Art. LIV.—Notes on the Waikato River Basins.
[Read before the Auckland Institute, 17th December, 1888.]
Plates XXXIV. and XXXV.
The Waikato River seems to have been subject to apparently abnormal changes in its course from an early period in its history. Incidental reference to these changes is to be found in several of the works on the geology and physiography of the country, but, so far as I know, the subject has never been dealt with in a comprehensive manner. At each change the river would appear to have left its natural valley, and, turning westward, to have found a new course through high mountainous country which separates one basin from the other. Thus it appears to have worked in a diagonal line across the country, from east to west, crossing three primary river-valleys. In consequence of these facts, the physiographical history of the basins, regarded as a description of the surface-configuration of the Waikato Valley due to a combination of the effects of volcanic action and planetary denudation, is of more than ordinary interest.
Unlike many of the large questions which geologists have to deal with, the study of the earth's surface-features is within the limits of our most familiar experiences, and requires no special scientific knowledge for its understanding. The plain
book of nature is laid open before us. In its most legible pages we may read—on the faces of the cliffs and on the terraces, in the steep or gentle slope of the valley towards the river, and in the character and condition of the soil—the half-hidden history of the past.
For years past my duties took me into every part of the Waikato's basins from its source to the sea, and I had an excellent opportunity to study its topography.
These notes refer to a comparatively recent period, when the surface-configuration of the country was very much as we find it now, and not to the geological ages of the past, during which the country rose gradually out of the sea and our river-valleys were first formed. Only the salient points of the subject can be touched in a short paper. The changes in the course of the Waikato seem to have been four in number, a long space of time intervening between each of them. The first took place at the Wai-o-tapu Valley, twenty miles below Taupo. The Wai-o-tapu has evidently been a large river-valley. It is, in fact, a continuation of that above, through which the Waikato River takes its course from Taupo. The direction and configuration of the valley lead to the conclusion that the Waikato River once flowed through it to the sea. From some cause its course was impeded: the waters were thrown back into the valleys above, which they occupied in the form of a serpentine lake or a lake-like river, with many arms spreading in between the spurs of the ranges. Round Taupo, Ngautuku, and other hills between Atiamuri and Taupo, may be seen the old lake-beds filled up with alluvial deposits. In the valleys between the hills immense beds of pumice and sand, sometimes 200ft. in depth, are seen in level plains through which the streams have worn their channels deep down to the bed-rock, disclosing stratified layers of drift, pumice, and light sands, enclosing the trunks of trees and carbonised wood. The worn, shore-like sides which surround these pumice-beds, cliffs of tufaceous rock often plainly water-worn, and the stratified character of the deposit, leave but little doubt that a large area in this part of the Waikato Valley was occupied by a lake. The waters found their next outlet through the ranges between Whakamaru and Titirau-penga.
The elevation of the outlet was at first about 300ft. above the present bed: gradually it was worn deeper through the barriers, and the waters of the lake drained off, each successive stage in the process of lowering being indicated by a well-marked regular terrace round the south end of the gorge and the lake-basins in the valley above. These terraces are of immense proportions, and range fully 200ft. above the present river-bed.
In the valley of the Waikato, near Atiamuri, where the Rotorua and Taupo Road crosses the river, a most excellent example of the terrace-formation, and of the wonderful power of the river in denuding its valley, may be seen. The height of the Waikato is 1,200ft. above the sea at its exit from Taupo Lake, where its outflow is 16,300 gallons per second—that is to say, about 3,500 tons of water passes through its channel every minute; it has therefore as much energy to expend in denudation as would be required to lift this mass of water 1,200ft. above the sea. Naturally, in its highest reaches, where the gradients are steepest, most of the dynamic forces are expended, and therefore here are shown the greatest examples of surface-denudation.
