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Other Faults.
An origin by faulting is implied for some of the longitudinal features of the Wellington Peninsula by Bell,§ and the line of the Makara Valley is included by McKay∥ among “active faults and earthquake rents.” The presence of many faults, and particularly of the last mentioned, is revealed in natural sections. The three faults which McKay¶ describes as “converging on … the capital of New Zealand” can be recognized, although it is difficult to see why they are to be regarded as the continuation of faults in the South Island. The stratigraphy of the district is too little known to allow an estimate to be made of the amount of movement on the fault-planes, and the period at which the main movement took place has not been ascertained. It can be confidently stated, however, for the whole of the area west of the Karori-Khandallah Valley that physiographic evidence of recent faulting is entirely lacking (see pp. 262–64). The boundaries of the subsided Port Nicholson block may next be investigated.
On the map of Port Nicholson given by Bell* there are indicated, in addition to the Wellington fault, five other fault-lines bounding the
[Footnote] § Loc. cit., section, p. 539.
[Footnote] ∥ “Reports of Geological Explorations, 1890–91,” map, p. 1; Wellington, 1891.
[Footnote] ¶ Loc. cit., p. 19.
[Footnote] * Loc. cit., p. 537.
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downthrown area. It is probable that these lines are only suggestions, for on the accompanying section giving probable faults* two given on the map are omitted and another is introduced. As mapped they are nearly parallel to one another, and appear to coincide with the strike of the rocks. For those bounding on the east and west the longitudinal ridges of Miramar Peninsula and the Kilbirnie ridge to the west of it there appears to be no evidence. The elongation of each ridge is satisfactorily explained as corresponding to rock structure. Neither ridge has, on either side, a straight or gently curved base-line, but sprawling spurs are given off (see fig. 6). Both shores of Lyall Bay (figs. 1 and 6) directly facing the ocean to the south are bounded by cliffs. That these are not fault-scarps there is abundant proof in the extensive rock platforms at their bases, which were raised above the sea by the small uplift of 1855. These prove a former long seaward extension of the spurs. Where the tombolo (fig. 6) connects Miramar Peninsula to the mainland this has afforded protection from marine erosion, and the spurs run far out, that from Miramar Peninsula almost meeting that from the mainland. To the north of the tombolo in Evans Bay, on both shores, smaller scarps are found, fronted by less-extensive rock platforms than those of Lyall Bay, all evidently the work of the waves on Port Nicholson, the energy of which is very much less than of those of the open sea. They are, however, sufficiently powerful,
Fig. 11.—Eastern Shore of Port Nicholson, Looking North-East From The Signal-Station On Miramar Peninsula.
urged by the prevailing strong northerly winds, to account for the destruction of the relatively small bulk of the spurs and slopes, the removal of which has resulted in the present scarped shore.
Similar arguments can be used against the probability of a fault bounding the harbour on the east. The shore-line is fairly straight for several miles in the entrance, but the obvious reason for this is that it is the side of a low narrow ridge, without lateral spurs, between two straight valleys. The shore is subject to powerful wave-action, as it is not sheltered from waves entering the harbour-mouth, and marine erosion has been able, by the removal of quite a moderate amount of material, to cut a continuous line of cliffs.
Farther north, towards the head of Port Nicholson, the land is higher, and no longer a narrow ridge. Torrent-gullies, opening to the harbour as small bays, are separated by tapering spurs which run down nearly to sea-level without change of slope. The points only of the spurs have been truncated by wave-action, and a marked decrease in the height of wavecut facets can be traced northward on successive spurs. This appears to correspond to the decreasing energy of waves, running along the shore, with
[Footnote] * Loc. cit., p. 539.
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increasing distance from the open sea. Before the delta of the Hutt River, at the head of Port Nicholson, is reached, effects of wave-action have shrunk to small dimensions, and the spurs which run down into the flats of the delta are not truncated at all.
It will be gathered from the above description and from fig. 11 that the eastern shore of the harbour presents characters similar to those of any ridge in highly inclined stratified rocks, determined by the resistant nature of the stratum of which it forms the outcrop. It is continuous with the ridge forming the divide east of the Hutt River. This divide runs for some distance parallel with and very close to the Hutt River; hence the tributaries entering the Hutt, or its continuation, Port Nicholson, can be only short, steep-grade torrents. The nearness of the divide to the Hutt at this point is explained by the fact that the ridge is composed of the strong greywacke with few joints, which is the hardest rock in the district. If, on the other hand, the ridge-face were determined by a line of recent faulting, and the ridge itself were composed of rocks of average or varying hardness, it might be expected that some of the streams of the fault-scarp would have worked through and captured the drainage at the back, as the streams of the Wellington fault-scarp have done. This ought all the more to be expected in the case under discussion, since, if it be a case of faulting, the actual scarp has reached a much more mature stage of dissection than the scarp of the Wellington fault.
The question of what actually is the eastern boundary of the Port Nicholson depression must for the present remain open.
There remains the line on the western side from Kelburne through the City of Wellington to the sea on the south. This is the line of one of McKay's faults (No. 3).* A section across this fault or a branch of it may be seen in the cuttings of the Brooklyn tramway, but the section gives no information as to the date of faulting or amount of movement. There is rather indefinite evidence of faulting in the steep scarp along the front of Kelburne and Brooklyn (the line AB in fig. 2). Evidence of faulting is much obscured owing to the fact that the line of fracture appears to have followed the course of a longitudinal mature valley in weak rock, the floor of which was very deeply weathered. The amount of movement appears to have been between 200 ft. and 300 ft. Farther south there is little evidence of a scarp, and the fault was perhaps replaced by a flexure.
[Footnote] * Loc. cit., p. 1.