The Makara and Karori Valleys and Their Bearing Upon the Physiographic History of Wellington.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 69, 1940, Page 401

The Makara and Karori Valleys and Their Bearing Upon the Physiographic History of Wellington. By Maxwell Gage, M.Sc. [Read before the Wellington Branch, July 14, 1937; received by the Editor, September 9, 1939; issued separately, March, 1940.] Introduction. The valleys named in the title of this paper exhibit numerous and varied terrace features from which evidence is obtained of a series of changes of sea-level, and at least one instance of tilting or warping of the land-surface of the south-western Wellington district. In his first general account of the physiography of Wellington, Dr. C. A. Cotton (1912, pp. 250–51) described the terraces of the Makara Valley, and the present writer has carried the study of these forms southwards into the Karori system and to the south coast. The sequence of events as visualized by Cotton is applicable to the southern area with certain modifications. Note on the Rocks of the District. The basement rock consists of the characteristic greywacke-argillite association generally regarded as belonging to the Hokonui System (Benson, 1922, p. 1), overlain by alluvial deposits ranging in age possibly from late Pliocene to present day. Except for an enigmatic occurrence at Makara of a littoral type of sediment with poor marine fossils (see later), Cretaceous and Tertiary strata are entirely absent. Cotton believed it possible that the Wellington area and contiguous portions of the South Island may never have been submerged since the post-Hokonui movements. Terrace Features. A most conspicuous feature of the middle reaches of the Makara Valley is the prominent set of alluvial terraces related to a base-level about 120 ft. to 140 ft. higher than present-day. In the neighbourhood of the junction of the Makara and Ohariu Streams only rather indistinct spur-shoulders are to be seen, but as we pass upstream, definite terraces appear at this level and increase in width until at the Makara Golf Course the present-day stream actually has its source upon the floor of the old valley, which continues with a broadly open cross-profile into the area drained by the modern Karori Stream. Upstream, the terraces again diminish rapidly in width, owing to lateral planation in the later cycle by the south-flowing Karori, disappearing finally near the confluence of the two branches of this stream. The last trace in this direction of features related to this 120 ft. base-level is probably represented by a curious rock-bench remnant some 230 ft. above the floor of the modern Karori Gorge at a point about 10 chains downstream from the junction. Its surface is rather irregular in longitudinal section, but it slopes evenly at

about 10 degrees towards the edge of a sheer drop to the modern valley floor, and evidently was sculptured by a near-source tributary of the once southwards-extended ancestor to the modern Makara. The prominent Makara terraces increase in height above sea-level, although more slowly than the modern valley floor, until at the Golf Course they are at 300 ft. From this point southwards, however, they fall steadily and a short distance upstream from the confluence of the Karori branches are 100 ft. lower. It is difficult to see how the terraces which have been described could have originated otherwise than by the work of a stream flowing north-westwards, for the cross-profile of the older valley as a whole increases in width in this direction, ancient alluvium at the same time becoming more extensive until near the Ohariu junction, where recent lateral planation by the Makara and its tributaries tends to eliminate it. Loss of height of the terrace surface southwards from the Golf Course at the present Makara-Karori divide is ascribed to warping of the surface, which will be treated more fully later. In the valley of the main branch of the Karori Stream, this system of terraces is represented by flanking deposits of alluvium in the vicinity of the Karori tram terminus. Near to its source, the modern stream appears to be flowing on the floor of a valley sculptured in an earlier cycle, as in the case of the Makara and Karori West Branch Streams, but as we pass downstream, it becomes rapidly entrenched to about 90 ft. at the end of the South Karori Road. At the same time it has carried on lateral planation sufficient to break up the alluvial terraces into discontinuous remnants and to eliminate them altogether beyond the end of the road. Downstream from here, the valley as a whole has become much narrower, a few spur-shoulders and a distinct change of slope of the hillside alone remaining until the junction with the West Branch is reached, but there is little doubt that these features correspond with the Makara terraces. In the lower reaches, the Karori is a strikingly youthful stream, its steep, unbroken valley-sides being devoid of high-level terraces, although raised marine platforms exist on the south coast. In the Ohariu Valley, similar but less complete dissection of the prominent terraces was noted by Cotton (1912, p. 251). Except for a short gorge near the junction with the Makara, the modern stream is not deeply entrenched in the earlier flood-plain. Indistinct higher terraces are visible near Karori Park, diminishing in relief northwards until their surface appears to merge into the floor of the important ancient valley system described by Cotton under the name of the “Long Valley.” (1912, p. 262.) The last system of terraces to be described is peculiar to the upper reaches of the Karori Stream and due to entrenchment, increasing downstream in both branches to a maximum of about 20 ft., below a sub-recent floodplain of moderate extent at the junction of the two main branches. Raised Coastal Benches. At Tongue Point, of the two distinct flights of elevated, wave-cut benches, the lower and more extensive was recognized by Dr. L. C. King (1930, pp. 500-2) as a continuation of a similar remnant a

