■2 Aithur R. Dwerryliouse on 



the main direction of ice flow. These are extremely plentiful 

 over the district under consideration, and several, including the 

 dlogh More near Rostrevor, were shown upon the screen. 



Attention was next directed to the effect of ice- sheets upon 

 the ordinary drainage of the country, and it was shown that the 

 advance of a glacier might interrupt the normal flow of rivers 

 and cause them to form lakes, which, by their overflow, might 

 cut channels or gorges in the rocks. When the ice had retreated 

 and the lakes been drained, these gorges were left as permanent 

 records of the levels of the water, and consequently of the 

 former position of the ice-front. 



In certain cases the overflow stream remained in the 

 temporary channel after the retreat of the ice, thus producing a 

 permanent diversion of the drainage. Examples of this were 

 cited in the case of the Bush River and the streams in Glen Aan 

 and Glen Dun, all in County Antrim. 



The lecturer then described his investigations of the glaciation 

 of the North of Ireland and first referred to the distribution of 

 erratics as being the most trustworthy indication of the direction 

 •of ice movement. In every assemblage of erratics there were 

 always some of which the origin was difficult or impossible to 

 determine, but there were usually others with such striking 

 characters that they could be identified readily. The most 

 useful of these in the district under consideration w^ere the 

 fine-grained granite of Ailsa Craig in the Firth of Clyde, and 

 the granites and pyroxenic rocks of an area extending from 

 Slieve Gallion in County Tyrone to the neighbourhood of Omagh, 

 while in the immediate neighbourhood of Belfast, the rhyolites 

 which occur at Tardree and at several other places on the 

 Antrim Plateau, were of considerable use. 



Some of the more important deposits of Glacial age and the 

 drainage channels and lake-terraces of the district were then 

 described and their bearing on the glaciation of the country was 

 explained. 



