Art. XLVI.—Notes on the Piako and Waikato Riverbasins.
[Read before the Auckland Institute, 23rd October, 1893]
I had the honour to read a paper before the Auckland Institute in December, 1888, on the Waikato River-basins* There was little time to discuss the subject on that occasion, and, as the changes in the course of the river are of an interesting and somewhat recondite character, I believe some of the members will be glad of an opportunity of discussing them. I have been enabled to add considerably to my previous notes and to include portions of the Waipa and Piako basins during recent journeys through the districts, and fresh evidence of changes in the level of the land and alterations in the courses of the rivers during comparatively recent times will be submitted to you.
It has been my fortune during many years past frequently to visit every part of the Waikato basins, and, in conducting the topographical survey of the country, I was afforded an excellent opportunity of studying its surface configuration. Unlike many of the great questions with which geologists have to deal, the study of the earth's surface-features requires no special scientific training—it is within the limits of our most familiar experiences. One needs but the time and opportunity, with some experience. Nature's features are bare before us—we can read on the faces of the cliffs and the terraces, in the steep or gentle grade of the valley to the river, and in the character and distribution of the surface-soils and water-laid materials, the half-hidden history of the past. These notes refer to a comparatively recent time from a geological view—a period during which the surface configuration of the country was very much as we see it at the present day
The Waikato River rises amongst the peaks of Ruapehu, and, flowing along the eastern base of the Tongariro and Ruapehu volcanic chain, receives the drainage of the great mountains and several considerable streams from the Kaimanawas, and enters Taupo Lake at Tokaanu. Taupo Lake is 24 ¾ miles in length, and 16 ½ miles in width; its average depth is about 400ft. It is bounded in most places by steep lava-cliffs and associated tuffs. Two well-marked terraces surround the lake and the valleys leading into it. One stands 100ft. above
[Footnote] * Trans. N.Z. Inst., vol. xxi., p. 406.
the present water-level, and the other 330ft., clearly showing that formerly the lake was at a higher level. The Waikato leaves Lake Taupo at Tapuaeharuru, and flows in a north-east direction down a well-defined valley for nearly twenty miles.
An inspection of the map will show that the Waikato basin here is bounded to the westward by a mountain-chain whose highest peaks are 3,800ft. above the sea. They run nearly parallel with the western shores of Lake Taupo. There is a general persistent trend in the range in a direction N. 40° Ei. Lateral offshoots diverge, but the general axis of the main chain has a dominant direction nearly north-east. This chain is continued on to Whakamaru and Tikorangi, and becomes confluent with the Patetere plateau and the high wooded country to the westward of Rotorua, where it disappears or becomes coalescent with the disjointed hills, Ngongotaha, Okohiriki, &c., which lie directly to the west of Rohorua, and stand from 2,500ft. to 2,600ft. above the sea. Twenty miles to the east of this main range lies the valley through which the Waikato River flows from Taupo Lake to Waiotapu Valley. The Waiotapu was evidently a continuation of the valley above through which the Waikato River flows from Taupo. The two together form the oldest topographical feature of the country, and along their course on either side are to be seen evidences of immense denudation as they were widened and deepened. To the west of them, extending to the main range, the ground is left projecting into high ridges and prominent isolated hills with valleys filled with alluvium, this general trend being from the range into the Waikato and Waiotapu Valleys. I think it is more than probable that the Waikato at one time flowed through the Waiotapu Valley to the sea on the East Coast, and that the main range before referred to formed the watershed of its basin on the northwest side. But its channel was obstructed, probably by subterranean disturbance or the volcanic action in the Rotorua district; it quitted the original valley, and eroded for itself a new channel in nearly a due west direction, through a pass of the range between Whakamaru and Titiraupenge.
The topography of the country and the land-sculpture in the new river-valley bear out this view. The waters of the river were poured back into the valleys, which they occupied for a time in the form of a serpentine lake or lake-like river with many arms spreading in between the spurs of the ranges. Round Tuahu, Ngautuku, and the other hills between Ateamuri and Taupo, are seen the old lake-beds filled with alluvial deposits. In the valleys between the hills are immense beds of pumice and sand in horizontal layers, sometimes over 200ft. in depth. Through these the streams have worn their channels 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 surrounded these pumice-beds, cliffs of tufaceous rock, often plainly water-worn, and the stratified horizontal layers of light drift pumice, leave but little doubt that a large area in this part of the Waikato basin was occupied by a lake through which the river flowed. In my opinion, deposits of light pumice sands such as are here found could only be laid down in still water.
