Art. XXVII.—Notes on Wellington Physiography.*
[Read before the Wellington Philosophical Society, 4th October, 1911.]
|Cycles of erosion||248|
|Forms of the Kaukau cycle||249|
|The Tongue Point cycle||250|
|The present cycle||251|
|The Port Nicholson area||251|
|The coast platforms||255|
|The Wellington fault||257|
|Nature of the movement||259|
|Changes in drainage of the Karori-Khandallah or “Long” Valley||262|
|Type of topography||264|
In the preparation of these notes a detailed examination has been made only of the district lying to the west of Port Nicholson, which for convenience will be referred to as the Wellington Peninsula. By means of hasty traverses and observations made from a distance, however, it has been possible to reach general conclusions which, the writer believes, hold true for the whole of the district represented in the locality-map (fig. 1).
With the exception of a few small patches of Recent sands and gravels occurring as beaches and river-flats, the rocks are a single series of sandy argillites and fine- and coarse-grained greywackes.† They are closely folded in a complex manner, but, owing chiefly to the unfossiliferous character of the rocks, the structure has not yet been unravelled. On any cross-section rapid changes in the direction of dip are the rule, but the attitude of the strata is so much more nearly vertical than horizontal that as far as their effect on topography is concerned they may be regarded as vertical. There has been no folding of any consequence in more than one
[Footnote] * When this paper was written the writer had not seen the criticism of Bell's paper by W. M. Davis in the Bulletin of the Am. Geogr. Soc. (vol. 43, No. 3, 1911, p. 190). Had he read that article earlier he would have been able to profit by several valuable hints given by Professor Davis.
[Footnote] † This thick, unfossiliferous series is correlated on hthological grounds with the Maitai system of New Zealand geologists, which, according to Marshall (“New Zealand,” Handbuch reg. Geol., 7 Band, 10 Abt., p. 35, 1911), is of Trias-Jura age. The period of folding is believed to be late Mesozoic.
direction. In other words, the strike is reasonably constant in direction, being very generally N. 15° E., but varying locally from N. to N. 30° E.
The rocks of the series are of very variable strength, the weakness of some bands being due, apparently, in great part to their shattered nature. The argillites are invariably traversed by innumerable joints, and so also are the greywackes as a rule, but in places they are unjointed except on a large scale. The greywackes with few joints are very strong, forming some of the highest ridges and peaks. They weather spheroidally, and, when broken, present an appearance similar to that of an even-grained igneous rock. The shattered greywackes have in some places been rendered equally strong by the deposition of interlacing quartz veins filling the joint-planes.
Corresponding to the regular strike and the steep dip of the strata and their varying strength, there is a well-marked arrangement of ridges and valleys parallel to the strike. This may easily be recognized on a map, and in the field it is found to be the dominant feature of the topography. In fig. 1 the straight and parallel courses of the Orongorongo and Wainuiomata are especially noticeable. Parallel to these the entrance to Port Nicholson and the Evans Bay — Lyall Bay depression will be noted. On the Wellington Peninsula itself (see also fig. 2) one continuous valley, occupied by the Karori, Makara, and Ohariu Streams, is well marked. The position of another is indicated by the settlements of Karori and Khandallah; it continues southward some distance, and its northward continuation is the Porirua Valley. Even the high bluff of Cape Terawhiti is almost cut off from the neighbouring land by a deep north-south valley.
The adjustment of stream-courses to structure, an arrangement which, with exceptions that will be noted, has been retained by existing streams, points to prolonged exposure to subaerial denudation for the duration of at least one nearly completed earlier cycle of erosion. The existing topography is composite, and has been developed during an uplift of at least 800 ft., and perhaps of 1,000 ft. or more. The amount of uplift seems to have been nearly uniform, although probably not quite uniform, over the area studied. During the uplift pauses occurred, some of which
were long periods of standstill. Further complications have been introduced by the subsidence of a block—Port Nicholson and the low-lying peninsula to the south of it (fig. 1)—resulting in piracy and obliteration of earlier topography in the high-standing block by vigorous new streams. The topography of a portion of the high-standing block unaffected by this
complication may be studied first. As a typical area may be taken that to the west of the north-south divide on which the peak Kaukau stands (fig. 1).
The relief in this area is moderate to strong, as may be gathered from fig. 2, the ridges in parts rising to 700 ft. or 800 ft., and in other places
to over 1,000 ft., while a few peaks reach to 1,500 ft. and more. The texture of dissection is medium to fine.
Cycles of Erosion.