There are evidences that the upper portion at least of the Patetere Valley, including the Tokoroa Plains, was once occupied by a lake. The stratified pumice-beds and the remains of horizontal terraces which are to be seen there indicate this. It may be that the Waikato River for a time flowed into the Patetere by the Mangaharakeke Valley, to the eastward of the Whakamaru Range; but of this there is not sufficient evidence. We have no well-defined river-bed which the Waikato might occupy, and I think it more than probable that the Tokoroa Lake was not formed by the Waikato River. I may here mention a somewhat strange tradition which was mentioned to me by the Assistant Surveyor-General as having been related to him by Mr. Lawry: That the Waikato River formerly ran into the sea near Tauranga; and that in the course of ages it changed its course and ran out into the Hauraki Gulf; and then, again, after a further lapse of time, it ran across by Tuakau and Mauku, and then into the Manukau Harbour; and thence into the sea at the mouth of the Wairoa River. It is strange what could give rise to this tradition. I do not think the Maoris are speculative in their deductions, and they would be unlikely to draw conclusions of this kind without something more than the surface-configuration of the land to guide them. However, it is not at all probable that the Waikato River flowed through Tokoroa. The lake owed its origin to other causes, and was drained by the Waikato River through the Kopokorahi Stream.
The whole surface-configuration of the Patetere Valley bears evidence of the immense effects of sub-aerial denudation. Tokoroa Plain is 1,220ft. above the sea; the fall from there to Matamata is pretty regular, and amounts to 1,050ft., being at the rate of about 26ft. to the mile: it is easy to imagine what the effects of denudation would be in a country with such an incline, and covered with loose materials, as we find the Patetere was. These effects are shown in the many deep water courses which furrow the valleys for miles. Most of these are
now dry. They are bordered with high water-worn cliffs of tufa, showing that they were once the beds of powerful streams. Two of the principal streams of the Patetere flow into the Waikato; the others, following the natural slope of the valley, find their way to the Hauraki Gulf.
The next remarkable change in the course of the Waikato, and that which was attended by the most serious results in the great middle basin, is that which took place at Piarere, about fourteen miles above Cambridge. Any one travelling the road from Cambridge to Oxford could scarcely fail to remark the well-defined broad valley, bordered by steep cliffs, which runs in a north-east direction by Hinuera towards Matamata. There is little doubt that the Waikato River once flowed down this naturally-sloping valley, and thence to the sea at the Hauraki Gulf. But from some cause it again left its old bed, and, turning to the westward, passed through the gorge between Maungatautari and Hinuera Ranges for six miles, and debouched into the great middle basin at Cambridge. Here, again, we have the same sequence of events recorded that took place at Whakamaru—the river formed a sinuous lake in the valleys above, extending backwards for a distance of eight miles, and covering the Waipa Plains, which were evidently the bed of a lake. We find the remains of a deep alluvial deposit, chiefly of light pumice-sands, which filled the valleys running in between the spurs in level plains. Through these, again, the streams in wearing out their deep channels exposed the strata of river-gravel, pumice-sand, and detritus, including large trunks of trees. This deposit fills the valley at Paeroa, where the Auckland Agricultural Company's homestead at Cranston is situated: it runs into the valleys between the ridges, varying in depth from 120ft. downwards. Following down the old river-bed towards Matamata, the deposit thins out like a wedge, and finally almost disappears four or five miles from the present river-bed.
The river, in working its channel deeper through the barriers in the Maungatautari Gorge, gradually drained off the waters of the lake, leaving behind, in the valley above, eight rows of terraces, which fringe the river on either side, indicating each a different stage in the lowering of the bed.
The height of the land through the gorge which the river now traversed was certainly over 200ft. above the present river-bed, and through this to the bed-rock the river has eroded its channel. We have now arrived at an area in the Waikato's basins where the facts to be recorded are of a perplexing and recondite character. The broad plain in central Waikato known as the “Waikato middle basin” has an area of five hundred square miles. We find an alluvial deposit all over the lower areas of this valley: in places it is 150ft. in depth. The
character of this deposit is unmistakable, as seen in the “washed-out” gullies so numerous all over the valley: the deposit is clearly stratified; it is made up mainly of rounded particles of pumice, interstratified with layers of clay and rhyolite sands, and enclosing the trunks of trees placed horizontally. That these deposits were brought down by the Waikato River seems unquestionable—pumice-drifts are not found in the valleys of any other rivers which flow into the Waikato middle basin; but how they came to be laid as we now find them is, in my opinion, the most interesting physiographical question which we have to deal with. Reference to the map (Pl. XXXIV.) will show the surface-height of the land at Cambridge to be 220ft. above the sea; at Hamilton it is 120ft.; at Ngaroto, 125ft.; at Morrinsville, 82ft.; and at Taupiri only 39ft. Now, we find the alluvial deposits have been carried by the Waikato waters to the Rotorangi swamps, eight miles almost in a southerly direction from Cambridge, whilst the natural fall of the country is in the direction of Taupiri, and over 7ft. to the mile. Dr. Hochstetter says of the middle basin: “The geological features of the basin are these: The lowest bed consists of layers of clay and sand, with bituminous shale, which, in some places, encloses trunks of trees changed to [ unclear: ] nite; the shale passes into argillaceous shale, containing numerous fossil plants; these and similar strata point to the fact that the whole middle Waikato basin was but recently a shallow bay of the sea, at the bottom and on the margin of which these layers were formed.”