The elevation of the outlet through the range was at first 300ft. above the present bed. A steep narrow channel was formed through the pass, the successive stages in the process of lowering being marked by horizontal terraces round the south end of the gorge and the lake - basins in the valleys above. These terraces are of immense proportions, ranging to 200ft. above the present river-bed. An excellent example of these terraces may be seen near Ateamuri Bridge, where the Rotorua-Taupo Road crosses the river. Below the gorge the river eroded a deep and narrow channel through the loose tufa country along the base of the wooded mountains of the King-country. Here one looks in vain for the characteristic features which mark an ancient valley. The country through which the river flows has the appearance of an old plateau, along the bases of the ridges and hills of which the water cuts its channel, and there is nothing to mark its course as an ancient feature of the landscape—no prolonged area of depression along the course of the river, and very little terrace formation.
Another remarkable deflection from its natural course would seem to have taken place in the Waikato at Hinuwera, fourteen miles above Cambridge. Here we have a broad, welldefined valley, bordered on either side by waterworn cliffs, from 30ft. to 40ft. in height, sloping down through Hinuwera in a north-easterly direction towards Matamata. It is confluent with the valley of the Waikato above. The trend and height of the river-terraces, the character of the alluvium, the waterworn cliffs, and the general topographical features of the land all point to the conclusion, I think, that this was its old natural valley, and that the Waikato once flowed down through it to the sea in the Hauraki Gulf. For the causes which brought about the change in the Waikato's course here we seek in vain amongst the topographical features of the land. There is nothing to show why its natural course was impeded, and why it is not now flowing down the Hinuwera Valley to the Hauraki Gulf, instead of taking the unnatural course it has through the ridges of the Maungatautari and Hinuwera Ranges.
Here, again, we have the same sequence of events recorded that took place at the Whakamaru. It would appear that the
waters of the river were ponded back into the valleys above, which they occupied in the form of a sinuous lake, extending upwards for over eight miles, and covering the Waipa Plains, which were evidently the bed of a lake. The topography of the country bears strong evidence of this. We find the remains of a deep alluvial deposit which filled the valleys running in between the spurs in level plains. Through these deposits the streams, in eroding this channel, exposed strata, horizontally laid, consisting of rhyolite sands, pumice, and detritus, including the trunks of trees. At Paeroa, where the Auckland Agricultural Company's homestead at Cranston is situated, this deposit has a depth of 130ft., as exposed by the washed-out gully of the Piarere Stream. It extends down the Hinuwera Valley, and almost disappears at Parakau, four miles from the present bed of the Waikato.
Reference to the map will show the elevation of the lake bed on the Waipa Plains to be 340ft. above sea-level. A terrace of about the same height fringes the valley of the Waititi Stream on the opposite side of the Waikato, and extends down the river-side to Piarere. It is a remarkable fact that the height of the old river-bed in the Hinuwera Valley is only 280ft. at Piarere, and the valley slopes gradually down towards Matamata. Were the contour and levels of the valley as we now find them there would be nothing to impound the water in the lake, the old Waipa lake-bed being 60ft. above the outlet in the Hinuwera Valley. I shall have occasion to refer to this question again in considering the causes which brought about the changes in the river's course.
The highest terrace in the Maungatautari Gorge has an elevation of 300ft. above the sea. Water-worn rocks appear on both sides of the river at that height, and between them on either side of the river are seven rows of terraces extending down to the present river-channel, which is cut deep and precipitous through the rhyolite rocks.