All the forms recognized appear to be due to stream-action alone. Three sets of forms are recognized, corresponding to different positions of base-level, and it is possible that forms are present corresponding to other pauses in the movements of uplift. It is evident that such pauses did occur, for remnants of coast platforms are found, in places, one above another (see p. 255). Shortness of a period of standstill during which erosive processes work is not in itself a reason why the period should not be dignified with the name of “cycle.” Huntington and Goldthwaite* have pointed out the analogy between the term “cycle” applied to an erosion period and the term “life” applied to the period of existence of an organism. “Life in one signification is the complete existence of a normal organism during which it passes from infancy, through youth, maturity, and old age to death. The life of man in this sense is seventy years. In another sense life is merely the actual period of existence of any specific organism. An animal whose life in the first sense of the word is fifty years may die the day that it is born, but nevertheless we say that it has finished its life. A cycle in the first sense is ideal and can never be realized, since infinite time would be required to reduce any land-mass to the condition analogous to death—that is, to a plain at absolute base-level. In the second sense any region that is subjected to erosion during a definite period, no matter how short, passes through a cycle and can be described in terms of age and development.” The term “chapter,” proposed by Davis† for an unfinished cycle, has not come into general use. A number of such brief cycles, corresponding to pauses during the earlier part of the period of uplift in the Wellington district, may have left traces on the topography not yet obliterated, and the number of nearly flat-topped ridges of varying height which lie spread out to the west of Kaukau Peak suggests that in the future, with detailed work and accurate mapping, some at least of them may be recognized with certainty. The writer, however, feels justified at present in grouping the observed features as belonging to only three cycles.
The earliest cycle of which a record is preserved by existing topographic features will be called the Kaukau cycle. Base-level stood perhaps 800 ft. or 900 ft. higher than at present.
To the next, or Tongue Point, cycle belong most of the broad features of the landscape as seen from a high point of view. Base-level stood about 250 ft. higher than at present, and during the cycle the most extensive of the elevated coastal platforms, including that at Tongue Point, were cut (see p. 255).
It was between the Kaukau base-level and the Tongue Point base-level that other pauses occurred that are mentioned above. They were, no doubt, brief, and the Tongue Point cycle itself was of relatively short duration. It is, however, of importance on account of the very recent date at which it was interrupted.
There is, lastly, the present cycle, with present sea-level as base-level.
[Footnote] * Bull. Mus. Comp. Zool., Harv., vol. 42, No. 5, 1904, p. 239.
[Footnote] † “Physical Geography as a University Study,” Journal of Geol., 1894, p. 66.
Forms of the Kaukau Cycle.
Kaukau* Peak (1,465 ft.) may be taken as a sample of a form belonging to the first, or Kaukau, cycle. An area of about 50 acres at the summit presents the appearance of mature topography, with gently rounded outlines, which abruptly give place to precipitous slopes, rocky crags, and torrent-ravines, forms of the next cycle. The small, gently graded valleys of the summit are transformed within a few yards into torrent-courses with rock beds and steep rock walls. There is no difference of rock-strength to account for the change, but summit and sides alike are composed of the most resistant type of strong greywacke, traversed by few joints.
In fig. 3 the slopes of the summit of Kaukau are seen in the foreground. The surface is littered with blocks of the greywacke, weathering in the manner generally regarded as characteristic of igneous rocks rather than of sedimentary rocks.
Fig. 3.—View of the East Branch of Ohariu Stream, looking Northward from the Summit of Kaukau Peak.
A graded reach, at its lowest point 250 ft. above sea-level, and incised about 50 ft. below the graded valley-floor of the earlier cycle.
Little is left of the surface belonging to the Kaukau cycle, and perhaps the most extensive remnant is a tableland nearly a square mile in extent, standing 950 ft. above sea-level, which exists on the divide west of the Makara Stream. In fig. 2 it is marked P. It has an undulating surface of mature valleys and rounded spurs, appearing from a distance perfectly plane. It is bounded on all sides by the slopes of young ravines eating into it.
Many of the higher ridges show very similar topography, though no other is so nearly plane. From these observations it appears that during the Kaukau cycle the stage of maturity was reached, and that this nearly plane area, P, stood not far above base-level. The height of Kaukau and other peaks above it shows that the relief remained fairly strong.
It is not probable that this cycle was the one which began when the folded range first rose above the sea, for planation might be expected to
[Footnote] * Locally pronounced Caw-caw, and spelt on some maps Kaka.
be much more complete. The longitudinal drainage corresponding to the strike appears to have been established during the Kaukau cycle and a hypothetical earlier erosion period, for the adjustment of stream-courses to structure which has been preserved in later cycles points to prolonged denudation, and in rocks presenting but slight variation in hardness it is unlikely that anything like complete adjustment could be attained in a period as brief as that occupied by later cycles.