If this be so, as the land rose and the sea receded a channel or channels would be left in the estuary, and through these channels the rivers and streams of the valley would naturally continue to flow into the Hauraki Gulf. We find, however, the places where the old estuarine channels might have been are filled with the fluviatile deposits, placed in such stratified form that they could only have been laid down by the action of very slowly-moving water in a lake or the sea. The depths of these deposits vary considerably: in the Rukihia Swamp, between Hamilton and Ohaupo, they are from 50ft. to 70ft.; in the Piako Swamp, from 40ft. to 60ft.; at Hamilton, from 40ft. to 70ft.; and in the neighbourhood of Taupiri, the lowest point in the basin, it is a remarkable fact that the deposit is lightest. Beneath these deposits in several parts of the valley the ancient land-surface can be seen. In the Waikato River, near Hamilton, are standing several trunks of maire trees, which appear to be standing as they grew. In several of the “washed-out” gullies the same may be observed—the trunks of trees lying horizontally and some standing erect on the old surface.
The most interesting example of this character, because
of the most recent occurrence, is that shown on Mr. E. B. Walker's property at Mona Vale, four miles south-west from Cambridge (Pl. XXXV.). A drain was cut about a mile in length through a neck of dry land, to drain the Mona Vale Swamp into a dry gully which led to the Waikato River. During a heavy flood some years ago a scour was started in this drain, which soon formed a gully from 60ft. to 70ft. in depth and in some places several chains across. At the bottom of this gully the ancient land-surface was exposed to view. It consists of a stiff, brown, marly-looking soil, apparently of excellent quality. The trunks of many trees are lying on the old land-surface, and some were found to be standing, with their roots penetrating the old soil, as they grew. The present land-surface is perfectly level, whilst the ancient surface is found at various depths from 30ft. to 60ft., showing the old contour of the land. The timber found here seemed in tolerably good preservation. This is suggestive of a very interesting thought: that, if the country was inhabited previous to this submergence, some relics of the animals or men who lived in it might come to light some day on the old land-surface.
Characteristic of the middle Waikato basin are the numerous funnel-shaped holes to be seen everywhere throughout the alluvial deposits. They were formed by the subterranean or soil waters in passing along beneath the surface of the earth. They create small caverns, and, finally, underground streams, which draw away the loose material from the surface, and frequently form symmetrical funnel-shaped holes—the “potholes” of the settlers. Probably the water, being charged with carbonic acid, was thus enabled to dissolve some of the river-gravel through which it passes, and by degrees to become a small running stream.
Now; it seems very evident that these deposits in the middle Waikato basin could never have been laid in the bed of a lake nor by the waters of the Waikato at all with the levels of the land as we now find them. The lake would have four outlets—one at Morrinsville, one at Hapukohe, one at Matahura, and one at Taupiri, all 100ft. below the level of the bed of the lake in the centre; therefore the waters would not be impounded to place the deposits. Neither is the action of the sea admissible: the character of the deposits, the mode of their distribution, and the levels of the valley as they now stand, preclude this, the western side of the basin being lower than the eastern side, where the estuary would have its outlet. To local movement, or oscillation in the level of the land within the basin, it would seem we must attribute the phenomena. The Waikato, on debouching through the Maungatautari Gorge, would probably occupy
about 350 square miles of the low areas in the middle basin, in the form of a broad shallow lake dotted with numerous islands, which are now the clay hills and ridges of the valley. At that time the land to the east, north, and western sides of the basin stood higher than it now does—sufficiently so to enclose the waters of the shallow lake: then were the alluvial deposits of the valley laid down, and subsequently a tilt or oscillation in the surface-level of the valley took place, emptying the lake.
