -foot change of level suggested by Canon ; 



Lett. ! 



If, however, there is any truth in Jamieson's theory of ' 

 isotatic recovery of the earth's crust from the depression 



caused by the ice-load, the warping must have been in : 



progress ever since the disappearance of the ice, and not \ 



merely since the Neolithic period. One would thus expect \ 



the total post -Glacial warping to be much greater than the \ 



post -Neolithic warping. Now, no Glacial or late Glacial \ 



shoreline has been detected in Ireland which would afford \ 

 a means of measuring the total post -Glacial warping of the 



country. There must of course have been a late Glacial : 



shore-line like the well-known ones in Scotland and Scan- , 



dinavia with their characteristic Arctic faunas and abundant 1 



