90 



Proceedings of the Royal Irish Academy. 



The passage of a glacier through the gap is a factor in this inter- 

 esting problem which cannot be omitted, to which in fact much 

 importance attaches. The abnormal deepening of the channel just 

 within the embouchure is almost absolute proof of its operation. For 

 corroboration and illustration we may refer to a standard instance 

 treated of by Dr. Hans Hess^ — that of Lake Iseo, in Lombardy. This 

 lake, about 12^ miles in length, by 2^ miles in width, and 172 miles 

 from the sea, is a rock -basin, with surface-level 185 metres above sea- 

 level, and more than 235 m. in depth : that is, its bottom lies more 

 than 50 m. below the present sea-level. Dr. Hess has been good 

 enough to send me a traced map of this interesting lake, with figures 

 indicating surface-level and soundings as given above. He has no hesi- 

 tation in attributing its formation to glacial erosion; in fact, he 

 regards the present valley-bottom as but a result of the latest stage 

 of the gouging action of glaciers, which successively occupied and 

 formed the entire valley. To this point it will be necessary to return, 

 noting here the two principal points for which this illustrative 

 instance is brought forward : — 1st, that a glacier has power to 

 produce, near the emhouchure of a valley, a remarkable deepening of 

 the river draining it, which could scarcely under the circumstances 

 of the Shannon gorge be attributed to water-erosion; 2ndly, that 

 deepest water hugs the concave side of the lake, where the ice-stream 

 turns westward in its course. 



The internal structure of glaciers, as described by Agassiz, Forbes, 

 Tyndall, and more recently by Hess, Chamberlin, Salisbury, and others, 

 especially the spoon-shaped curving of planes of deposition frequently 

 assumed at glacier-ends, is strongly suggestive of scooping action, 

 such as would account for the hollowing of valley-bottoms near their 

 emlouchures. A view of the Erenva glacier of the Mont Blanc group 

 is given by Hess,^ and an ideal section through the centre of a glacier^ 

 along its course, in both of which the upturning of the layers of 

 deposition at the glacier end is clearly shown. Chamberlin and 

 Salisbury* also refer to this feature, and give instances : for example 

 the glacier on the south side of Orliks Bay, and that of Bowdoin in 

 North Greenland. They, moreover, write as follows : — " It is merely 

 necessary to assume that the gravity of the accumulated mass is 

 sufficient to produce minute temporary liquefaction at the points of 



1 Op. cit., p. 356, and plates. 



2 Op. cit., p. 169. 



3 Op. cit., p. 336. 



* Geology, vol. i., Physical Processes, pp. 281, 300, and 303. 



