Sri:u;H'i\ Cockayxe, Laixc — Motmt Anoirsnullt Dixlncf. :-j87 



and became corrie glaciers. The process of erosion begun by the snow 

 was continued by the ice, but on different lines. The walls and sides were 

 sapped back and the basins were enlarged till the glaciers they held became 

 snuill ones of the valley type. As this went on, the floor was eroded deeper 

 and deeper at the head, while little erosion went on near the lip, so that 

 Avheii the ice disappeared the hollow left behind was usually occupied by a 

 lake. This seems to be the ordinary course of the development of a corrie 

 glacier and the hollow usually associated with it, according to the most 

 advanced school of physiographers ; but there is a weighty body of opinion 

 totally opposed to the idea that corrie glaciers are potent agents in modify- 

 ing landscapes. Although the present writer is somewhat chary of express- 

 ing a dogmatic opinion on a subject which has lead to so much controversy, 

 his experience in the glaciated districts of this country, notably in the Sovmds 

 regioii, leads him to think that the course of development outlined above 

 , accounts best for the phenomena that occur. 



By granting the capacity of glaciers of this type to sap back their heads 

 one can explain the formation of the jagged and razor-backed ridges which 

 so frequently separate the head of one tributary glacier from that of its 

 neighbour across a divide. The usual sequence of events in such a case 

 appears to be as follows : First there is a ridge, more or less rounded, with 

 two shell-like hollows containing snow on either side. In process of time 

 a corrie glacier forms. Then sapping goes on, and the divide becomes 

 narrower and narrower ; then it is a mere wall, and finally this collapses 

 and a saddle results. At the head of the Rakaia there are existent glaciers 

 which show all stages of this development and furnish some idea of the 

 modification which results from their action on moimtain-ridges ; but in 

 those parts from which the glaciers have retreated the landscape-forms 

 resulting from this action can be readily noted and studied in detail. We 

 see here all the stages from the shell-like hollows, through the razor-backed 

 ridges, to the final " pass " form. The latter are usually U-shaped in 

 cross-section, but they tend to become parabolic by the accumulation on 

 their floors of detritus shed from the walls. It seems possible that the 

 isolated ridges which so frequentty occur in all the great valleys of Canter- 

 bury owe their dissection to this process, especially as the main valleys run 

 across the strike and the cross-valleys are developed in the soft beds parallel 

 with it, these beds furnishing an opportunity for snow to weather rapidly 

 the shell-like hollow where thicker and more persistent drifts can gather. 

 When the lowering of the divide has been accomplished by this action the 

 main glaciers occasionally ]3ass through them, especially if they are on the 

 increase ; they are cut down further still by the usual methods of glacier 

 erosion. The occurrence of parallel or subparallel ridges in the motm- 

 tainous district of Canterbury are of such a frequent occurrence, and such 

 an important feature of the landscape, that their formation appears to 

 be connected with the former glaciation, and the explanation I have given 

 seems to me to be the most satisfactory way of accounting for their exist- 

 ence. 



Note. — Since writing the above I have seen an interesting paper by 

 Professor W. H. Hobbs, of Michigan University, published in the Geo- 

 ijraphical Journal for February, 1910, which emphasizes the important 

 effect produced on the mountain topography by the sapping-back of the 

 walls of corrie glaciers. Most of the landscape-features mentioned by 

 Professor Hobbs are exemplified in that part of the mountain district of 