Cape Terawhiti, increasing in altitude from 120 ft. there, to twice that figure at Tongue Point, and thence dipping steadily eastwards. East of Karori Stream both sets disappear, seemingly as a result of rapid coastal recession along a section of the coast where fault-crushed rocks favour wave-attack. Still farther east, resistant rock again occurs, but the benches do not reappear; their eastward inclination alone would be sufficient to have caused them to plunge beneath the sea and be masked by sediments near the entrance to Port Nicholson. Meagre remnants of other elevated wave-cut platforms occur at Te Kaminaru Bay and at the southern extremity of Ohariu Bay, differing not greatly in height in either case from the Terawhiti platform. Elsewhere they appear to have been destroyed by vigorous cliff recession, which has also betrunked a former tributary of Makara Stream known as Smiths Gully. Physiographic History. Although Cotton (1916, p. 41) considered that the land to the north-west and to the south of the present Wellington Peninsula subsided beneath the sea, he did not attempt to fix the dates of the founderings. So great are the recognizable physiographic changes since this commencement of the evolution of the modern landscape, despite a preponderance of resistant rock, and so long the lapse of time involved that the writer feels justified in assuming the great subsidence to have taken place during a crisis of the Kaikoura Orogeny. The Kaukau Cycle. The wide-open valleys and gentle relief of the earliest recognizable erosion cycle, the Kaukau Cycle of Cotton (1912, p. 248), were only in part controlled by structure at the close of the period. They might be termed sub-senile, and the rock beneath the old surface in the few places where exposed is very deeply weathered. In addition to Cotton's examples, other discontinuous fragments of the Kaukau surface exist on the ridge between Karori and Waiariki Streams, as well as on others nearby and on the divide between Island and Ohiro Bays, all at different heights above sealevel. A few eminences such as Mt. Hawkins, rising abruptly above the general level of the ridges and surface remnants must represent monadnocks of Kaukau times. In carrying out surface reconstructions based on present-day relations between fragmentary remains, it is fully realized that these may have been disturbed by deformational movements of which we have no knowledge. Nevertheless, from higher viewpoints indications can be seen of an ancient valley system, now occupied by the Makara and the Karori. A branch, separated from the main valley by the modern Makara ridge, is occupied by the east branch of the Karori. Northwards from Karori Park, traces of this branch of the Kaukau Cycle valley are indistinct, mainly owing to dissection by the “Long Valley” system. The valleys of the Kaukau Cycle extended well beyond the present land, but it is difficult to tell in which direction the stream flowed in the ancestral Makara-Karori Valley, as the remaining traces may have been displaced by non-uniform earth movements. The lowest areas of the Kaukau surface are near the mouth of Makara

