Art. XXV.—Examples of Readjustment of Drainage on the Tararua Western Foothills.
[Read before the Wellington Philosophical Society, 8th October, 1919; received by Editor, 31st December, 1919; issued separately, 15th June, 1920.]
The Major Physiographic Features of the Tararua Range.
Topography of the Arapaepae Ridge and of “The Heights” Basin.
Changes of Drainage in “The Heights” Basin.
Changes of Drainage on the Poruriri Ridge.
In the long-settled and more-closely-studied countries of the Northern Hemisphere the processes and events of physiographic history genetically connected with the present geographical regime have been more or less thoroughly worked out, but in New Zealand the subject has been dealt with in detail in only a comparatively few isolated areas. During the last few years, however, New Zealand physiography has attracted ever increasing notice, and has come to be regarded here as elsewhere as being of considerable importance in the deciphering of the detailed geologic history of a country. I have therefore been led to contribute the following notes on the physiography of an area which has hitherto received but scant attention. While it is not claimed that the course of events outlined in section 4 is in any way unique, the constricted area in which the diverse changes took place may be deemed somewhat remarkable, comparable to some slight extent with the classical instance of drainage-readjustment in the district round Chur, in Switzerland (Heim).
2. The Major Physiographic Features of the Tararua Range.
The Tararua Range is that portion of the structural axis of the North Island extending from the headwaters of the Hutt River to the Manawatu Gorge, a distance of fifty-seven miles. The range consists of a series of parallel and subparallel longitudinal ridges so disposed that they collectively form an obtuse angle or wide arc convex to the west; thus it is that from south to north their trend changes from north-north-east to north-east by north. In addition to the longitudinal ridges there is a subsidiary series of transverse ridges which link the former together.
Another feature of the Tararua Range is its asymmetry. The highest ridge—that bearing the peaks of The Mitre and Mount Holdsworth—lies well to the eastern side of the range, and on that side the altitudes of the foothill ridges decrease more abruptly towards the subjacent lowlands than on the west. In general, both towards east and west, the longitudinal ridges decrease in height in succession as the respective piedmont lowland areas are approached, the outermost foothill ridge on either side being usually the lowest of the respective series. The whole is suggestive of a high-standing tilted earth-block, having a steep eastward-facing scarp and a back-slope declining towards the west. The initial intense compression and crumpling of the Trias-Jurassic (Marshall, 1912, pp. 127–29, 208) strata (the post-Hokonui deformation) was probably succeeded by peneplanation (Cotton, 1916, p. 246; Thomson, 1917, pp. 399–400), and
this was followed by secondary folding (the Kaikoura deformation) accompanied by block-faulting on a large scale, and also by the deep dissection of the penultimate erosion-cycle. The secondary folding corrugated the Tararua earth-block into broad anticlinal and synclinal forms, and these appear to have determined the trend of the principal drainage-lines, and to have guided the agents of erosion in the production of the high relief of the present topography. The initial drainage-pattern is thus considered to be mainly consequent on the secondary deformation, and only to a very slight extent due to adjustment to the original structure.
With reference to the hills near Wellington City, which may be regarded as a south-western extension of the Tararua Range, and perhaps also to the main range itself, the longitudinal ridges have been explained (Cotton, 1918, pp. 213–14) as being bands of resistant rock reinforced by a network of secondary mineral matter sealing the joints; and the valleys between the longitudinal ridges as belts of shattered rock—shattered by the ancient folding of the strata—along which the erosional action of the streams has been more effective.
While this hypothesis of resistant reinforced bands of rock is accepted as a probable contributory cause of the development of the longitudinal ridges,* it appears to be inadequate when confronted by the notable linear persistence of the longitudinal ridges of the Tararua Range, and also by the associated physiographic features thereof—viz., the subsidiary transverse ridges,† and certain arresting characteristics of the hydrography. The present writer's explanation of the genesis of these land-forms may be presented in detail later, and it will suffice to state here that there are numerous and seemingly sound reasons for the belief that orogenic folding and uplift, synchronizing with the production of the existing stream-sculptured relief, is the most satisfactory explanation of the origin of the longitudinal and transverse ridges, and also of the notable peculiarities of the present hydrographical regime. Under this explanation the principal rivers must be classed as anteconsequent and in part antecedent.
3. Topography of the Arapaepae Ridge and of “The Heights” Basin.
On the western side of the Tararua Range the outermost foothill ridge is divided into sections by the vents of the rivers that issue from the mountains on to the plain. Locally the ridge-sections bear names usually corresponding to the trigonometrical stations situated on their highest points. The Arapaepae Ridge, on which is situated the more striking of the two examples of drainage-readjustment that form the subject of these notes, lies between the vents of the Ohau and Mangaore Streams, which cross the plain near Levin and at Shannon respectively.
