Chaulesworth — Glacial Geology of North- West of Ireland. 303 



the eoiuitry to the south of Omagh, produciug a lake of uncertain extent, 

 which extended over a considerable area south of Oniagh and drained by the 

 valley east of Bessy Bell. Most probably two not inconsiderable valleys 

 existed pre-glacially on this line, the one falling southward, the other north- 

 ward, the latter tributary north-west of Newtownstewart to the main stream 

 flowing along the Baronscourt valley. The contour of the country is 

 suggestive of such a stream arrangement. 



The glacial flood waters lowered the watershed and deepened the valley 

 to such an extent that by the time the ice had shrunk sufficiently westward 

 to uncover the mouth of the Baronscourt valley, the floor of the Strule valley 

 was below tlie critical level of the intake of its western neighbour. Hence 

 the westward withdrawal of the ice could not aff'ect the new direction of 

 drainage, and a resumption of the earlier line of flow was in consequence 

 rendered impossible. 



The Fairy "Water winds its way over an extensive boggy and alluvial flat, 

 varied only by a number of irregularly-shaped morainie mounds. It would 

 seem that pre-glacially the drainage of the area south of Omagh found its way 

 westward along this flat plain to the Baronscourt valley, and that over this 

 stretch of the Fairy Water a reversal of drainage direction has taken place ; 

 the present Fairy "Water enters the Strule Eiver with its entrant angle acute 

 up stream, tliough this anomalous confluence is largely the I'esult. as already 

 pointed out, of the obstruction offered by the Mountjoy moraine. 



The great morainie accumulations and associated fluvio-glaeial deposits 

 at the head of Barnesmore Gap have altered the course of the Barnes liiver. 

 This stream would seem to have originally passed into the broad and open 

 valley now occupied by the Monrne Beg Eiver ; this old course being blocked 

 by the moraines, the river has been diverted into Barnesmore Gap. 



The Culmore stretch of the Eiver Foyle below Londonderry is cut in rock. 

 It was possibly initiated and to some unknown extent carved out by snb- 

 glacial streams, and later by waters streaming fiom the ice-front. A buried 

 valley of uncertain depth wordd seem to run beneath Lough Eanagh to the 

 east of this, and may be the pre-glacial course of the Eiver Foyle. 



Post-glacial denudation has on the whole effected so little that almost all 

 the details of the present surface are due to ice action, either erosive or 

 accumulative. The remarkable freshness of the striated and polished surfaces, 

 the preservation in their original form of the coastal gravel-spreads bordering 

 Lough Foyle, and of the drumlins of Donegal and the moraines of Fermanagh 

 and Tyrone are, in view of the incoherence of the deposits, eloc[uent of the 

 recency of their formation. Changes have been chiefly confined to the 



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