The almost direct course of the Waikato River from Cambridge to Ngaruawahia, and the absence of a wide river-valley, may be taken as indicating the rapid formation of the riverbed. In a flat alluvial valley we should naturally expect to find a winding river and a broad valley, instead of which we find that the Waikato River has cut its course almost straight in a north-westerly direction until it is stopped by the Hakarimata Ranges, along the base of which it flows in a northerly direction to Taupiri.
Mr. James Stewart, in his paper on “Evidences of Recent Change in the Elevation of the Waikato District,”* shows proof of subsidence as follows: “The proofs of subsidence we at present adduce are two. The first lies in the sunken forest in Lower Waikato: thus we find at a distance of forty-five or fifty miles from the sea the remains of an ancient forest, the trunks of whose trees are standing as they grew. They are found as snags where their roots are of a certainty far below the level of high water in the ocean.” These trees, of course, could never have grown in that position.
Again, Mr. Stewart shows that the cylinders of the railway-bridge at Ngaruawahia are sunk several feet below low-water mark in the Auckland Harbour, and at this depth river-pebbles and shingle were found, indicating an ancient river-bed, which must of course have been higher than it now is to allow the river to flow to the sea. The same evidence was found in sinking the cylinders for the Hamilton railway-bridge, alluvium and river-gravel being found in a position considerably below the level of the water in the ocean.
In a section of a bore for coal at the Huntly coal-mines, large gravel was found at 94ft. below the surface, or about 60ft. below sea-level; pumice was found at 32ft. below sea-level. In the valleys of the lower basin of the Waikato trunks of large trees are to be found in positions where their roots would certainly be below sea-level. On the clay hills in the swamps near Rangiriri, water-worn blocks of pumice are to be seen deposited in little depressions and on small terraces 20ft. or 30ft. above the level of the swamps—positions to which only the
[Footnote] * “Trans. N.Z. Inst.,” vol. viii., p. 430.
waters of the lake could take them. Underneath the trig. station at Pukeotoka, near Miranda, a large mass of boulders, rounded and water-worn, is found 200ft. above the neighbouring valley, through which the head-waters of the Maramarua River flow: these boulders evidently mark an old river-bed of a time when the country was 200ft. lower than it now is.
The facts above quoted, whilst they prove first depression and subsequent elevation of the land, do not, of course, show that either movement was partial or local. This is always most difficult to prove, although it is well known and an admitted fact that earth-movements are variable—here a depression, there an elevation; and the complicated forms of our stratified rocks very clearly show it.
The surface-configuration of the central Waikato basin, especially on its western side, would appear to show evidences of local subsidence. The spurs on the eastern side of the Hakarimata Range, looking southwards from Taupiri by Ngaruawahia, bear, I think, the appearance of a scarp along their base. It would be interesting to ascertain whether the Taupiri Gorge itself marks a line of fault; but a close examination of the strata on either side would be necessary for this purpose.
There is little doubt that the waters of the middle basin had their outlet by Hangawera-Hapuakohe Valley, and also through the Waitakaruru Valley for a time; subsequently they flowed through Matahura into Waikare Lake and the lower basin, and finally the Waikato drained them through the Taupiri Gorge. Remnants of the old lakes still remain in the lakelets, lagoons, and lake-like swamps which occupy the depressed areas in the valley, and many of them are fast drying up.
An interesting feature in the lower Waikato basin is the deep, wide valley which lies on the western side of the Hopuakohe Ranges. The Matahura and Wangamarino Rivers rise in it. Their head-waters are separated by a low saddle, one flowing to the north and west, and the other to the south. This valley did not, evidently, owe its origin to the streams which now occupy it: it was a great river-valley in the past, and possibly the course of the Waipa when the Waikato River discharged itself into the Hauraki Gulf.
There is no trace of pumice—the characteristic of the Waikato's alluvium—to be found in this valley. Its outlet was at Pukorokoro, into the Hauraki Gulf. The low saddle which separates the waters of the Maramarua from those of the Pukorokoro is not, I think, more than 60ft. above the level of the sea.