The broad plain in central Waikato, in which the towns of Cambridge, Hamilton, Ngaruawhia, &c., are situated, has an area of 500 square miles. All over the lower areas of this plain we find an alluvial deposit, varying in depth from 150ft. downwards. The character of this deposit is unmistakable—it is no doubt the alluvium of the Waikato River, and differs in no way from that at Waiotapu. Whakamaru, and Hinuwera. Pumice-drifts are not found to any extent in the beds of any of the other rivers which flow into the Waikato middle basin. How these deposits came to be laid down as we now find them is an interesting physiographical question. The surface-height of the land at Cambridge is 220ft. above the sea, at Hamilton it is 120ft., at Ngarato 125ft., and at Morrins-
ville 82ft.; whilst at the Taupiri Gorge the elevation is only 39ft. Yet we find that the Waikato carried its alluvium to the Rotorangi Swamps, eight miles in a southerly direction, against the present grade of the country, the natural fall being north and east, towards Morrinsville and Taupiri, at the rate of 7ft. to the mile. It becomes at once evident, on considering the relative heights of the land in the middle basin, that it never could have been occupied by a lake, nor could these deposits have been laid by the Waikato at all if the levels of the country were as we now find them. The lake would have four outlets: one at Morrinsville, one at Hapuakohe, one at Matahura, and the present river-bed through the Taupiri Gorge (supposing it to have existed at that time), all considerably below the bed of the lake. It may be suggested that the deposits are estuarine, and were laid when the middle basin was a bay of the sea. The whole topography of the country and the character of the alluvium are against this theory. There is no trace of marine deposits to be found, and if any existed they would easily be discovered amongst the detritus which is scored and exposed by the streams, frequently to great depths, all over the plain. Neither is there any trace of a sea-beach found fringing the swamps round the clay-hills in the plain which were islands dotting the lake. Hochstetter says of the Middle Waikato basin: “The whole basin was, previous to the last elevation of the North Island, a bay of the sea, extending from the Hauraki Gulf far into the interior. The steep margins of the surrounding hills continue to this day displaying the seashore of old, and the singular terrace formation on the declivities of the hills and the river-banks within the basin is the result of a slow and periodical upheaving.” This being so, as the land gradually rose and the sea receded, tidal channels would have been left within the estuary, through which the rivers and streams would continue to flow out of the basin to the sea in the Hauraki Gulf—that is, if no change of the surface took place to prevent the natural course. We now find, however, that the valleys where these estuarine channels might be looked for are filled with an alluvial deposit of the detritus of the volcanic country brought down by the Waikato River, and placed in stratified horizontal beds, as they could only be laid in very slowly-moving or impounded water.
The depth of these deposits varies considerably. In the Rukuhia 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 the deposit in many parts of the basin the old land-surface may be seen.
In the bed of the Waikato River at Hamilton several mairetrees appear standing as they grew, the present land-surface being 70ft. above them. In many of the gullies eroded by the streams trunks of trees are found lying horizontally, and some standing, their roots penetrating the old land-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, near Cambridge. A drain was cut about a mile in length through a spur of dry land to drain the Mona Vale Swamp into a gully which led to the Waikato River. During a heavy rainfall some years ago a scour started in this drain, which soon eroded a gully 70ft. in depth and several chains wide. At the bottom of this gully the ancient surface, consisting of a brown marly-looking soil, is exposed to view. The trunks of trees are seen lying on the old landsurface, and some are standing, their roots penetrating the old soil where they grew.
The Waikato flows in almost a direct course from Cambridge to Ngaruawahia in a north-westerly direction, crossing, the former estuary diagonally. In a flat alluvial valley like this we should naturally expect to find a winding river and a broad and terraced river-valley, instead of which there is a comparatively narrow valley, a deep-cut channel, and the river runs in a direct course, until it is stopped by the Hakarimata Ranges, near Ngaruawahia; it then follows the base of the mountains to the Taupiri Gorge.
Five miles and a half east of the Waikato River, below Huntly, there is a wide valley extending from the Manga-whara Stream, in the middle basin, to the Matahura, in the lower. It is very evident from the topography of the country that this was at one time the outlet for the middle basin, and the Waikato flowed through it and down the Matahura Valley, through Waikare Lake and the Whangamarino Flats. That the Taupiri Gorge, through which the river now flows, was formed subsequently I think there can be very little doubt. Captain Hutton, in his paper “On the Alluvial Deposits of the Lower Waikato, and the Formation of Islands by the River,” says: “There appears, therefore, to be no geological evidence of the sea having been in the Lower Waikato Valley since the upheaval of the Waitemata series—that is, since it has had any existence. I therefore think that the fact of the presence of several littoral plants in the Lower Waikato basin, brought forward last year by Mr. Kirk, may be best explained by supposing that they have spread down the river from the Middle Waikato basin, after the formation of the Taupiri Gorge.”* The evidence of changes in
[Footnote] * Trans. N.Z. Inst., vol. iv., p. 334.