Towards its northern end the Arapaepae Ridge is markedly asymmetrical. On its western side the spurs are deeply truncated, the hillside presenting a steep face composed of short blunted salients. These features mark the position of a former coast-line, they being ancient sea-cliffs (Adkin, 1911, p. 509; 1919, p. 109). On the eastern side of the ridge a different
[Footnote] * The topographic details as etched out by erosion are undoubtedly due to the presence of bands and patches of rock of varying resistance.
[Footnote] † The transverse ridges are not mere erosional features occurring at haphazard intervals. In some cases they extend from lowland to lowland right across the mountain-system, and therefore must be attributed to a more profound causation. Nor can it be due to mere chance that the principal transverse ridge of the Tararuas, if produced eastward across the Wairarapa lowland, there coincides with the water-parting from which the drainage of the lowland and adjacent mountains is directed north and south respectively.
set of land-forms occur: long fully-developed lateral spurs run out from the main divide like great buttresses for a mile or more, and between them lie broad, open stream-valleys, all diversified by a dendritic drainage-pattern. One of these broad valleys, locally known as “The Heights” basin, is situated at the junction of the Arapaepae Ridge with the transverse ridge that connects it with the inner ridges of the range. By virtue of its situation, this basin possesses such change-favouring features as superior altitude and shallowness to a greater degree than the adjacent eastward-facing lateral valleys of the Arapaepae Ridge.
“The Heights” basin, in which the changes of drainage took place, lies near the crest of the Arapaepae Ridge, its flat alluvial bottom (Plate XI, fig. 1) having an average altitude of 1,015 ft.—only 200 ft. below the trigonometrical station, Arapaepae No. 3, 1,210 ft., located at its north-western corner. The basin is roughly rectangular in shape, and about a mile and a half across from north to south. Forming its rim are hilly ridges, varying in height from a few feet to more than 400 ft. above the alluvial flat: on the west the Arapaepae Ridge, on north and south two lateral buttressing spurs of the same, and on the east the western end of the transverse connecting-ridge. To the north lies the catchment area of the Mangaore Stream; to the south and south-east that of the Makahika, a tributary of the Ohau River; and to the west that of the Koputaroa. Formerly “The Heights” basin had only one outlet; now there are no less than three.
4. Changes of Drainage in “The Heights” Basin.
Originally the whole of the drainage of “The Heights” basin was discharged through a comparatively narrow outlet situated at its south-east corner, by a single stream, tributary to the Makahika River. The origin of the basin is, in the present state of knowledge, somewhat problematical; but a tentative hypothesis is that it was formed by the denudation, first by the subaerial agencies in general and afterwards principally by stream erosion, of the crest of a broad anticlinal structure of crumpled strata possessing but little primary variation in hardness. Under this conception the weakened crest of the anticline was widely opened, while the limbs retained a sufficient degree of compactness to restrict erosion to narrower limits. This hypothesis conforms to the conception of the orogenesis of the Tararua Range briefly outlined in section 2; the general topography of the Arapaepae and adjacent ridges is also favourable to its adoption.
An identical origin must be ascribed to the neighbouring eastward-facing lateral valleys of the Arapaepae Ridge (fig. 1)—the Waireka (Plate XI, fig. 2), Wainui, & c. All these valleys are of the basin-like, bottle-neck type, being gorge-like and narrow at their outlets, and broad and open above. Valleys of similar form occur in the Blue Mountains* due west of Sydney (Taylor, 1919, p. 177).
It may be suggested that in the case of the Tararua foothills the broad arching of the secondary folding would be quantitatively insufficient to produce so sharp a distinction in the resistance to erosion of the crest and limbs of a fold as to determine the ultimate form of the bottle-necked valleys. For the present this may remain an open question, though the
[Footnote] * Professor David's and Griffith Taylor's explanation of the Blue Mountain bottleneck valleys appears to be applicable to those of the Tararua foothill ridge, though in the former locality this particular type of land-form was produced by a single rock stratum in the limb of a large anticlinal fold, and in the latter by the compacted and stronger strata of the anticlinal limb itself as compared with the weaker crest.
Fig. 1.—“The Heights” basin, looking north-east, showing topography and present distribution of drainage. Remnant of original stream in foreground (centre to right), outlet of same on right beyond margin of view. Spill-over outlet on left. Gorge and trenches of pirate stream in distance (right to centre). Trig. station, Arapaepae No. 3, 1,210 ft, in distance on left. Fig 2—The valley of the Waireka Stream, looking east towards its narrow outlet. The old alluvial flat is now trenched by the slight rejuvenation of the stream.
available evidence appears to indicate that it was sufficient. There was, however, a contributory cause. The rejuvenated Makahika River has cut its present trench-like channel at the foot of the long eastward-trending lateral spurs of the Arapaepae Ridge, truncating them; and to maintain accordant junctions the Wainui and Waireka Streams were thereby forced to cut shorter, deeper gorges than they would have done had the Makahika River cut its present trench in the middle or on the opposite side of its former widely-opened valley. Of itself the action of the Makahika was quite insufficient to produce the bottle-neck, basin-like valleys of the tributary streams referred to, but, in conjunction with the broad arching of the secondary deformation, conditions came into existence that were favourable to the production of the tributary valleys in their present form.
