F. R. Cowper Reed — Carries of Cotneragh Mountains. 231 



and in general features of cross-section, hanging mouth, and pre- 

 cipitous walls it resembles Coumgorra and the other corries. 



The difference between the north and south sides of the Nier 

 Valley is striking, for corries or lateral valleys of the type described 

 are conspicuously absent on the northern flanks. 



Origin of the Corries. 



In striving to find an explanation of the origin of these corries 

 and lakes in the Comeragh Mountains we are met at the outset by 

 the still unfinished controversy as to the method of formation of all 

 corries and rock-basins. There are current the theories of glacial 

 erosion, subaerial erosion, marine erosion, and tectonic movements. 



Thoroughgoing glacialists, like the Eev. Maxwell Close, ^ have 

 ascribed the carving out of the Irish corries themselves, as well as 

 the hollowing out of their floor, to the unaided action of ice. Others 

 believe that the main or preliminary processes in their formation 

 were effected by the ordinary subaerial agencies, and consider that 

 ice has only played a subordinate part, superinducing certain peculiar 

 features by which we can distinguish true cirques from the funnel- 

 like heads of ordinary valleys of erosion.^ Others maintain that 

 typical cirques occur even in mountain regions which we have no 

 reason to believe were ever glaciated,^ and that the origin of the 

 corrie was to a large extent independent of and prior to the 

 formation of the hollow holding the lake, whether the latter lies in 

 a rock-basin or a barrier-basin. 



Kinahan * has urged the primary importance of faults, joints, and 

 local dislocations of the strata producing lines of weakness along 

 which marine erosion has acted ; and he compares the cwms to the 

 ' cooses ' on the Galwaj' coast, the shape of which has been 

 determined by the intersection of various small faults. 



Finally, tectonic movements, causing local subsidences, have been 

 considered sufficient to account for their features, which indeed recall 

 those of calderas ® and of the breached craters of Auvergne.^ 



The most acceptable theory seems to be that of a modified head of 

 a valley of erosion. The head of an upland glen, be it short or long, 

 tends to have a half funnel shape, owing to the convergence to 

 a central point of the various streams descending its slopes ; and, 

 given certain favourable conditions,'' such a valley head may be 

 converted into a true cirque. One of these conditions is the moderate 

 horizontality of the strata over which the streams fall. Another is 

 that the rocks must be of such a nature as to allow the formation 

 of cliffs, and for this purpose well-jointed bedded limestones or 

 sandstones of considerable toughness and durability are especially 



1 Maxwell Close: Journ. Eoy. Geol. Soc. Ireland, vol. ii (1871), p. 236. 



* Eichthofen: " Fiihrer fiir Forschungreisende, " 1886, pp. 255-259. 

 3 James Geikie : " Great Ice Age," 3rd ed., 1894, p. 236. 



* Kinahan : " Valleys and their Eelations to Fissures, etc.," p. 126 ; London, 1875. 



5 Eichthofen: op. cit., p. 255. 



6 Howorth : " Ice and Water," vol. i (1905), p. 535. 



' Bonney: Quart. Journ. Geol. Soc, vol. xxvii (1871), pp. 312-324; ibid., 

 vol. xxix (1873), pp. 382-398. 



