F. R. Cowper Reed — Corries of ComeragJi Mountains. 233 



torrential rivers arising from the rapid melting of the ice and 

 ■snow, but the glaciers in the upper parts of the side valleys would 

 Btill persist and protect their beds from erosion.^ The result in 

 ■either case would be the same ; the heads of the lateral valleys 

 would be left hung up above the main thalweg when the period 

 of excessive erosion was ended. Such may have been the first stage 

 in the evolution of the Comeragh corries. Any new descent of the 

 snow-line on the return of colder conditions would cause the 

 lateral glaciers to advance and lead to the accumulation of mounds 

 or dams of glacial debris in or beyond the mouths of the lateral 

 ' hanging valleys.' 



It is not without significance that the vertical distribution of hanging 

 valleys along the sides of a main valley has in certain instances been 

 found to bear a definite relation to the thalweg. Similarly, in many 

 districts corries have been observed to possess a certain uniformity 

 of level; but this is held by some geologists to indicate a former 

 snow-line. Successive tiers of corries in bolts (which are not 

 unknown in some regions) are accordingly regarded as marking 

 successive stages in the retreat of the snow-line.^ 



In the Comeraghs there is only one belt or zone of corries recog- 

 nisable, and this lies between the contour-lines of 1,250 and 1,500 

 feet. Carvill Lewis (op. cit.) noticed that these mountains were 

 glaciated up to a height of about 1,000 feet, and we may perhaps 

 imagine the mountain-mass above this level to have stood up as 

 a nunatakkr above the great ice-sheet during the period of maximum 

 glaciation.^ A lobe of the ice may have forced its way over the 

 low col by Coumgorra and descended the Nier Valley. The heads 

 of the lateral valleys which were not invaded by the ice would be 

 modified during this period by the action of frost ; their rocky sides 

 would be splintered into cliffs, helped by the bedding, lie, vertical 

 divisional planes and character of the rocks. 



When the maximum cold and the ice which swathed the lower 

 hill-slopes departed, these rock- walled niches were found left as the 

 sole representatives of the former lateral valleys. Some were short 

 and wide, others were long and complex, their size and shape 

 having been determined by the various local conditions which 

 affected the flow, pressure, and height of the ice-stream which 

 crossed their mouths. On the return of the cold, though in a less 

 severe form, an ice-cap probably formed on the mountains * and sent 

 tongues down into the incipient corries on its flanks. The corrie- 

 glaciers thus originating would produce the modifications in the 

 already formed hanging valley-heads which were needed to convert 

 them into true cirques. The U-shaped cross-section which we notice 

 in them is characteristic of glaciated valleys. The frequent widening 

 towards their head, though beojun before the formation of the 

 •corrie-glaciers, was increased by the scouring out of the accumulated 



1 Bonney: Q.J.G.S., vol. Iviii (1902), pp. 590-701. Garwood: id., pp. 703-714. 



* Penck: op. cit., pp. 307-309 and references. 



» Maxwell Close: Journ. Roy. Geol. Soc. Ireland, vol. i (1867), p. 228. 



4 Hull: Phys. Geol. Geogr. Ireland, 1878, pp. 103, 263. 



