140 Transactions. 



by comparison of the lengths of the drowned portions with the average 

 gradients of similar valleys. For this purpose the contoured map of the 

 district has been used. The result shows that the depth of submergence 

 at Gollan's Valley may be as much as 200 ft., but cannot be more. The 

 Koangapiripiri valley is so small that the result obtained by comparison 

 with the small-scale map available is not very reliable, but the submergence 

 indicated does not seem to be greater than in Gollan's Valley, though 

 indications of greater submergence might be expected, seeing that it is half- 

 way between the latter and the deeply-drowned entrance to Port Nicholson. 



The evidence of the drowned valleys supports the hypothesis of fairly 

 even tilting towards the Port Nicholson depression. The position of the 

 hinoe-line is not definitely indicated. It is possible that the mouth of 

 the Wainui-o-mata valley has been very slightly drowned and quickly 

 filled again with alluvium by the river, but this is doubtful. Farther 

 east, however, at the mouth of the Orongorongo, there has been no sub- 

 mergence, for this river is cutting on bed-rock at the mouth. 



Some of the rejuvenation of which there is evidence in the Orongorongo 

 valley is in all probability a result of the uplift of the eastern side of the 

 tilted area which caused the emergence of the Turakirae coastal plain. 

 Hence the rejuvenation of the eastern valleys is not such good proof of 

 tilting as the drowning of the western valleys. 



Evidence from Regraded River-vaUeijs. 



Near the coast the larger streams flow without exception in courses 

 parallel with the hinge-line of tilting inferred from the evidence aheady 

 described. They are close together, and have only very small, steep- 

 graded tributaries. In this part of the district there are, therefore, no 

 streams that would be particularly sensitive to tilting in a west-north- 

 westerlv direction towards the Port Nicholson depression. Degradation in 

 the Orongorongo and Wainui-o-mata valleys, of which there is evidence hi 

 the presence of low terraces, might be the result of purely regional movement, 

 and distinct terraces can be correlated with uplifted strand-lines on the 

 Turakirae coastal plain, which lies across their mouths. The aggradation 

 in Gollan's Valley, which has previously been described, can be accounted 

 for by the drowning of the valley-mouth. Nevertheless the western branch 

 of Gollan's Valley is aggraded quite to the head, as though as a result of 

 headward tilting, which would be the result of the general downwarping 

 of the surface towards Port Nicholson. 



There is a suggestion of aggradation also in some parts of the valley 

 of the Wainui-o-mata, which is somewhat winding. It is a mature valley, 

 with a flood-plain, and this widens out considerably in places, where the 

 valley bends to the west. Though such expansions are due in part to the 

 development of large curves by lateral planation and their later abandon- 

 ment when the stream returned by a cut-off to a straighter course, they 

 appear to be partly the result also of aggradation in response to backward 

 tilting in certain reaches, either because these are below westward bends 

 in the sinuous valley, or else on account of transverse warping, such as 

 certainly occurs farther north, corrugating the general slope of the country 

 towards the Port Nicholson depression. 



These aggradational effects are, however, much less definite proof of 

 tilting than some which are found farther north, opposite the Hutt delta 

 and the upper Hutt Valley. Here, though the principal valleys still trend 

 north-north-east or south-south-west (Cotton, 1914), a number of tributaries 

 of moderate size enter them from the west. The western branch of the 



