50 Transactions. 



now lies has been entirely clue to glacial erosion. In my opinion, the hollow 

 is primarily tectonic, but the surface so formed has been modified by stream- 

 action, succeeded by glaciation, and that now a new cycle of stream erosion 

 has commenced. 



The form of the notches cut in these spurs is also characteristic of ice 

 erosion, since glaciers always appear to exert their maximum erosive effect 

 at the base of the valley-sides or shelves along which they move. Thus the 

 notches have the backward slope which results from this mode of action. 

 When this becomes more pronounced and ice-action has been more prolonged, 

 the outstanding portions of the ridge tend to become rounded eminences. 

 If a stairway was attacked further the notches would become a string of 

 knobs, gradually getting higher as the spur is followed upwards. This 

 stage of development is seen in the Waimakariri Valley to the west of the 

 Cass River, where Moimt Horrible and Mount Misery owe their rounded 

 form to the great Waimakariri Glacier crossing a spur which runs parallel 

 with the present Cass River and enters the main valley nearly at right 

 angles. (Plate IX.) 



The formation of a well -developed series of notches generally occurs 

 where the spur has great length ; but if it is shorter in the pre-glacial stage 

 only one or two notches may be cut, and the resulting form becomes a semi- 

 detached knob or titan beehive noted by Andrews m the Sounds region. 

 (Plate VIII, fig. 2.) This form is typically developed in the Upper Rakaia 

 Valley at Mem's Knob and Jim's Knob, the latter being formed by 

 the Ramsay Glacier passing over the terminal spur of the Butler Range. 

 (Plate X, fig. 1.) Numerous illustrations in all stages of development can 

 be seen in the Upper Waimakariri Valley, especially where the action of 

 the main glacier has not been interfered with by the weight of the ice 

 issuing from a tributary comparable in size to the main stream. When 

 the tributary becomes large the modification of the spur is attributable 

 chiefly to its action, and not to the erosion of the main stream. 



From the slight difference in the form of the notches in the higher part 

 of the series as compared with those at floor-level it is evident that all the 

 notches were cut during one period of ice-advance. Had there been more 

 than one ice-flood, reaching various levels, there would have been some 

 difference in the form of the higher members of the series from that of the 

 lower. As the lower members would have experienced more than one 

 ice-flood, their stage of erosion would have been more mature. Also, if the 

 ice had not reached so high in the later floods as in the earlier the exposed 

 notches or knobs at higher levels would show more the effects of subaerial 

 erosion, by rain, frost, &c. For example, if the first flood were the highest, 

 then while the lower levels were being subsequently glaciated the higher 

 and exposed levels would have been differentially modified by subaerial 

 erosion, and glacial erosion of the lower slopes would have been carried to 

 a more mature stage. If the last ice-flood had been the highest, the 

 modification of the higher levels would have been different in that the glacial 

 surface would have been juvenile, while the lower would have been mature. 

 If an intermediate flood had been the highest, a differential modification 

 partaking of both characters would have occurred, depending on the relative 

 importance of the two phases. But the only difference — and that is a very 

 slight one — is that which might have been expected in the lower parts 

 of a glacier, where, under the influence of greater weight of ice, abrasion, 

 plucking, sapping, and other glacial agencies are more intense. 



The knobs of the Cass Range show very markedly the modifying effect 

 of frost erosion, as their plant covering is of the scantiest — in marked contrast 