While some adjustments may have been completed in the Tongue Point cycle, there is no doubt that most streams in the initial stage of that cycle followed subsequent courses.
Owing to a peculiar set of circumstances, referred to elsewhere, the captures that have taken place during the present cycle have to some extent destroyed rather than completed the earlier adjustment. It is possible that some of these retrograde changes took place as far back as the Tongue Point cycle.
The Tongue Point Cycle.
The stage reached in the Tongue Point cycle was adolescence or early maturity. In the streams of the Makara-Ohariu system (fig. 2), for example, the stream-courses were graded, and the valley-floors occupied by
On the foreground and on right and left are benches of the flood-plain of the Tongue Point cycle.
broad flood-plains, of which abundant traces remain as benches along the sides of the valleys now trenched by the deep, young valleys of the revived streams, and scored across by the young ravines of insequent tributaries.
Fig. 4 represents the valley of the Makara. The sketch was made from a broad bench of the flood-plain of the Tongue Point cycle. Portions of this are seen also on the other side of the valley. In Plate XVIII, fig. 1, a view is given, looking southward, up the valley from about the same point. It shows the elevated flood-plain of the Tongue Point cycle on the left, and in the centre the later, steeper-grade flood-plain developed by the stream, in a graded reach, in the present cycle. By lateral swinging and migration of meanders on this flood-plain the stream has cut back the valley-slope on the right to a steep scarp.
Divides in the Tongue Point cycle have in some cases been reduced to a fairly low altitude. Where they stand only 600 ft. to 800 ft. above present sea-level they have been rounded and their slopes graded, and rock outcrops are few. Higher-standing ridges are more rugged; with rock outcrops and sharpened summits, except where they are flat-topped, and forms of the Kaukau cycle remain.
The broad upland features in fig. 3 belong to the Tongue Point cycle. The Ohariu and other streams, however, shown in the figure have been revived, and reaches have been graded and widened with the formation of flood-plains. These belong to the present cycle.
The Present Cycle.
Forms of the present cycle comprise the steep lower slopes of valley-sides and the flood-plains developed along portions of the courses of the larger streams. The Makara-Ohariu system may still be retained as an example (figs. 3 and 4). The streams are not yet graded throughout their length, but a number of flat-floored graded reaches have been worked out, the flood-plains of which are extensive enough to be cultivated. These reaches are invariably strictly parallel to the strike of the rocks. The long graded reach of the east branch of the Ohariu shown in fig. 3, for example, is incised only to a depth of about 50 ft. below the flood-plain of the earlier cycle. Where it turns sharply to the south-west its bed is 250 ft. above the sea. It then follows an entrenched meandering course in a young gorge diagonally across the strike, and falls 240 ft. in two miles.
The present cycle, therefore, cannot be said to have passed its early youth.
The Port Nicholson Area.
East of the ridge upon which Kaukau Peak stands there is a complicated topography, the result of subsidence of the Port Nicholson block. The writer is inclined to believe that either the original boundaries of the subsided block were broad flexures rather than faults, or, on the other hand, the original subsidence took place so long ago that topographical evidence of faulting has been obscured by subsequent denudation. The numerous fault-lines suggested by Bell* run parallel with the strike, and for this reason old faulting along these lines would not be rendered recognizable by revival of erosion.
The north-western portion of Port Nicholson was, however, un doubtedly let down by faulting at a relatively recent date, for along the north-western shore of the harbour there is a clearly defined fault-scarp (see fig. 9). Faulting along this line (the Wellington fault, see p. 257) took place perhaps along with, but more probably after, the submergence of the main portion of the Port Nicholson block. The fault and, in general, the subsidence of the whole block have provided the drainage of the whole area with a much shorter and therefore steeper descent than it formerly had. Two of the active torrents which descended the steep slope quickly succeeded in cutting back so as to tap the drainage of a broad mature valley, the floor of which stood 500 ft. and more above present sea-level (see “Changes in the Drainage of the Karori-Khandallah Valley,” p. 262).
[Footnote] * Trans. N.Z. Inst., vol. 42, 1910, map and section, pp. 537, 539.
