246 Proceedings of tlie Royal Irish Academy. 



depiessiou to the \vest«iu shoulder oi Graiguamadtly. This lake was drained 

 by the gigautic tiench of " Barnes Gap " (B.G). It intakes at about 760 

 feet, O.D., and falls to 600 feet, O.D., which is the level of the Invev channel. 

 At its enti"auee into " Lake Glenelly " a fine flat-topped terrace was built up. 



The southern ice-support of "Lake Goitin," which extended up the 

 valleys of the Owenreagh and Owenkillew rivei-s, swung from the north-east 

 shoulder of Mvdlaghearn, across the Owenieagh valley,' on to the southern 

 slopes of Crocknamoghil, over the eastern end of the hill of Croekanboy, 

 across the Owenkillew valley and on to the slopes of Crockyneill and 

 Mullaghturk. 



The greater part of Mullaghcarn would at this stage project above the 

 ice which fitted closely on its southern and eastern flanks, as no trace of 

 marginal drainage was observed at the height of 900 or 1,000 feet, which in 

 this area must have been approximately the altitude of the line of contact of 

 ice and hilL 



Hence, " Lake Gortin " was largely held up by the ice-face on west and 

 south-east. Its greatest depth exceeded 500 feet, while its water-surface was 

 approximately 20 square miles. 



The water-worn sands and gravels exposed in the sides of the Glenawtsk 

 Bum and in the bottom of the Owenreagh Kiver are its lake deposits. 

 Similar lacustrine sands and gravels occur plentifully in the valley of the 

 Owenkillew, and are extremely well exposed in the sides of the Glenlark 

 vaUey up to, roughly, 750 feet, O.D.- 



There can be veiy little doubt that, as a comparison of the positions of the 

 ice-fronts at the second and third stages represented in the map (PI. YIII ; wdl 

 readily show, the retreat of the ice from rhe earlier to the later position was 

 effected by the splitting of the ice-sheet into two portions, the line of division 

 occuning between Barnes Gap and Muliaghcam. The lake occupying the area 

 between the ice-fronts gradually increased in extent, as the ice withdrew 

 westwards to Gortin and beyond, and southwards to the " Omagh-Draperstown 

 corridor." This splitting of the ice-sheet is manifestly to be ascribed to the 

 shelter afforded by the increasing size of the Mullaghcarn nunatak. 



A slight recession of the ice westward, along Glenelly, beyond the second 

 stage represented in the map (PI. VIII), uncovered Eden HiU !977jand the col 

 at the head of the large tributary of the Edeu Ptiver. Iliis threw the 



> The large ridges running towards the river from the valley sides, just south of Rusky, 

 probably mark its position at this stage. 



- The mounds in the higher part of this glen, and on its southern side (indicated by 

 the three outliers of the ], 000-foot contour of the one-inch O.S. map), represent the 

 deposits of an earlier moraine. 



