104 Transactions. 



certainty the general direction of the streams that flowed through this 

 tract of country anterior to the glaciation. In addition to the two main 

 branches of the river a large stream rising near the head of Lake Sumner 

 followed the course of the Lake Taylor Valley, parallel with the South 

 Branch ; this entered the North Branch about half-way between Lake 

 Sumner and the junction of the two main branches. A small tributary 

 entered this valley on the north side, rising near the head of Lake Taylor 

 and following the course of Lake Sheppard. Another small stream rose 

 near Lake Katrine and joined the North Branch below the outlet of 

 Lake Sumner. In pre-glacial times the ridges dividing these valleys would 

 be more or less entire, though they might have saddles at their heads. 

 It is impossible to reconstruct such features exactly, but the description 

 just given affords a fairly accurate view of the stream conditions which 

 obtained in this tract of country before it was modified by glaciation. 



Whatever was the prime cause which promoted glacier extension, it is 

 reasonable to assume that it was gradual in its incidence. Snow would 

 slowlv accumulate, glaciers would be formed at higher altitudes and slowly 

 extend down the valleys. Thus the heads of the small valleys would pro- 

 bably be filled with corrie glaciers, while the glaciers of the first order would 

 be extending down the main valleys. These would help to lower the divides 

 in the way suggested by Matthes.* As the ice-flow increased in volume the 

 main streams would be filled, and in time overflows would take place over 

 the lowest part of the divides, which would be lowered at the same time 

 by active ice abrasion. It is significant that the greatest amount of lower- 

 ing has taken place near the head of Lake Sumner. This would be due to 

 the marked overflow of ice from the main Hurunui Valley, no doubt due to 

 the narrowing of the cross-section of the valley at Lake Sumner, which 

 caused the ice to overcrowd into the headwaters of the neighbouring streams, 

 as it has done in several of the valleys of the Canterbury rivers. The full 

 force of this would be felt at the head of the Lake Taylor Valley, and thus 

 its divide has been completely reduced. The headwaters of the intermediate 

 tributary valleys were also invaded and the saddles at their heads reduced. 

 Thus a clear passage for the ice was opened down these valleys past the 

 site of the Lakes Station in the direction of the south-easterly reach of the 

 North Branch below Lake Sumner, while the main stream of ice followed 

 down the valley now occupied by this lake. 



In addition to the overflow toward Lake Taylor, a powerful stream passed 

 over into the tributary which runs into the South Branch from the north. 

 The saddle at the head of this stream was thus reduced, but not so much 

 as its neighbour, which was more in the line where the ice-stream would 

 impinge on the valley-wall. If, however, glacier action had continued this 

 saddle would have been reduced and the mountain ridge to the north of 

 the South Branch would have been completely isolated. It is possible that 

 ice also overflowed into the valley of this stream near the Lakes Station, 

 and as at the height of the glaciation the country in its vicinity would have 

 the form of an intermontane basin, and would be an efficient gathering- 

 ground, overflows from it took place along several lines from the front of 

 the ice-sheet in the direction of the valley of the North Branch. These 

 would produce the breaks in the valley-wall between the South Branch 

 and the country to the north which occur immediately up-stream from the 

 junction of the two branches. 



* F. E. MATTHES, Glaciation of the Biu Horn Mountains. U.S. Geol. Sun: 21st Ann. 

 Rep., 189U--1900. 