The downthrown area is partly covered by the harbour of Port Nicholson, which occupies the seaward portion of the drowned valley of the Hutt River, and also portions of some smaller valleys which appear to have been tributaries of the now dismembered Hutt. The date of subsidence, whether before or after the beginning of the present cycle in the high-standing block, has not been deduced with certainty from the outlines of the partly submerged Miramar Peninsula and adjoining ridges. Mature slopes are now the rule, and younger slopes, if they have existed, are submerged. The deep water that is to be found over the greater part of the harbour (fig. 5) is an indication either of a great amount of sinking of the submerged block, or, on the other hand, of the recent date of the subsidence. Enormous quantities of waste must have been delivered to the harbour by the streams which enter it along the Wellington fault and have cut their gorges in the post-faulting period. Since tidal currents are insignificant to prevent silting, the range being only 3 ft. to 4 ft., the existing freedom from shoals must therefore be taken as an indication of great initial depth and large initial capacity of the basin. The Hutt River, entering at the northern end, has already built an extensive delta of sand and gravel, but the enormous loads of waste tipped over the fault-scarp by the Kaiwarra and the Ngahauranga have not been revealed even by the uplift of 5 ft. which took place in 1855 (see p. 259). Fig. 5, which is a rough contour-map of the harbour-floor, gives an idea of the manner in which sediment is being evenly spread out as a flat layer in the deep water of the harbour. It will be noted that the shallowest water is near the entrance, where a dredge is at work lifting sand and shells. The shallow water at the entrance appea [ unclear: ] rs to be due to the accumulation of waste broken by wave-action on the outer coast.
The material furnished by marine erosion on the outer coast has completely blocked one former entrance to the harbour. A bar of sand, or tombolo,* has converted a former island into a peninsula (Miramar Peninsula), and divided a former channel into two bays (Lyall Bay and Evans Bay). A good example is here afforded of the manner in which a coast-line is straightened (regularized) by wave-action, as described by Davis† and by de Martonne.‡
[Footnote] *See F. P. Gulliver. “Shore-line Topography,” Proc. Am. Ac. of Arts and Sci., vol., 34, No. 8, 1899, p. 189.
[Footnote] † “The Outline of Cape Cod,” Proc. Am. Ac. of Arts and Sci., 1896; reprinted in Geogr. Essays, 1909, p. 690.
[Footnote] ‡ “Géographie physique,” p. 685; Paris, 1909.
The diagram (fig. 6) is an attempt to explain graphically the evolution of Miramar Peninsula. It does not appear that the channel thus blocked had ever the importance of the present entrance, which has from the first been the main channel, and is the continuation of the Hutt Valley.
Mr. Elsdon Best has drawn the writer's attention to an authentic Maori tradition, first put in writing about 1850, which relates some episodes in the history of the locality some seventeen generations ago (i.e. about the end of the fifteenth century). It appears that before that period Evans Bay and Lyall Bay were connected by a channel, which was probably kept open by the tide through the growing sand-bar. The tradition relates that, when a party of Maoris had retired to the island (Miramar Peninsula is clearly indicated) with all the available canoes, another party, pursuing them, were compelled to build rafts to effect the crossing. An account is given also of an event which appears to have been an earthquake accompanied by elevation of the land. By that movement the channel was finally closed.
Fig. 6—Diagram of Evolution of Miramar Peninsula (A Land-Tied Island).
In the lower diagram Evans Bay (opening to Port Nicholson) is on the left, and Lyall Bay (opening to the ocean) on the right. Spurs running down both to Evans Bay and to the ocean have been out back by marine erosion, and rock platforms indicating their former area have been exposed by a recent movement of elevation. These are much more extensive at the seaward end, but even on the shore of Evans Bay a moderate amount of cutting has been done by the waves raised on the waters of Port Nicholson by the prevailing north wind. The sand-bar joining the island to the mainland must have been formed at an early stage, for the spurs running down into it have been almost completely protected from marine erosion. The upper diagram is a restoration of the initial form of Miramar “island.”
The case of Miramar Peninsula is therefore one where island-tying has been assisted by a slight movement of the land.* It seems probable that without a slight movement of elevation a shallow channel would always have been kept open through the bar by the tide.
In a quaint paper by Crawford,† entitled “Port Nicholson, an Ancient Fresh-water Lake,” the view was advanced that the present entrance had been opened quite recently by the sea, and that over a dam of boulders in the Evans Bay—Lyall Bay channel the waters of a fresh-water lake formerly escaped and cascaded down to join the “great Cook Strait liver.”
The small channel appears to have been formed by the drowning of two small streams, one flowing north and the other south, separated by a low divide which is evidently not deeply buried, for the spurs running down from opposite sides into the sand-bar are not widely separated.
[Footnote] * See Gulliver, loc. cit., p. 200.
[Footnote] † Trans. N.Z. Inst., vol. 6, 1874, p. 290.
