Charlesworth— Glacial Geology of North- West of Ireland. 193 



AltahuUion, across the Owenbeg and Oweureagh Kivers, and along the 

 southern slopes of the Benady Glen. Favtlier east it was fonnd impossible 

 to trace the boundary, as the drift becomes a purely local stony accumulation 

 — eg., at Eden ];odge and in Glenshane Pass — formed almost entirely of 

 basalt debris and without doubt the product of both the Scottish and Donegal 

 ice-sheets in this upland region. 



In the coimtry lying generally to the south of the Sperrin Mountains, the 

 boundaryagain becomes fairly distinct. It runs southfrom east of Draperstowu, 

 along the eastern slopes of the Beleevnamore range, and to the south-east of 

 Pomeroy, continuing along the southern slopes of the O.E.S. ridge and across 

 the Clogher valley on to the eastern flanks of Slieve Beagh, and to the 

 neighbourliood of the town of Monaghan. 



The western limit of this glaciation is approximately laid down on t;he 

 map (PI. IX). 



It is significant that, especially in the southern area, the eastern erratics 

 are frequently to be found in moraine accumulations and outwash sands and 

 gravels formed during the retreat of the Donegal ice. This association 

 indicates the redistribution of these erratics at a later period by the Donegal 

 glaciers, though to what extent this has taken place it is impossible to 

 determine. As these glaciers were, in general, moving in a direction contrary 

 to that taken by the Scottish ice, the true limit of the eastern drift, when 

 first deposited, and therefore of the Scottish ice-sheet, must have lain to some 

 extent outside the limit of the present distribution of the erratics as just 

 delineated. 



probably derived from local sources, just S.W. of their outcrop, than froQi more distant 

 sources, such as the metamorphic complex of N.E. Antrim, as suggested by this writer. 

 This supposition of a local origin is strongly borne out by the huge " Meskan erratic " 

 (10 X 6 X 6 feet) occurring ca. 1 mile W. of Claudy, and petrographically described in the 

 Memoir. Here this large erratic is said petrographically to be " quite unlike any rock 

 hitherto found in Ireland." It can, however-, have had only a local origin. That the 

 outcrop of this rook, as of many others found as erratics in the drifts, has not been dis- 

 covered is not surprising in view of the great thickness and extent of the bouldor-clay 

 and the terracic deposits. 



The absence of chalk or chalk-flints from the glacial deposits south of this line, none 

 being recorded by Professor Seymour or having been found by myself, would be difficult 

 of explanation had this area been earlier overridden by ice from the east. (The "large 

 flake of white flint " found by Professor Seymour " on the hill-slopes " of Curryfree Hill 

 (Londonderry Memoir, p. 75) can scarcely be regarded as suflicieut proof of this earlier 

 glaciation.) Should fliuts, chalk, or other undoubted eastern erratics be later discovered 

 in the actual glacial deposits in the area south of the approximate line represented on the 

 map (fig. 1), a southward shifting of this margin in conformity with the find or finds 

 would have to be made. Any such slight adjustment of the boundary would, however, 

 not make any fundamental change in the scheme of glaciation as elaborated in this 

 paper. 



