The following account may serve to supplement the “proof of the great fault along the western side of Wellington Harbour” given by Bell.∥ In fig. 1 the line of faulting is indicated as “Wellington fault” (see also fig. 9, a sketch of the fault-scarp as seen from Kelburae, and Plate XIX, fig. 2, a photograph from Petone). For the length of this line, about six miles, the Port Nicholson depression is bounded by an abrupt scarp with a base-line almost perfectly straight, the departure from perfect alignment consisting of two very gentle curves, concave towards the shore, separated by a similar convex curve of very
[Footnote] ∥ Loc. cit., p. 539.
wide radius. The average direction of the base-line is N. 50° E. It makes a decided angle with the strike of the rock strata. Where road-cuttings have been made parallel with the line of the scarp, rock outcrops run up the face obliquely in one direction or the other, according to the dip of the beds. Sloping down to the even base at an angle of 55° is a flat and even face, separated into triangular facets by a number of ravines. The mouths of some of these ravines overhang the shore, as if a period or periods of standstill accompanied by erosion had separated periods of movement the last of which took place at a very recent date. There are, however, no traces of wave-cut shelves along the scarp such as one would expect if the movement had been one of elevation of the landward block. It would seem rather that the movement was altogether a subsidence of the harbour block. Clay terraces overhanging Tinakori Road, which were regarded by Bell* as beach deposits on a rising block, are clearly remnants of the floor of a mature valley which was cut across obliquely by the fault.
An alternative and perhaps the correct explanation of the hanging ravines on the fault-scarp is that the ravines were developed when the boundary of the Port Nicholson depression lay farther out, before the final movement on the plane of the Wellington fault. By the final faulting movement they would then be truncated. This hypothesis gains some support from the fact that tributaries of the larger streams, the Kaiwarra and the Ngahauranga, which cross the fault-scarp show evidence of recent revival.
Fig. 10.—Truncated Valley Overhanging The Ngahauranga Gorge.
The line of the straight fence gives a oross-profile of the upper part of the valley.
These two larger streams have been sufficiently active to capture the drainage of a longitudinal valley at the back. The changes in their courses are described in a later paragraph (p. 262). Both streams in their lower reaches, where they cross the fault-scarp, flow in narrow, young gorges (see Plate XX, figs. 1 and 2).
Fig. 10. a sketch of a little valley truncated by the Ngahauranga, gives an indication of the depth to which the latter has incised its course below an older surface of moderate relief.
The Kaiwarra, which is the larger stream of the two, has graded its course, and for a distance of a mile from its mouth has worked out an extremely narrow strip of flood-plain (Plate XX, fig. 1). The Ngahauranga is not graded. A fall in its lower course is illustrated in Plate XX, fig. 2.
There is no doubt that both these streams are of extremely recent origin. Their lower courses are consequent upon the slope of the faultscarp, or, at least, of the boundary of the Port Nicholson depression.
Next to the extremely young character of the streams the most important piece of evidence in favour of faulting is the abrupt manner in which the ridges separating them are terminated as a straight line of cliffs at the harbour side. If the theory of faulting is not entertained these must
[Footnote] * Loc. cit., p. 539.
be regarded as having extended at least a mile out into the waters of Port Nicholson, enclosing between them the continuations of the present gorges; and the coast must have been cut back to a straight line by wave-action.
The problem may be attacked in two ways: (1.) Search for the rock platforms which should remain to indicate the former extension of the spurs. A glance at fig. 5 shows that these are absent, and that the deepest water of Port Nicholson comes close to this shore Rock platforms, if they existed, ought to have been actually raised above water by the 5 ft. uplift of 1855, but for nearly the whole length of the scarp rocks are not exposed at low water more than 50 yards from the foot of the cliffs. (2.) Comparison with other parts of the coast-line where marine erosion has been more or less effective in cutting back the coast. The coast of the seaward end of Miramar Peninsula (fig. 6) may be considered. Here, indeed, bluffs have been cut back to the extent of a mile, as the exposed rock platform at their base shows, but the coast has by no means been rendered perfectly straight. Moreover, compared with its activity on the outer coast, wave-action within the harbour is extremely feeble. A safe comparison can therefore be made only with another stretch of coast within the harbour. When the eastern shore is examined it is found that wave-action has succeeded only in shaving off the ends of points. Fig. 11 represents the eastern shore as seen from the signal-station on Miramar Peninsula. Its irregular base-line may be noted on the maps, figs. 1 and 5. It should be noted that this side of the harbour is bounded by a strike ridge, and that no spurs of any magnitude run down from it. So a shoreline originally nearly straight has been rendered but little straighter by wave-cutting. Moreover, the increasing height of cliffs towards the harbour - entrance shows that the greater part of the work has been done by waves rolling in from the open sea. The western shore of the harbour, on the other hand, is affected only by waves raised on the harbour itself. The effect of waves raised within the harbour is seen on the shore of Evans Bay (on the left in fig. 6).
The conclusion reached is that the scarp bordering the harbour on the north-west, with its straight base-line, cutting at an angle across the strike both of the rock strata and of the drowned ridges to the south of it, with its faceted spurs and its steep-grade gorges, is the result of recent faulting. Fig. 9 may be compared with the sketches and photographs of the Wasatch Range given by Davis,* and also with the diagrams illustrating his theoretical discussion of the dissection of the face of a faulted block.†
[Footnote] * W. M. Davis, Bull. Mus. Comp. Zool., Harv., vol. 42, No. 3, 1904, p. 153, and pl. 4; and vol. 49, No. 2, 1905, fig. 2, and pl. 1, A
[Footnote] †Loc cit., vol. 42, No. 3, 1904, figs. 6–9.