From the above description and from fig. 6 it will be gathered that the coast-line of the downthrown Port Nicholson block is a normal drowned coast, passing through the normal cycle of littoral erosion which has reached the early mature stage. It is thus in strong contrast with the coasts of the neighbouring high-standing blocks described in the next paragraph.
The actual outline of the coast of the high-standing block is the result of marine erosion working back from an earlier coast-line almost certainly bounded by fractures. This seems to be the only view tenable, for the amount of marine erosion necessary to cut back the present coast, with its line of lofty cliffs, from a coast-line of any other form would be enormous, and seems out of the question when a comparison is made with the recently revived condition of the land-drainage. There is no evidence of a slow sinking of the land such as would be required to keep up the activity of wave-action on a receding coast. The depths of hundreds of fathoms recorded within a very few miles of the southern coast indicate that the block to the south has sunk, and the closeness of the hundred-fathoms line to the western coast indicates subsidence in that direction.
The hypothesis of a fracture-bound coast gains further support from the relation of the coast-line to stream-courses on the land-surface. The Silver Stream (see fig. 2) rises at a height of 1,000 ft. only three-quarters of a mile from the southern coast, and flows northward. The western coast also cuts in along a north-easterly line, making an angle with both the strike of the rocks and the stream-courses. The Ohariu Stream, on the north-west, like the Silver Stream on the south, rises almost on the coast, and flows inland.
Cook Strait, which bounds the Wellington Peninsula on the west and south, has been generally regarded as the result of faulting since the time of Hochstetter, whose views were followed by Hutton and more recently by Park. Hochstetter's early view* was that one island had been thrust laterally past the other—that is, that the movement was of the nature of a “flaw.” As has been pointed out by Suess, however, Hochstetter's later view† was that Cook Strait owed its origin simply to the subsidence of a mountain block or blocks, and he was aware that the continuation of the North Island ranges is to be found on the same line of strike in the Kaikoura Mountains of the South Island. This relation is brought out by Marshall's‡ maps of physical features of New Zealand.
The west and south coasts present similar features. The only projecting points are those composed of resistant rock, usually bands that are hardened with interlacing veins of quartz, filling joints. The intervening, less resistant rock bands recede as bays of gentle curvature, bounded by imposing cliffs. The larger streams emerge at beach-level, in gorges revived and steepened by the rapid recession of the coast,
[Footnote] * Lecture on the Geology of the Province of Nelson, 1859, reprinted in “Geology of New Zealand” (Auckland. 1864), p. 106; see also Park's “Geology of New Zealand, 1910. p. 262.
[Footnote] † ‘Novara,’” 1864, Geol. vol. 1, p. 2.
[Footnote] ‡ Loc. cut., pp. 10, 11.
while the smaller ravines are truncated, appearing as notches, hanging at various heights on the cliffs. Wherever the lower reach of a stream makes a small angle with the coast the spur separating it from the sea has the form of a razorback, due to lateral cutting by the stream on one side and the sea on the other.
The coast-line is, therefore, a continuous line of stupendous cliffs, rising in places on the south coast, where the coast-line cuts across the highest ridges, to 700 ft. or 800 ft. In Plate XIX, fig. 1, a portion of the south coast is seen eastward from Sinclair Head. The triangular cliff-facet photographed is 400 ft. or 500 ft. in height. To the west the height of the cliffs increases.
The Coast Platforms.
Along parts of the coast no relics remain of elevated platforms cut by the sea during pauses in the movement of uplift. They have either been completely cut away by the waves or cut off by faulting along new lines of fracture. At other places extensive shelves remain. The most prominent begins at Tongue Point and extends some distance west-ward (see fig. 7, and Plate XVIII, fig. 3). The shoreward edge of this shelf
appears to indicate the base-level at the time when the streams of the district developed the greater part of the existing upland topography. For that reason the writer has named that erosion cycle the Tongue Point cycle.
The height of the shelf at its inner edge at Tongue Point is 240 ft. Its slope seaward is at first 10°, but rapidly decreases, and at the end of Tongue Point, where the shelf is half a mile broad, it runs gently out at an angle of 2° or 3°.
The upper surface of the shelf is covered by a veneer. 6 ft. or 8 ft. in thickness, of gravels similar to those of the present beach. They vary irregularly from beds of coarse roughly rounded gravel and boulders, material similar to what is being supplied to-day in large quantities by the smaller streams, to layers of fine flattened discs of beach-shingle varying from the size of a threepenny-piece to that of a penny. A layer of the coarser gravel is seen on the right in Plate XVIII, fig. 3.