When “The Heights” basin had been opened to nearly its present extent a period of alluviation followed. This alluviation covered the former valley-bottom to a considerable depth, and ultimately formed a wide alluvial flat (Plate XI, fig. 1) having a very gentle surface slope to the south-east. The alluvium consists of stiff cream-coloured and yellow clays resting on a thick mass of fine gravel. Towards the close of the deposition of alluvium many of the exposed ends of the half-buried spurs round the rim of the basin were levelled off by lateral corrasion, and the flat was thus further enlarged. The current of “The Heights” stream was at this time extremely sluggish, flowing in meanders of small radius, and the surface of the flat must have been diversified by cut-off ox-bows and deserted swampy channels (fig. 2). On the whole, the topographic form of the basin at this stage was one of late maturity or even old age.
Alluviation of a somewhat similar character took place also in the Waireka, Wainui, and other adjacent bottle-necked valleys. Thick masses of clay containing scattered angular fragments of rock, often of large size, were laid down, the master streams became sluggish and winding, and the topography reached an advanced stage of maturity.* The alluviation of these tributary valleys took place in harmony with the alluviation of their trunk valley, that of the Makahika: this is proved by the corresponding accordant levels of the surfaces of the valley-fill in the several parts of the valley-system. The alluviation of the large river-valleys of the Tararua Range was caused by the failure of the rivers to transport the excessive amount of waste derived from the then more extensive alpine and subalpine areas in which these rivers took their rise during the “glacier period,” the snow-line (and consequently the timber-line) being at that time, by reason, in part, of the greater elevation of the country, relatively very much lower than now.† In my previous papers (Adkin, 1911, pp. 497–98, 520; 1919, p. 112) it was shown that the building of the valley-plain of the Makahika and Ohau Valleys, and also the construction of the Ohau fan, took place during that period of great elevation of the North Island—viz., in the early Pleistocene (Park; 1910, pp. 156–57, 250; Marshall, 1912, p. 210). The initial opening - out by rosion of “The Heights” basin and the other adjacent bottle-necked valleys is therefore of some antiquity—certainly of not later date than middle or perhaps late Tertiary times.
The alluviation of “The Heights” basin was followed by further changes. The sources of a tributary of the Mangaore Stream, situated on
[Footnote] * The last physiographic event in each of these valleys except that of “The Heights” was rejuvenation, by which their alluvial bottoms were trenched to depths up to 100 ft.
[Footnote] † For fuller reference to this subject see “The Discovery and Extent of Former Glaciation in the Tararua Ranges, North Island, New Zealand,” Trans. N.Z. Inst., vol. 44, p. 315, 1912.
the northern side of “The Heights” basin, were actively extending their sources headward into the outer slopes of its rim. One of these streams finally cut back completely through the rim, and tapped a portion of “The Heights” stream. Invigorated by this success, it still further extended its course within the confines of the basin, ultimately capturing half of the drainage-lines therein. Proof of this act of piracy and the consequent reversal of drainage is furnished by the fact that the stream now flows in a direction contrary to the slope of the area it drains, as shown in fig. 1. (Note altitudes—corrected aneroid readings—of the surface of the old alluvial flat.)
Simultaneously, or approximately so, a further change took place, this time in the south-west part of “The Heights” basin. There, one of the aggrading streams so far raised its bed that it at last overtopped a low place in the main Arapaepae divide, and a spill-over course resulted, by which its waters were diverted westward into the catchment area of the Koputaroa Stream. This type of stream-diversion has been described by Gilbert as “diversion by alluviation” The alternative possibility, that the spill-over course at “The Heights,” and also that on the Poruriri Ridge (described in section 5), were caused through capture by the headwater erosion of streams rising outside the basins of the diverted streams, is rejected on the following grounds: (1.) Both the Arapaepae and Poruriri Ridges have fairly even, unnotched crest-lines, and headwater erosion of the streams draining their western slopes does not now, or at any previous period, appear to have caused, or even tended to cause, diversion by piracy. (2.) The original surfaces of the alluvial flats in the basins of the two diverted streams referred to above still overtop the former low parts of the Arapaepae and Poruriri ridge-crests respectively. In each case the notch cut by the spill-over course can be differentiated from the low part of the ridge-crest.
