1 92 1. HiNCH — Post-Glacial Climatic Optimum in Ireland. 



been named the Yoldia Sea. As regards climate all the 

 evidence points to arctic conditions, and the presence of much 

 floating ice. 



Towards the end of this Yoldia period a considerable 

 elevation of western Europe set in and portion of the Yoldia 

 Sea was converted into a great fresh-water lake, inhabited 

 by animals and plants requiring a much greater degree of 

 warmth than those which had existed in the preceding 

 Yoldia period, and among which is the small fresh- water 

 mollusc Ancylus fluviatilis, from vvhich the name Ancylus 

 Lake is derived. The elevatory movement which converted 

 the Yoldia Sea into Lake Ancylus is considered to have been 

 very extensive and to have effected the area of the British 

 Isles, and probably the coast-line of Europe existed some- 

 where near the present loo-fathom line. The climate of 

 the period is regarded as a dry, fairh^ warm, continental or 

 mainland one during which the Fir was the dominant forest 

 tree and during the later part of which the Oak also was 

 found. 



At the end of the Ancylus period a considerable depression 

 took place and when the Belt and the Sounds had been 

 submerged the fresh- water Anc3dus area was flooded by the 

 warm and salt water of the Atlantic Ocean. The climate 

 of this Littorina period, as it is called from its most charac- 

 teristic mollusc in the Baltic, is considered to have been a 

 warm damp insular one, with forests of Oak in which the 

 Beech was already making its appearance. 



The Climatic Optimum may be considered to have 

 occurred in this period, during the later stages of the Ancylus 

 Age and the earlier stages of the Littorina Age, and it is not 

 easy from the nature of the case to set definite limits to 

 either its commencement or its ending. The land was then 

 occupied to its fullest extent by the animal and plant life 

 now found fossil in the peat-mosses and in the lacustrine 

 deposits, while in the soa the marine fauna flourished in 

 immense numbers and exuberant size. We ma}^ consider 

 that the maximum was reached when the Hazel had reached 

 its most northerly fossil boundary, and was not making any 

 further progress as a fruit-producing tree. After a shorter or 