In the foregoing notes I have endeavoured to give some of the evidences of the changes which have taken place in the
Waikato basins. The cause of the changes is a physiographical question of great interest. There can be little doubt that the lakes which are seen to have occupied the lower areas in each basin successively were caused by the impounding of the waters of the river. By what was the impounding caused? It seems to me to be accounted for by either of two causes—the damming-up of the old river-bed or oscillations in the level of the land. We have ample evidence that at least a very large portion of the North Island has been submerged, and again rose above the sea. Speaking of the changes of level at the Thames Captain Hutton says, “It would thus appear that when the alluvium, full of boulders, found on top of the hill near Shortland was forming, the land was 1,000ft. lower than at present; that it then gradually rose until it was at least 100ft. higher than now; and at that time the Thames ran further north than Shortland. The land then sank 10ft. or 12ft. lower than now, and subsequently has again risen to its present level.” Now, if these movements of elevation and depression were uniform throughout the island, when the land was 1,000ft. lower than it now is very little of the North Island was above the sea, only the high country in the interior, with our other high hills, appearing as islands off the coast.
Mr. Percy Smith has shown us very clearly that elevation has been the latest movement. A very clear case of an elevation of at least 15ft. is shown by him to have occurred in recent times at Miranda, in the Hauraki Gulf, and the settlers there are of opinion that the land on the flat referred to by Mr. Smith continues to rise gradually. One settler informed me that he was enabled to sink a drain 1ft. lower than he originally sank it, twelve years ago, and he feels convinced the tide does not now rise in the drain within a foot of its former height. We may now imagine the land (having sunk) again gradually and uniformly rising from the sea. The Waikato River may have occupied the bed it now does before the submergence. We should then expect its old valley to be filled with the detritus and alluvial materials. In the subsequent elevation the river might have first found its way to the sea on the east coast, through the Waiotapu Valley. As the land rose gradually and uniformly, the river would erode its bed deeper into the loose materials, and we might imagine, as the high land to the eastward came above the sea, its elevation being greater than the low valleys in the upper basins, the water to be impounded and a lake formed until such time as the river, having resumed its old course, by degrees removed the detritus which filled it, and so emptied the lake. If we still imagine the same to have occurred at Hinuera, where the second change took
place, the Waikato would for a time flow through the Hinuera Valley, but subsequently would resume its former course into the middle basin, when surface-denudation had washed the detritus from the old bed. But, as we have before seen, the events in the great middle basin cannot be accounted for without the hypothesis of local oscillation in the level of the land. On the other hand, if we can believe that gradual general elevation has been going on, and that it was accompanied by local movements on a smaller scale, it would be easy to account for the Waikato's changes. If, for instance, the axis of upheaval was along the main range in a southeast direction, from Te Aroha to Rotorua (which seems probable), with a slight anticline to the westward, we should find the Waikato first filling all its valleys as a lake, through elevation to the eastward, and the water seeking a new outlet in the lowest or weakest point in the gorges to the west.
That the movements in the earth's crust are complicated—here an upheaval, there a depression, faults, crushing, and corrugation of the rocks on the surface, the efforts of the earth's crust to adapt itself to the form of the cooling nucleus—seems to be the doctrine of our wisest geologists; but whether these movements are applicable to areas so small as those we have been describing is a matter of conjecture. Charles Darwin, whilst contemplating great events in South America, came to the conclusion that to volcanic action must be attributed the force by which mountain-chains are elevated; and that the efforts of the earth's crust and the contracting nucleus to conform themselves to one another by deforming the spheroid, counteracted by the earth's rotation acting to maintain the spheroidal form, cause most of our volcanic phenomena. Elie de Beaumont, the great apostle of secular refrigeration, defines these volcanic phenomena as “a struggle between the deformation of the spheroid by the loss of internal heat and volume, and the earth's rotation, which constantly tends to cause it to revert to the true spheroidal figure.”
Mr. W. L. Green says, in his work “Vestiges of the Molten Globe,” published last year, “When we find, in one short experience, that Chili and its Cordillera can be jerked up several hundred feet at one stroke, we may well be careful how we limit the magnitude of such catastrophes in all past time.” It is in reference to these great changes of surface-configuration, so copiously noted elsewhere in the world, and to which sources of information New Zealand might contribute a good deal, that I hope these notes may be of some interest. If the conclusions I have drawn are not satisfactory, the simple facts recorded will still remain independently, as
Links in the chain of evidence from which others may draw their own conclusions.
Explanation of Plates XXXIV. and XXXV.
Plate XXXIV.—Map of the Waikato River basin.
Plate XXXV.—Section of Walker's Gully.