Cotton. — Warped Land-surface at Port Nicholson. 133 



to the east, whether or not each is confined to the outcrop of a single 

 formation, may be described with perfect propriety as subsequent in origin. 

 Prior to the formation of the Port Nicholson depression it appears that 

 the whole district was maturely dissected by the subparallel streams of 

 the system just described, and by their numerous small insequent tribu- 

 taries, with a relief of about 1,500 ft. 



The erosion-scarp descending to the shore of Port Nicholson and to 

 the Hutt Valley, as will be shown below, is not the boundary of the whole 

 depression, this being found to be a relatively broad strip of strongly- 

 warped land-surface. 



The other boundaries of the depression call for passing reference only. 

 On the north-western side the immaturely dissected scarp of the Wellington 

 fault, mentioned above, meets the warped eastern slope obliquely in the 

 Hutt Valley, which is thus a fault-angle depression. South-westward this 

 fault-scarp extends inland about a mile and there dies out, and thence 

 southward to the sea-coast the western side of the depression is apparently 

 a warped surface, though evidence of the exact nature and extent of the 

 warping has not yet come to light. Seaward, to the south, the depression 

 is open to the Pacific Ocean. 



General Tectonic Features of the Depression. 



The foregoing features of the Port Nicholson depression, taken in 

 conjunction with the observation that the Wellington fault-scarp follows 

 a pre-existing line of weakness — a very prominent shatter-belt extending 

 north-east and south-west, somewhat oblique to the system of subsequent 

 features previously referred to, and marked by prominent subsequent fault- 

 line valleys (Cotton, 1914.) — would seem to indicate that faulting must 

 be regarded as merely an incident in the formation of the depression. 

 The principal event appears to have been the sharp downwarping of a 

 belt of land about thirty miles long elongated in a north-north-east and 

 south-south-west direction (and extending an unknown distance farther to 

 the south-south-west beneath the sea). The depth of downwarping that 

 must be assumed is variable, the maximum being perhaps in the neigh- 

 bourhood of 1,500 ft., or perhaps rather more, where the broadest part of 

 Port Nicholson now is. The width of the strip affected also varies in 

 different parts, but is at least ten miles where Port Nicholson is widest. 

 Both depth and width diminish, though irregularly, to the north-east up 

 the Hutt Valley.* It is as though the sagging-down in synclinal fashion 

 of an ill-supported superficial flake of the lithosphere crossed fortuitously 

 by the shatter-belt marking a pre-existing fault had resulted incidentally 

 in the formation of the more modern Wellington fault, the scarp of which 

 replaced part of the warped border of the depression. f When the evidence 

 of the features of the neighbouring coasts are taken into account, however, 

 it appears unlikely that the harbour-depression can be accounted for so 

 simply as by mere downward sagging owing to lack of support. The 

 deformation of the ancient strand-lines may be ascribed to compressive 

 folding, or the warping of the land to the east and south-east may be described 

 as tilting of an earth-block, for. as shown below, the warping or tilting has 



* If, as Adkin (1919) has suggested, the drowning of Porirua Harbour and the 

 formation of Port Nicholson are clue to the same movement, the downwarped strip must 

 become wider northward, or must send out a branch towards Porirua Harbour. 



t Similar synclinal warping with one side partly replaced by a fault occurs in the 

 Aorere district, in northern Nelson (Cotton, 1916b). 