The varying height of the outer scarp of this marine terrace as seen from the sea is clearly due mainly to the varying breadth of the portions that have withstood the action of the sea, the seaward slope of the shelf being regarded as nearly constant. At the extremity of Tongue Point it comes down to 170 ft. Beyond the next creek to the west, where there is a well-preserved but narrower remnant, the outer edge bounded by the present scarp is, as might be expected, higher. It is evidently this apparent variation in the height of the shelf that
led Park* to remark that he had satiafied himself “that it was not an uplifted marine platform of erosion.” It may be remarked that a section, even on a vertical plane through a coastal platform, parallel to the average direction of the coast must not be expected to yield a perfectly horizontal crest. It ought to show a convex crest opposite to bluffs, where the old coast approaches it, and a concave outline opposite bays, where the old coast recedes. To this initial irregularity there may be added slight variations in the amount of subsequent uplift. Remnants of this terrace extend nearly to Cape Terawhiti, and it may be seen also at Te Kaminaru Bay, on the western coast.
There exists also a higher shelf, which was examined at Tongue Point. It may be seen in fig. 7. Its height is about 450 ft., and, like the lower shelf, it is covered with a layer of water-worn pebbles. Its width at the point examined had been reduced by the cutting of the lower shelf to about 50 yards.
At Baring Head, on the coast south-eastward of Pencarrow Head, at the entrance to Port Nicholson, similar shelves occur,† and again at Cape Turakirae. They may be seen from the deck of a steamer entering Port Nicholson. The sketch, fig. 8, represents them as seen from
Fig. 8.—The Elevated Coast Platforms Between Pencarrow Head and Baring Head, as Seen From The, Signal-Station On Miramar Peninsula.
Pencarrow Head in centre, Baring Head on right.
the signal-station on Miramar Peninsula. They are cut through by the revived Wainuiomata. The writer has not examined these platforms closely, but believes they correspond in a general way to those at Tongue Point, the sunken area of Port Nicholson lying between. The highest platform at Baring Head appears to be about 500 ft. above sea-level. It has been shown above that the general outlines of the coast appear to be determined by subsidence of land blocks, but, on the other hand, it cannot be assumed that the whole of the uplift of which we here have evidence is differential uplift along these lines of fracture. At many places on the New Zealand coast marine platforms and raised beaches are known, indicating uplift of varying amount.‡ McKay has recorded Recent shells on a beach at a height of 500 ft. at Amuri Bluff, about eighty miles south-west of Wellington. If this beach can be correlated with the highest shelf at Wellington it may indicate that the stretch of land between has moved as a whole. The latest movement, which took place in 1855, and was described by
[Footnote] * Trans. N.Z. Inst., vol. 42, 1910, p. 586, and fig. 3.
[Footnote] † See Park, loc. cit., p. 585, fig. 2.
[Footnote] ‡ See Marshall, loc. cit., p. 31.
Lyell,* affected both sides of Cook Strait. It was, however, a tilt to the west, which depressed the western shore of the strait and elevated the Wellington side as a whole—that is, the area shown in fig. 1—by an amount varying from zero on the north-west to about 9 ft. on the south-east. The raised beaches of the Wellington coast which owe their elevation to that movement have been described and figured by Bell.† They may be seen also in Plate XVIII, fig. 2, and Plate XXI, fig. 2. Both views are of parts of the eastern shore of Miramar Peninsula.
There is some evidence that this tilt is a continuation of an earlier tilting movement in the same direction, the axis of the movement lying a little to the west of Wellington. On the south-east a series of very fresh raised gravel beaches at Cape Turakirae, the highest being 90 ft. above the sea. are mentioned by Aston.‡ On the north-west there appears to have been a downward movement of small amount subse-quently to the general movement of elevation the proofs of which have been given. This movement, which has drowned the lower reach of the Porirua Stream, does not appear to have been more than 30 ft. or 40 ft. The stream had previously developed a broad strip of flood-plain, and this has been drowned to a distance of about a mile and a half from the sea. At Porirua there appears to have been little or no movement either up or down in 1855. Raised rock platforms similar to those at Wellington are not found. This agrees with the accounts of eye-witnesses given in substance by Lyell.§
The Wellington Fault.
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] * “Principles of Geology,” 10th ed., 1868, vol. 2, p. 82.
[Footnote] † Trans. N.Z. Inst., vol. 42, 1910, p. 538, and pl. 41 and 42.
[Footnote] ‡ B. C. Aston, this volume, p. 208.
[Footnote] § Loc. cit.
[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.†
Nature of the Movement.
