Chaklesworth — Glacial Geology of North-West of Ireland. 299 



the Donegal Ice sldvted the hills rising from the eastern bank of the Elver 

 Foyle. 



The retreat of the Donegal Ice from the mouth of the Foyle estuary to the 

 position south-east of Londonderry, as represented on the map above (fig. 4), 

 is a measure of the interval separating this recession from the re-advance 

 of the Scottish Ice. 



The Scottish Ice was now less powerful than in the earlier phases of the 

 giaciation, and was no longer able to surmount hills of any considerable 

 altitude, but merely to skirt the coast, to spread over any comparatively 

 low ground, and to protrude into the mouths of the larger inlets. It is 

 significant that the area of re-advance lies in the direction of the maximum 

 thrust which was exerted by the Scottish ice during its earlier invasion. 



IX. — Glacial Stream Diversions. 



Upon' the disappearance of the ice from this region, the streams resumed 

 their flow, cutting rapidly into the superficial smear of incoherent glacial 

 deposits, in some places succeeding in reaching the pre-glacial rock floor, in 

 the majority of cases, however, still flowing well above this. Their courses, 

 in position and trend, are in general coincident with those of their pre-glacial 

 predecessors. Some streams, meandering over the glacial accumulations 

 which, after the withdrawal of the ice, thickly covered the floor and sides of 

 the valleys, even to their complete obliteration, have failed to discover the 

 pre-glacial bed, and have cut into the rock, while others have been diverted by 

 morainic barriers. A few of the more prominent instances of such stream 

 diversions may be briefly noticed. 



Above and below the Carrick Rocks the valley of the Roe is broad and 

 open. At the Rocks the river passes through a very deep gorge, hemmed in 

 between sheer walls of gnarled and contorted schist. The old valley site is 

 just to the east, as suggested by the drift-hollows running up the sides of the 

 valleys at the abrupt bends in the stream-course. 



As noted by Portlock,^ traces of terraces are preserved above this gorge ! 

 these are possibly to be attributed to the slow down-cutting of the River Roe 

 -through the schist barrier. The River Roe has deviated from the line of its 

 pre-glacial valley at two other places below this point, namely, at the Dog's 

 Leap, and at O'Oahan's Rock. 



The " Ness Waterfall,"- in County Londonderry, is due to the wandering 



1 Those terraces mentioned by Portlock (Report, p. 633), as occurring below the 

 " Ness Waterfall," are lake deposits of " Lake Claudy," their material derived in large 

 measure from the receding ice. 



- A description of this waterfall is given in the Londonderry Memoir, p. 82, 