In this manner the former coalescent drainage of “The Heights” basin became divided into three distinct parts, each of which possesses either inherited or newly-acquired topographic characteristics. The remaining undiverted portion of the original drainage bears all the signs of advanced age, and still pursues a meandering course on the surface of the alluvial flat before flowing south-east to join the Makahika River. The streams captured by the pirate stream flow in narrow youthful trenches at a depth of from 15 ft. to 100 ft. below the old alluvial surface, and then plunge into the deep and narrow gorge that forms the breach in the northern rim of the basin, and join the Mangaore. In one place part of this entrenched drainage lies only about 7 chains from the old meandering stream, and further captures are thus imminent. The stream diverted by alluviation has also entrenched itself to a slight extent into the alluvial flat, and then cascades down the steep western face of the Arapaepae Ridge to augment the waters of the Koputaroa. The changes described bear the marks of extreme youth, and further adjustments must eventuate before even a moderate state of stability is attained.
5. Changes of Drainage on the Poruriri Ridge.
The Poruriri Ridge is another section of the outermost western foothills, lying between the vents of the Mangaore and Tokomaru Rivers; and linearly it is the northward continuation of the Arapaepae Ridge. The diversion of drainage which took place on the Poruriri Ridge was similar to but less complex than that at “The Heights,” since in the former locality the act of piracy was not committed. In the former locality the change was due to diversion by alluviation causing a spill-over course (now known as the
Mangaharakeke Stream: fig. 3) which produced the highest waterfall* yet recorded in the Tararua Range, and a ravine exhibiting all the criteria of extreme youth—precipitous, crumbling rocky sides, and a narrow ungraded stair-like bottom down which the stream plunges in a series of falls and cascades. The main fall is situated at the head of the ravine, and descends the 300 ft. of its height in three leaps, separated by narrow rock ledges.
The northern end of the Poruriri Ridge is even more asymmetrical than the Arapaepae Ridge, its spurs being so deeply truncated on its western side as to present an almost unbroken face, the exceptions being the gash-like Mangaharakeke ravine and some minor gullies. Like the Arapaepae Ridge in the vicinity of “The Heights,” the Poruriri has long branching lateral spurs on its eastern side, and physiographically the two ridges have much in common.
Considered as a single feature, the former and present catchment areas of the Mangaharakeke Stream have a topographic form intermediate between that of the Waireka Stream (Plate XI, fig. 2) and that at “The Heights” (Plate XI, fig. 1). Some of the topographic details of this dismembered catchment area are of considerable interest, but only one which has a direct bearing on my argument can be touched on here. The sudden spilling-over of a fair-sized stream like the Mangaharakeke liberated an enormous amount of erosive power, with the result that the spurs on either side of the ravine were shorn away longitudinally, leaving them as half-spurs—i.e., having a concave precipice on the one side and the normal form on the other. None of the spurs enclosing any of the neighbouring minor gullies possesses a similar configuration, a fact emphasizing the special origin of the ravine.
Formerly, the Mangaharakeke Stream took its rise on the northern side of the Poruriri Trigonometrical Station, and flowed north and north-east into the upper valley of the Tokomaru River. By the excessive alluviation of the upper part of its course a state of great instability ensued, and while swinging to and fro on its alluvial flat the stream found a low place in the main Poruriri ridge-crest and flowed down the western slope, there producing the ravine and falls described above. A sluggish shrunken remnant of its former trunk, tributary to the Tokomaru, still drains the eastern part of the deserted alluvial flat.
List of Papers cited.
Adkin, G. L., 1911. The Post-Tertiary Geological History, of the Ohau River and of the Adjacent Coastal Plain, Horowhenua County, North Island, Trans. N.Z. Inst., vol. 43, pp. 496–520.
—— 1919. Further Notes on the Horowhenua Coastal Plain and the Associated Physiographic Features, Trans. N.Z. Inst., vol. 51, pp. 108–18.
Cotton, C. A., 1916. The Structure and Later Geological History of New Zealand, Geol. Mag., dec. 6, vol. 3, p. 246.
—— 1918. The Geomorphology of the Coastal District of South-western Wellington, Trans. N.Z. Inst., vol. 50, pp. 212–22.
Heim, A. Quoted by Lord Avebury, 1902, in The Beauties of Nature, pp. 159–62 and maps.
Marshall, P., 1912. Geology of New Zealand.
Park, J., 1910. The Geology of New Zealand.
Taylor, Griffith, 1919. The Physiographic Control of Australian Exploration, Geog. Journ., vol. 53, p. 177.
Thomson, J. A., 1917. Diastrophic and other Considerations in Classification and Correlation, & c., Trans. N.Z. Inst., vol. 49, pp. 397–413.
[Footnote] * From the Wellington-Manawatu Railway line just south of Tokomaru this fall is a conspicuous and striking object.