The fact that the portions remaining of the scarp along the faultline are inclined back at an angle of about 55† may indicate that the surface along which movement took place had that inclination. On the other hand, if the fault-plane were steeper the slope would quickly be reduced by slipping along the crest of the high 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.
From the absence of slipped material along the base of the Wasatch Range, in Utah, Davis* argued that the slope of the spur-facets now found there gives the inclination of the plane of faulting. In the case of the Wellington scarp, however, it is uncertain whether a scree of slipped material exists or not beneath the water and silt of the harbour. Nor can the very even slope of the facets throughout the length of the scarp be taken as an indication that they represent the actual plane of faulting. Their slope appears rather to be “the angle of rest for the products of decay” of the material of which they are composed. The writer cannot agree with Bell† that the slope is steeper than the angle of rest. It is clear that many, if not all, of the clearly defined, sharp-edged facets owe their actual form to waveaction at their bases, the extent to which the scarp has been thus cut back being indicated by a narrow wave-cut platform at its foot. This, however, seldom reaches a width of 40 or 50 yards, and part of it may represent a levelled-off scree of slipped material. It is now almost entirely covered by the railway-embankment along the shore.
Reasons have already been given for believing that the actual movement has been subsidence of the block to the south-east (p. 258). It was assumed by Bell‡ that the faulting movement was one of block elevation and tilting towards the north-west, and the Porirua Stream was cited as an example of a stream flowing down the tilted back slope of the block. There is no doubt, however, that the Porirua followed its present course before faulting took place. It follows one of the old strike valleys. In the valley there is evidence of recent revival, but not such as would be required by a tilt of the magnitude assumed; it appears to be due solely to the general movement of uplift which has affected the Wellington Peninsula, although perhaps not everywhere by exactly the same amount. The drowning of the lower Porirua may be ascribed to a less-extensive later tilt of a much larger block of country (see p. 257).
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] * Bull. Mus. Comp. Zöol., Harv., vol. 42, No. 3, 1904, p. 158.
[Footnote] † Loc. cit., p. 536.
[Footnote] ‡ Loc. cit., p. 539.
[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.
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.
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.
Changes In Drainage Of The Karori-Khandallah Or Long Valley.
This old valley might be called the Karori-Khandallah Valley, from the names of two important settlements in it. For the sake of brevity, it is here called the “Long Valley.” Its line is now followed by the Silver Stream, the Kaiwarra and its tributaries, the upper Ngahauranga, and the Porirua. In fig. 2 the line of the old valley is indicated as a double broken line, and farther north by the line of the Manawatu Railway. Starting at the head of the valley and following it northward, we may note the changes that have taken place. At the head of the Silver Stream, which occupies the southern end of the valley, the divide is now 1,000 ft. above the sea, and the old valley appears to have continued still farther southward, the divide now being rapidly pushed northward by the activity of torrents of the south coast. Two miles and a half from its source the Silver Stream turns very sharply
[Footnote] * Loc. cit., p. 1.
to the west, and finds its way to the sea as a tributary of the Karori, having thus a roundabout course eight miles in length. As indicated in fig. 2, the capture of the Silver Stream by the Karori is a double one, two branch ravines of the Karori tributary having successively tapped the coarse of the Silver Stream. The floor of the old Long Valley here stands about 840 ft. above sea-level. The deepening of the captured stream at the elbow of capture is 400 ft. or 500 ft. Northward from this divide the Kaiwarra, which here occupies the Long Valley, descends somewhat rapidly in a trench incised in an older mature valley-floor. At the upper reservoir (U.R. in fig. 2) it follows entrenched meanders of small radius, and a portion of the old flood-plain on which the meanders originated remains as a bench far above the present stream and at a height of 660 ft. above the sea. At this point a mature dry valley on a level with the old flood-plain bench, evidently the old stream-course, swings off to the north, while the course of the Kaiwarra, flowing north-east, is a young gorge. The sketch, fig. 12, shows the old valley and the young gorge of the Kaiwarra.
Following the old dry valley mentioned above, we find ourselves in the broad mature valley occupied by the settlement of Karori. It has been invaded by the head of the Karori Stream from the south-west, as well
Fig. 12.—Capture Of The “Long Valley” Stream By The Kaiwarra.
Upper reservoir on the left; young gorge of the Kaiwarra below the upper-reservoir dam on the right.
as by the Kaiwarra from the north-east. The north-eastward continuation of the now broad and mature Long Valley through Ngaio and Khandallah is evident, but between Karori and Ngaio the floor of it has been almost completely gouged out by the numerous young deep-gorged tributaries of the middle Kaiwarra. Overlooking the Kaiwarra there are, however, abundant stream-deposits in Karori, and a bed of gravel on the western slope of the Tinakori hills at a height of 600 ft.
