260 Proceedings of the Royal Irish Academy. 



representatives of this phase. The large valley, which enters the Mourue lUver 

 near Douglas Bridge and intakes at about 270 or 280 feet, O.D., discharged 

 into the Mourne and Foyle di'ainage the overilow waters of the now greatly 

 shrunken remnant of " Lake Gortin-Grenelly " ; on the abandonment of this 

 valley the lake ceased to exist. The morainic country stretching from south 

 of Douglas Bridge on the north to the right bank of the Mourne Eiver, just 

 north of Ifewtownstewart in the south, marks the position of the ice-edge 

 at this period. 



The retreat of the ice from the Sperriu Mountains has now been briefly 

 outlined. The mode of recession shows most unmistakably the progressive 

 melting of the ice from the main watershed aud the gradual increase of the 

 ice-free surface as the glaciers receded down the mountain flanks. Through- 

 out almost the whole period of the retreat, the drainage, held up in a series 

 of large and small extra-glacial lakes, was discharged northwards, proving 

 incontestably the pressure of great ice masses from the south and west. 



2. — Tlie Tyrone Glacier. 



Even after the ice had partially imcovered the Sperrin Mountains, great 

 glaciers still continued to pour along the " Omagh-Draperstown Corridor" to 

 Draperstown and beyond, and over the country east of this, sloping towards 

 the Lough Xeagh basin. At the maximum phase of the glaciation, the highest 

 summits of the pyroxenic and metamorphic range of Beleevnamore, extending 

 S.-W. from Slieve Gallion, were buried beneath the ice-sheet. As the thick- 

 ness of the ice diminished, these summits began to project above the ice-surface, 

 until finally the ice masses east and west of the range were connected only 

 by tongues thnist through its larger gaps. 



Through these, the ice proceeding eastwards along the great corridor to 

 the north squeezed large glaciers, which formed in retreat a series of creseentic 

 morainic ridges. At one stage they were united to the east of the range, but 

 split up into individual separate glacier tongues with the recession of the ice. 

 The arrangement of these ridges, their creseentic form, the convexity directed 

 to the east and south-east, indubitably prove the retreat towards the west or 

 north-west, i.e. towards the " Omagh-Draperstown corridor."' 



The finest of these gaps and associated moraines is probably the northern- 

 most, situated between the western flanks of Slieve Gallion and Fir Mountain. 

 Here the curving of the lobar recessional moraines across the valley is 

 extremely well shown, the drainage from the melting ice-front being carried 

 off by the Lissan water towards Draperstown. The waters pouring laterally 

 from the ice, when its edge stood on the line of the magnificent moraine just 

 south of Lough Fea, excavated a valley some 50-60 feet deep, now quite 