Stream, while in the South Karori district points which must have been near the middle of the old valley-floor are 200 ft. to 300 ft. higher. On the other hand, the manner in which the branch valley joins the main one suggests a direction of flow from north to south. Post-Kaukau Uplift. The sub-senile land-mass was elevated about 600 ft., and immediately active degradation was resumed during a fairly long stillstand until the streams, following courses inherited from the previous cycle, became graded in their lower reaches with respect to a base-level not greatly different from that of to-day. Cotton, however, postulated a change of level equal only to the vertical interval between the Kaukau surfaces and those of his Tongue Pont Cycle, represented by the major Makara terraces and the lower Tongue Point platform. The reasons for favouring the greater uplift involve consideration of the Makara fossils which will now be dealt with. The Makara Fossils. Fragments of massive, marine, worn and broken oysters are contained in a fine conglomerate consisting of fairly well rounded greywacke and argillite debris in a sparse, muddy matrix which outcrops in a small branch of the Makara Stream. The deposit was first described by A. McKay (1877, p. 50), who considered it to be Miocene in age. F. K. Broadgate (1916, p. 78) thought that, on the physiographic evidence alone the beds could not be younger than Pliocene, and that they are a remnant of a once extensive Tertiary cover preserved from erosion locally at Makara by down-faulting; but he cited no facts in support of his contention. Dr. Marwick informed the writer that merely from the massive type of oyster that the indeterminable shell fragments represent, and from the degree of induration of the matrix, he would infer a probable Pliocene age for the deposit. Against Broadgate's faulting hypothesis is the absence of signs of the implied post-Pliocene faulting in the vicinity, although zones of intense faulting show in the greywacke nearby. It is unlikely that a portion of a widespread Tertiary cover has been preserved in a depression of the undersurface as other relics would surely have been found. The deposit is confined between fairly steep greywacke walls, but the actual contact with the basement rock is not exposed. A fossiliferous bed about 2 ft. thick underlies 6 ft. of barren conglomerate, while at another locality, 200 ft. distant across the valley, poorly rounded fine conglomerate indistinguishable from the barren layer merges upwards without visible break into consolidated gravels such as make up the bulk of the main Makara Valley terraces. Stratification is poor, but neither folding nor tilt were detected. Re-submergence. The writer considers that the post-Kaukau Cycle elevation was sufficient to enable the sculpturing of youthful valley systems below the previous cycles to a lower level than the floor of the present Makara Valley, but that before much widening was effected a reversal of movement permitted estuarine waters to invade the land, the Makara shell-bed being deposited at the head of an estuary. Submergence continued, but was either intermittent or slower than the initial plunge, so that bay-head progradation was able

Tongue Point-Cycle Terraces in the Makara Valley

Divide between Karori West Branch (left) and Makara Stream (right) on Tongue Point-Cycle flood-plain surface at Makara Golf Course.

Fig. 1.—Mature spurs carved from Tongue Point-Cycle valley-floor at Makara-Ohariu confluence. Fig. 2.—Smiths Gully, Shortened by cliff-recession under vigorous marine erosion, viewed from across Ohariu Bay.

Fig. 1.—Mouth of Karori Stream and Tongue Point, showing elevated, wave-cut platform. Fig. 2.—Lower reaches of Karori Stream. Compare width of flood-plain with middle reaches of Makara, plate 51.

Fig. 1.—Gravel beds on valley-side, South Karori Road. Fig. 2.—Surface of southernmost gravel terrace in Karori Valley, end of South Karori Road.

Fig. 1.—Spur-shoulders in Karori Valley downstream from terrace shown in plate 55, fig. 2. Fig. 2.—Entrenchment of modern Karori Stream, ascribed to acquisition of volume following on captures.