The lower Kaiwarra leaves the Long Valley by a steep-walled gorge, and crosses the scarp of the Wellington fault. The north-eastward continuation of the valley is occupied next by a short obsequent stream, a tributary of the Kaiwarra. Farther on, at Khandallah, it is crossed by a stream which joins the Ngahauranga near its mouth. Still farther to the northeast the valley has been invaded by the Ngahauranga, a stream which, owing its activity to its position on the fault-scarp, has worked back in a profound gorge along a nearly straight course at right angles with the
fault-line until reaching the Long Valley. It has reversed the drainage of the Long V [ unclear: ] alley for a mile and a half. It follows a winding course, but the tapering shape of the spurs on the concave sides of the meander-curves indicates that the winding character is due, at least in part, to lateral cutting that has accompanied the deepening of the gorge. At one point a narrowed and almost cut-off spur is a conspicuous feature in the Ngahauranga Valley. Plate XXI, fig. 1, is a view looking north-east across this spur and up the valley. The height of the narrowed neck above the stream on the downstream side is 200 ft., and on the up-stream side 90 ft. Its breadth is about 100 yards, while the distance roundabout by the course of the stream is three-quarters of a mile. Beyond the divide, 500 ft. above sea-level, at the head of the obsequent Ngahauranga, is the head of the Porirua Stream, which, robbed of two-thirds of its ancient length, still occupies the northern end of the Long Valley. Probably this was the outlet at the close of the Tongue Point cycle.
The cause of most of the captures in the Long Valley is, as has already been indicated, the subsidence of the Port Nicholson block, particularly along the line of the Wellington fault, giving a short descent to sea-level. With regard to the Silver Stream, it seems remarkable that its capture had not taken place earlier and in a less roundabout way than the present outlet to the Karori Stream. A reasonable explanation seems to be that in earlier times, when streams followed the Long Valley and the other main longitudinal valleys of the Wellington Peninsula, the peninsula formed part of a land-area extending to the north-west and to the south far beyond its present limits. Reasons have already been given for the writer's belief that the present coast was determined by fractures after the main lines of the present drainage were established.
Type of Topography.
A consideration of the courses of streams and the elongation of ridges of the Wellington district leads to the conclusion that, apart from local complications due to unequal vertical movement, the topography of the south-western end of the North Island mountain-chain is of the Appalachian type—namely, an old, folded range subjected for a sufficient time to denudation to bring about longitudinal drainage by subsequent streams adjusted to structure, not following original synclinal folds, and afterwards elevated sufficiently to allow dissection by revived streams to produce a surface of strong relief. The analogy with the Appalachian Mountains must not be pushed too far. For example, planation in the earliest cycle seems to have been far from complete, and the absence of transverse streams following antecedent courses is especially noticeable. Their unfortunate absence accounts for the difficulty of railway-construction between Wellington and the western coast. In spite, however, of the obvious differences the remarkable similarity of our range to the Appalachians is brought out by a comparison with Lesley's map of Pennsylvanian topography, repeated by de Lapparent,* or with the detailed maps of smaller areas given by Salisbury and Attwood.† It may be noted that the “great Cook Strait river” of Crawford,‡ it if existed, must have been transverse for part of its course; but reasons have been given above for believing that Cook Strait is not a drowned river-valley.
[Footnote] * “Lecons de Géographie physique,” 1907, p. 613.
[Footnote] † U.S. Geol. Survey, Prof. Paper 60, 1908, especially pl. 5 and 56.
[Footnote] ‡ Trans. N.Z. Inst., vol. 7, 1875, p. 448.
The following conclusions have been reached:—
(1.) The south-western extremity of the North Island of New Zealand is probably a horst isolated by subsidence of land blocks on the west and on the south, and possibly on the east also.
(2.) The drainage-system has been developed by normal processes during a long period of elevation punctuated by pauses, the amount of elevation being at least 800 ft., and probably more.
(3.) The nature of the longitudinal drainage suggests that adjustment to structure was established in an earlier erosion period.
(4.) A prominent feature, Port Nicholson, has been produced by the subsidence of a block along lines which, with one notable exception, have not been clearly recognized.
(5.) This exception is the line of the Wellington fault, along which fault scarp topography is well developed.
(6.) Recent changes of drainage have had the effect of destroying, rather than completing, previous adjustment to structure.
(7.) This is attributable to the activity of transverse streams on and near to fault-scarps.