508 Prof. Cole dt' T. Hallissy—The Wexford Gravels. 



IV. An Ihteeglacial Epoch in Ireland. 



It would not have been surprising if the outlying region of the 

 British Isles, exposed to considerable precipitation, had revealed no 

 evidence of an interglacial epoch. While considerable shrinkage was 

 going on in the Scandinavian ice-naass, the central parts of Ireland may 

 have remained covered by a stagnating glacier, which melted so slowly 

 that it was still in existence when glacial conditions again set in. 

 We cannot, however, overlook the evidence of extensive melting, 

 of the flow of broad bodies of water, and of the washing and 

 denudation of the boulder-clay deposited by the ice which descended 

 from Scotland along the Irish Sea. The deposition of that boulder- 

 clay implies the reduction of the great glacier by melting from its 

 surfaces. The waters from the residual masses swept across the 

 moraine matter, which till then had been englacial, and gave rise to 

 the Wexford gravels and their associates along the eastern coast. 



The later boulder-clay represents the spread of Irish glaciers during 

 an epoch which failed to reproduce the conditions of the previous 

 ice extension. The large blocks of Leinster granite, which are so 

 well seen in the new cuttings for the Dublin and South-Eastern 

 Railway near Shankill, and which are so frequently observed in other 

 places on both sides of the Leinster Chain, are among the most 

 distinctive features of the second morainic sheet. 



Harkness ^ observes that the Wexford gravels represent an epoch of 

 less rigorous climate in Middle Pleistocene times. While Hull calls 

 the Dublin gravels interglacial, and Praeger & Sollas ^ observe that 

 they mark " a great change of conditions", Lamplugh states that they 

 were formed during a recession of the ice.^ The difference between 

 these authors is evidently one of degree. The same relative positions 

 are taken by those who advocate and those who deny that glaciers 

 are great agents of erosion. No one really overlooks their erosive 

 action, and the question becomes one of how many metres may be 

 removed from a valley-floor during an ice age of unknown duration. 

 The evidence in Ireland for the disappearance of the Scottish ice, 

 whether from Antrim and Londonderry or Dublin and Wexford, is 

 clear enough. The question is, could the local ice at the same time 

 have undergone a great extension, or was there an interval which 

 may be fairly styled interglacial?* If cold is the main factor in the 

 production of a glacial epoch, warmth must be the main factor that 

 brings it to an end. Changes in the amount of precipitation cannot 

 be involved in this case. If glacial cold prevailed during the 

 formation of the Wexford and Dublin gravels, the water necessary 

 for their production could not have existed in the liquid state. 



English geologists are meeting with the same question in the survey 

 of their eastern drifts. While G. W. Lamplugh consistently maintains 



1 " On the Middle Pleistocene Deposits " : Geol. Mag., 1869, p. 549. 



^ "Notes on Glacial Deposits in Ireland. I. The Bray Eiver " : Irish 

 Naturalist, 1894, p. 198. 



'^ Mem. to Sheet 112, 1903, p. 45. 



* Cf. G. A. J. Cole, "Glacial Features in Spitsbergen in relation to Irish 

 Geology " : Proc. Eoy. Irish Acad., vol. xxix, sect. B, p. 207, 1911. 



