296 Proceedings of the Royal Irish Academy. 



J. E. "Ki I roe's view of tbe glaciation of the area, which postulated, as has been 

 already observed, the retreat of the ice v/p the valleys and towards the main 

 watershed of the Sperriu Mountains. Had such been the mode of recession, 

 there can be little doubt but that vast quantities of sand and gravel would 

 have beeu swept out of the ice and deposited as great outwash fans in the 

 valleys and along the estuary; the huge quantities of sand and gravel 

 deposited in "Lake Claiidy " are sufficient proof of this. In this paper it 

 has, however, been proved that the ice shrank off this mountain group and 

 retreated doivn the valleys : the marginal phenomena observed in this region 

 and sketched in an earlier section can, it is submitted, have no other 

 interpretation. 



The whole appearance of this great sheet is indeed that of a deposit laid 

 down in the tranquil waters of a lake : the terrace is still remarkably fresh 

 in outhne and has not assumed the moimdy form associated with topo- 

 graphical maturity. It is suggested that the materials were accumulated in 

 late-glacial times by the rivers draining from the Speriin Mountains to the 

 south, and charged with a superabundance of sand and gravel, obtained from 

 the rapid demolition of the terraces formed at higher levels in the temporary 

 ice-dammed lakes. Indubitably some of the material, though what proportion 

 it is impossible to estimate, was deposited . on the ice-retreat by the rivers 

 issuing from the lowest of these lakes. From the great development of 

 the higher terraces it is quite clear that vast quantities of sand and gravel 

 were thrown down in these settling tanks, and that the greater bulk of the 

 material brought down by the streams — those pouring out of still higher 

 lakes, or flowing normally down the hill-sides, or again issuing from the ice 

 itself — was, during the retreat of the glaciers, effectually trapped, and unable 

 to escape to lower levels until the noi-mal drainage had been reopened. It 

 would seem, therefore, that erosion of the valleys, carrying the surplus waters 

 of the lowest lakes and the streams pouring off the ice standing in the Foyle 

 estuary, were the sole contributors to any accumulations formed prior to the 

 complete withdrawal of the ice from these hills. 



A shelf-like feature corresponding with this great accumulation extends 

 discontinuously along the Inishowen coast of the Foyle estuary, e.g., about 

 Ture and at Greeiibank. The best of these deposits is the terrace of gravel, 

 which stretches along the coast for about three miles S.-W. of Mo^^lle, the 

 materials of which were derived from the drift which masked the hUl-flanks 

 behind. A few of the streams, moreover, flowing down the Inishowen slopes of 

 Lough Foyle, have built up deltas at the outfall of their valleys, e.g., the well- 

 marked terrace composed of well-stratified sand and gravel observed at Muff, 

 a few miles N. of Londonderry. This fan was, without doubt) laid down at the 