the level of the land, and alterations in the courses of the river in the Lower Waikato basin, are numerous and interesting from the surface-geologist's point of view, and a great deal might be written about them, but it is not my intention to enter fully into a description of this basin at present. One of the most interesting features of the Lower Waikato basin is the Matahura-Whangamarino Valley, which lies to the westward of the wooded Hapuakohe Ranges. The Matahura and Whangamarino Rivers rise in the centre of it, the former flowing to the south and the latter to the north-west. Their sources are a very short distance apart, and are separated by a low saddle. This has evidently been the valley of a large river at some period, and does not owe its erosion to the small streams which now occupy it. There are evidences of vast erosion high up on the sides of the valley, the remains of ancient river-terraces now worn and wasted by the elements. This was probably the course of the Waipa River when the Waikato flowed into the sea in the Hauraki Gulf. The alluvium of the valley is quite different from that of the Waikato; it is an argillaceous deposit, free from pumice or the rhyolite sands which characterize the deposits of the Waikato. It is very fertile. The lands in the Matahura and Waerenga Valleys derive their rich qualities from it, the latter being some of the most fertile in the Waikato district. It is the same alluvium which characterizes the rich lower terraces of the Waipa River, from Ngaruawahia up into the limestone land in the King-country, and is traceable along the Mangawhara River into the Matahura Valley.
Passing now from the Waikato to the Piako basin, we find again evidences of changes, as shown by the raised beaches, ancient terraces, and the river-alluvium. The level, swampy plain extending from the Hauraki Gulf to Te Aroha is plainly a combination of the bed of the gulf: the base of the hills on the western side of the valley shows distinctly the remains of the former coast-line. At Maukoro, twenty miles inland from the sea, close to the western bank of the Piako, and near the confluence of the Waitoa and Piako Rivers, an old raised beach, standing about 17ft. above high-water mark, is to be seen. There is a consolidated slag or marl deposit containing shells and unmistakable crab-holes, such as we see on the soft beaches at present. Along the banks of the Piako there are numerous sand-banks and banks of sea-shells, clearly showing that at no very remote time the Lower Piako Valley was a shallow bay of the sea. The immense quantity of pumice and other detritus from the volcanic districts laid down as river-alluvium in this large valley shows that it was the valley of a great river flowing through a volcanic country; and I have no doubt but the Waikato brought down the alluvium seen in the
upper portions of the plain, by Matamata. The alluvial beds of the lower areas of the Piako basin from the confluence of the Waitoa and Piako to the sea differ in a very remarkable manner from those higher up, and in the Waikato basins. Here we find a heavy argillaceous deposit similar to that of the Waipa River. A large area on the western side, and between the Piako and Thames Rivers, is covered with it. The land is extremely fertile, and if it could be drained would be some of the best soil in the country. The source from which the alluvium came is an interesting question. That it should be found in the lower area of the valley, whilst higher up, on the plains of Piako, Te Aroha, and Matamata, the alluvial deposits are of pumice and sands similar to those in the Waikato basins, is not easy to explain. The tributaries of the Piako may have brought down clay-alluvium from the Maungakawa Ranges, but it would necessarily be limited in quantity, from the small area of the ranges which they drain. I think it best explained by supposing it to have been brought down by the Waipa River from the limestone land in the King-country previously to the great upheaving and changes in the Waikato basins, and laid down in the lower part of the Piako Valley as an estuarine deposit when the Hauraki Gulf extended further inland. There is some evidence to favour this supposition. The Mangawhara Stream rises near Hoeatainui, within three miles of the Piako River at its confluence with the Waitoa. A low saddle in the fern-ridge which separates the head waters of the Mangawhara from the Piako Valley would seem to have been eroded by a river. Terraces are traceable on either side of the saddle. At Hoeatainui, and all along the Mangawhara Valley to the junction of the river with the Waikato at Taupiri, the alluvium is of similar character to that of the Lower Piako Valley. It seems probable, therefore, that at one time the Waipa occupied the Mangawhara Valley, and flowed into the Hauraki Gulf at Maukoro.
