A. Harker — Ice-Ero>iion in Ski/e. 37 



The author points out that while the carving out of the main 

 features of mountain and valley was due to aqueous erosion in 

 pre-Glacial times, the actual details of the relief are due to the 

 action of ice and frost during the Great Ice Age. Before glaciation 

 the drainage system was a fully established one, and erosion was 

 practically at a standstill, as it is under present conditions.' Hence 

 the effects of ice and frost action remain practically without 

 modification by later agencies ; and almost every square foot of 

 the surface bears the stamp of glaciation. It is clear, as the author 

 remarks, that the ice has been in close contact, throughout its whole 

 extent, with the subjacent rocks, and has forced its way into hollows 

 and openings, vertically and horizontally, in a fashion which argues 

 effective plasticity in its lower layers. 



He points out that it is practicable to distinguish the features due 

 to glacial from those due to aqueous erosion. The work of the ice 

 was something more than a mere excoriation of the surface, and 

 in comparing the work of ice and of water he observes that 

 " a sand-grain gripped in the sole of a glacier, or of an ice-sheet 

 thousands of feet in thickness, must be incomparably more efficient 

 as a graving tool than the same grain rolled aloug the bed of 

 a stream." Ice-action, then, must be rapid compared with that 

 of running water ; its work is done, moreover, with little or no 

 chemical co-operation, and instead of the relief produced by ordinary 

 weathering agents, features are carved out of the rock-complex in 

 a fashion wholly irrespective of lithological differences or geological 

 structure. Lateral erosion, too, comes in unfettered by any con- 

 sideration of 'base-level,' and valleys may be widened as well as 

 deepened and their sides straightened. 



Attention is drawn to the formation of cirques or corries and 

 of small rock-basins on their floors. The erosion of these was 

 dependent upon an adequate supply of debris or abrading material 

 at the under surface of the ice, and as the excavation of the higher 

 cirques proceeded the dividing ridges became intensified. On the 

 principal ridges, which acted as ice-sheds, ice-erosion necessarily 

 failed for want of a tool to work with. 



The author observes that if we take the Cuillin district as a type, 

 it appears that ice-erosion does not, like water-erosion, work con- 

 stantly towards the establishment of an even gradient along a valley 

 in which it operates. Erosion will be most efficient when the 

 pressure below the ice is greatest, that is, when the thickness is 

 greatest, and differential erosion will operate so as to exaggerate 

 inequalities. This would be arrested when the lower layers of 

 the ice begin to be ponded in the lee of a strong feature, and the 

 upper layers slide over them. Thus, rock-basins in valleys are 

 dependent not merely on glacial erosion, but on glacial erosion 

 operating under certain local conditions and in more than one way. 

 Detailed observations and soundings made by the author and 



^ See article by A. Harker on Subaerial Erosion in the Isle of Skye : Geol. Mag., 

 1899, p. 485. 