not only to maintain the shoreline, but evidently also to cause it to advance down the valley. Delta deposits and then flood-plain alluvium were laid down in turn over the littoral material, deepening as depression of the land continued, until base-level was raised to the level of the highest set of marine erosion benches on the south coast. Then occurred a pause long enough to allow of the sculpture of these features. The Tongue Point Cycle. At the time of the above-mentioned pause, valleys carved earlier from the Kaukau surface were deeply filled with alluvium. This is the interpretation placed upon the terrace gravel deposits of the middle reaches of the Makara Valley, of the “Long Valley,” and perhaps of Tinakori Valley, laid down in eroded depressions below the level of the Kaukau Cycle remnants, although the Tinakori example is obscured by the Wellington Fault. The nearby important valley system of the Hutt contains a great thickness of gravels which may have been deposited under similar circumstances. Following upon the stage of maximum submergence, a series of earth movements, probably with intervening halts, then brought sea-level to its present position. A major halt in uplift saw the formation of the wide, lower, wave-cut bench at Tongue Point, re-excavation of the Makara Valley, and its widening to a mature cross-profile with walls outside the ancient gravel-filled trough. If a Pliocene age may be accepted for the Makara material, it would follow from the above discussion that the Tongue Point period of erosion occurred since that time, but that the Kaukau Cycle is earlier. Makara-“Long Valley” Capture. The high-level terrace remnants near Karori Park mark the most southerly traces of the floor of the “Long Valley” of Cotton. A south-westward flowing branch of the Makara River of Tongue Point times, as mentioned earlier, worked back northwards and captured the headwaters of the “Long Valley” which also then evidently possessed a widely flaring cross-profile, but was dominated over by its westerly neighbour, which probably had the advantage of a more direct course to the sea. Cook Strait Foundering. A glance at the Admiralty chart of Cook Strait (N.Z. No. 695) shows extreme irregularity of the sea floor off the south Wellington coast, except where masked by sediments opposite the entrance to Port Nicholson and the Wairau River mouth, strongly suggested a drowned land surface (Cotton, 1918, p. 325). There is a rather abrupt change to shallower depths about the 50-fathom line, inshore from which is a more even submarine bench, free from sediments in many places, and ascribed to wave-planation acting at a depth somewhat greater than usual owing to the effect of strong tidal scour and the prevalent stormy conditions of the Strait preventing accumulations of protective debris. The line of sudden deepening is believed by the writer to mark approximately the position of a fracture separating the subsided from the upstanding-block. The drowning occurred after the Kaukau Cycle, but necessarily before the uplift of land preceding the Tongue Point Cycle to account for the raised wave-cut platforms. He favours a period co-eval with the sinking discussed in connection with the Makara

fossils, and considers that the subsidence of the southern block may have continued after the northern block halted, with dislocation along the fracture referred to and that the submerged area did not take part in the subsequent rise of the peninsula. During each pause in uplift, the coast has receded from the fracture under wave-attack. To-day recession is proceeding rapidly, and the smaller streams flowing to the south coast cannot cut down rapidly enough, reaching the shore in cascades. Cotton considered (1912, p. 254) that the west coast of the peninsula also is derived from an initial fault coast. It is thought, from the relatively more advanced development of the streams flowing to the west coast compared with those flowing to the south, and from the more mature condition of the west coast, that the foundering of land to the west occurred at a more distant date.* Dr. L. C. King's paper (Trans Roy. Soc. N.Z., vol. 68, pp. 544–569) on the origin of Cook Strait had not appeared when this paper was read. Karori-Makara Capture. Following on the formation of a new south coast much farther north, rapid south-flowing torrents dissected the older valley-floors, steadily pushing the divides northwards. These streams had the benefit of a short steep course to the new coast, and more rapidly experienced the advantages of recurrent uplifts of the land; so it is not surprising that the largest and most successful of them, the Karori, should in time break through into the Makara system, and effect an important capture. One by one the headward tributaries of the Makara were diverted to the south coast, and finally the important branch that effected the “Long Valley” capture vas claimed, giving rise to the Karori Stream as we now know it, flowing in a deep valley far below the remaining traces of the earlier Makara southward extension. A western tributary of the Karori became established in the main Makara Valley, causing the migration of the divide farther northwards, and the formation of the higher set of terraces south of the Makara Golf Course, the terraces along the South Karori road, and the change of slope of the walls of the narrow upper Karori Valley. Post-Tongue Point Uplift and the Port Nicholson Subsidence. King (1930, p. 502) could not make out whether a uniform uplift terminated the Tongue Point Cycle, followed by a later deformation that tilted the coastal platforms and Makara terraces. It seems logical to associate these tiltings with the movement that depressed beneath the sea the Port Nicholson area (Cotton, 1912, pp. 250-4) as phases of the same disturbance. The effects would be produced by a rotation about a north-north-east axis a little east of Karori Stream, lowering to the east and elevating to the west. The Karori Stream had no extensive floodplains such as would leave terraces to bear witness to the uplift in the lower reaches. The western tributary dissecting the Makara Valley had its gradient increased by the warping. The main stream would have been hindered as it flowed obliquely up the dip of the warp, but its steep gradient coupled with rejuvenation must have offset the adverse effects. Cotton

