Speight. — Unrecorded Tertiary Outlier in Basin of the Rakaia. 359 



the mountain region of the Southern Alps. It is therefore evident that 

 the relative height of the central portions of that range compared with the 

 parts to the east was at one time greater than it is now, unless there are 

 faults of whose existence nothing is known at present which worked in the 

 other direction. 



This is not the only instance that the locality furnishes of a valley 

 determined in all probability by structural movements, for some five 

 miles to the south-east there is an exactly analogous valley in the Upper 

 Porter leading to Coleridge.. Pass, a low saddle in the Craigieburn Range; 

 and on the Lake Coleridge side of the pass there is a marked break in the 

 parallel ridges which are such characteristic features of that part of the 

 Rakaia region. I have been informed that pieces of coal have been found 

 in this locality, but have never been able to come across any myself, though 

 it is not at all improbable that it exists, and that another small outlier of 

 the Tertiary series exists in that part of the lake-basin. If this is really 

 so it would explain, just as the Harper fault explains, the existence of the 

 isolated blocks into which these parallel ridges are cut. However, many 

 glaciated regions show the presence of such isolated remnants of ridges, 

 and it is hardly safe to explain the Rakaia phenomena in this way without 

 further evidence. It may be noted that Gregory in his book. The Nature 

 and Origin of Fiords, notes in numerous places the frequent occurrence of 

 isolated mountain blocks in glaciated regions, and explains them as the 

 result of cross-fractures and not as a product of glacier erosion. 



The question of the age of the Harper River fault is a matter of some 

 interest. In its initial stages it is certainly of pre-glacial origin, since the 

 valley has been glaciated. An overflow from the "Waimakariri basin came 

 in from the neighbourhood of the Bealey, and came down the unnamed 

 tributary referred to previously, and left traces all down the valley to the 

 junction with the Avoca, where it merged into the great ice-streams which 

 have so profoundly modified the landscape of this region. The result of 

 glacial action on a valley of fault origin will, of course, be considerably 

 different from that in a normal stream-eroded valley. In the former there 

 will be no overlapping spurs, and therefore there will be. after ulaciation, 

 no truncated or semi-truncated ends, no beehive forms, but the valley- 

 walls will exhibit complete alignment, a feature well exemplified in the 

 Harper and Cass Valleys. Unless the formation of the valley has ante- 

 dated the glaciation by a long period there will be little sign of the- develop- 

 ment of tributaries, so that hanging valleys will be absent. Further, if 

 the sides have all irregularities removed, the flat faces left when the glacier 

 retreats will be stable under the action of erosion agents, and will be 

 preserved much longer than those valley-sides on which the stream-valleys 

 are already organized. 



The Cass Valley exhibits a well-developed system of overlapping spurs 

 on its floor, but these are no doubt due to the overdeepening of the flat 

 floor of the glaciated trough by stream erosion after the retreat of the ice ; 

 and the same remark applies to the Upper Harper, but to a more limited 

 extent. 



Although the fault formation of the valley must be pre-glacial. there is 

 evidence of disturbance of drainage in the case of Lake Coleridge which may 

 be attributable to recent movements along the same line of fracture. The 

 circumstances are as follows : Round Lake Coleridge there is a well- 

 developed old shore-line about 60 ft. above the present level of the lake. 