(1912, p. 264) has described captures of portions of the “Long Valley” system by the Kaiwarra and Ngahauranga, vigorous new streams flowing down the tilted surface into Port Nicholson, and their subsequent rejuvenation through the formation of the Wellington fault, across which they flowed towards the downthrown side. Another effect of the warping was the immediate diversion of all the water of the Makara Valley from as far north as the present divide at the Golf Course into the Karori system. This may in part account for the present entrenchment of as much as 20 ft. at the junction of the Karori branches, but in all probability the chief cause of this was cutting down of the west branch necessary to maintain an accordant confluence with the main branch, which had been further rejuvenated by another capture to be described. Silver Stream and Kaiwarra Captures. The Silver Stream, an insequent tributary of the Karori, tapped the headwaters of an earlier high-level stream which followed the shatter-belt of an ancient fault. This may have been part of a lost stream of the Hutt system whose course is now indicated by gravel and silt beds on the east side of Tinakori Valley, as described by Cotton (1912, p. 258). The increased flow of water following upon this capture is believed to be the cause of entrenchment of the Karori. The portion of the old valley below the elbow of capture of the Silver Stream has been added as a result of the Kaiwarra capture to the Kaiwarra system (Cotton, 1912, p. 262). The Uplift of 1855. The writer has little to add to the account of the raised beaches given by Bell (1909, p. 538). It was noted, however, that there are numerous breaks in the continuity of these features, as for example between the Karori mouth and Tongue Point, due either to subsequent removal of the material by the waves, or to the fact that prior to the uplift, the waves at high tide may have reached to the base of the cliffs, preventing the building of the characteristic storm beach. Adjustment TO Structure. The Wellington Peninsula may be regarded as consisting of north-north-east-striking belts of resistant rock alternating with weak zones of fault-crushed material. The zig-zag pattern of the modern streams reveals the influence of these structures upon their courses. Short, gorgy, transgressive reaches are characteristically joined by lengths of wider valley with more open cross-profiles, more or less parallel with the general strike-direction of the rocks. A similar tendency is exhibited by the Tongue Point Cycle forms in the Makara Valley. Again, the Waiariki Stream passes from one weak belt to the next through narrow transgressive water-gaps. The general obliquity to structure of the Makara-Karori lineament as a whole, however, is ascribable to the controlling influence of an ancient valley of an earlier cycle of erosion, within the limits of which it lies. The question then arises as to why this ancient valley disregarded the structural trends. Superposition may be eliminated, as there is no evidence for a stripped Cretaceous or Tertiary cover. We are left with the

remaining alternative suggestion that the common tendency in subsenile phases toward widespread lateral planation and consequent destruction of earlier adjustment to structure was in this case promoted during the Kaukau period by the upsetting of gradients through a diastrophic movement. The crooked course of the Karori is no doubt largely due to its being made up of a number of sections of different streams connected by a succession of captures. Acknowledgment. Before concluding, the writer wishes to express his thanks to Mr. A. W. Hampton, who re-drew the locality plan for reproduction. Literature. Bell., J. M., 1909. Geomorphology of Wellington, Trans. N.Z. Inst., vol. 42, p. 538. Broadgate, F. K., 1916. The Red Rocks and Associated Beds of Wellington Peninsula, Trans. N.Z. Inst., vol. 48, pp. 76–86. Cotton, C A., 1912. Notes on Wellington Physiography, Trans. N.Z. Inst., vol. 44, pp. 245-65. —– 1914. Supplementary Notes on Wellington Physiography, Trans. N. Z. Inst., vol. 46, pp. 294-98. —– 1916. Structure and Later Geological History of New Zealand, Geol. Mag., Decade 6, vol. 3, pp. 243-9, 314-20. —– 1916. Fault Coasts in New Zealand, Geogr. Rev., vol. 1, no. 1, p. 41. —– 1918. The Outline of New Zealand, Geogr. Rev., vol. 6, no. 4, pp. 321 40. King, L. C., 1930. Raised Beaches and Other Features of the South-East Coast. North Island, New Zealand, Trans. N.Z. Inst., vol. 61, pp. 498–523. McKay, A., 1877. Reports on Geological Explorations in 1874-76, Rep. N.Z. Geol. Survey, 1877, p. 50.