From the foregoing observations it would appear that the Waikato River for a long period of its history has undergone successive changes in its course At each change it would appear to have left its natural valley, and, turning to the westward, found a new course through the ranges which separate one valley from the others. Each basin would appear to have had extensive lakes situated within it. The present topographical features of the country do not afford sufficient evidence of the causes which effected these changes. If the levels of the country were as we find them to-day there would be nothing to impede the Waikato River in its old and natural course by the Hinuwera Valley to the Hauraki Gulf. And the contour of the Middle Waikato basin would not permit of the existence of the lake: neither would the Waikato River
occupy its present course directly across the plain, cutting through a series of clay-ridges, whilst the fall of the country from Cambridge is in a northerly direction. To subterranean movements altering the surface-elevation of the land it is evident these changes in the river's course are due; and these movements of elevation of the surface were of a paroxysmal character, or, at least, too rapid to allow the river to erode its channel deeper as the land rose. The evidences of elevation and depression we have are numerous. Captain Hutton proves submergence at the Thames by finding at a depth of 30ft. below sea-level, near Shortland, kauri-gum, raupo, and pieces of wood. He continues as follows: “It would thus appear that when the alluvium full of boulders found on top of the hills (near Shortland) was forming, the land was 1,000ft. lower than at present; that it then gradually rose until it was 100ft. higher than now; and at that time the Thames ran farther north than Shortland. The land then sank to about 10ft. or 12ft. lower than now, and subsequently has again risen to its present level.” I would here remark that when the land at Hauraki was 1,000ft. lower than at present—assuming that the movements of elevation and depression were unequal in the different parts of the district (a supposition of which we have some proofs, as shall be shown later on)—the Waikato River would flow down the Hinuwera Valley to the Hauraki Gulf, which would at that time extend far up the valley. As the land rose until it was “100ft. higher than now,” probably the axis of elevation would be along the main range from Cape Colville to Rotorua, with a contemporaneous subsidence to the south-west. The changes of level may be supposed to have ponded back the Waikato River into its valley above Hinuwera until it had graded a new channel through a pass in the Maungatautari Gorge. And the same explanation may be applied to the phenomena in the Middle Waikato basin. When the land in the Hauraki was “1,000ft. lower than now” the Middle Waikato basin was probably a shallow bay of the sea. As the land rose along the main range until it was 100ft. higher than now, the waters of the Waikato were impounded in the middle basin, covering all the lower areas of the valley as a shallow lake, which were then filled with the alluvial deposits we find there now. The formation of the Taupiri Gorge would probably have taken place at this period. The direct course of the Waikato River from Cambridge to Ngaruawahia, and the absence of a wide valley, may be taken as indicating the rapid formation of the riverbed, which was probably the result of the changes of level. Mr. James Stewart, in his paper on “Evidences of Recent Changes of Level in the Waikato District,” gives the following: “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 far below the level of high water in the ocean.” The cylinders of the railway-bridge at Ngaruawahia are sunk several feet lower than high-water mark in Auckland Harbour, and at this depth river-pebbles and shingle were found, indicating an ancient river-bed.
In a section of a bore for coal at Huntly large gravel was found 60ft. below the level of the sea.
On the clay-hills in the swamps at Waikare Lake waterworn blocks of pumice are deposited in saucer-like depressions on top of the hills, 30ft. above the level of the lake, positions to which only the water of the lake at a former elevation could take them. Near the trig. station at Pukeotaka, near the Miranda, a mass of rounded boulders appear, which mark an old river-bed when the country was very much lower than now, and when the stream from the Lower Waikato basin flowed into the Hauraki Gulf at Pukorokoro. That elevation is plainly shown along the western side of the Hauraki Gulf within recent times is proved by Mr. Percy Smith in his paper on “Changes in Level of Coast-line in North New Zealand.”* Whether these subterranean movements are going on at present or not we have no conclusive proof. The records of the height of the recent flood at various places, as compared with that of 1875, would appear to give some evidence of recent depression in the Lower Waikato basin. Thus, in the recent flood the water rose in the Waikato River at Mercer 2ft. higher than it did in the flood of 1875; at Hamilton it was 5ft. lower; at Ngaruawahia the same; and the Waipa at Alexandra did not rise within 6ft. of the flood of 1875. It may be that the flooded state of the Whangamarino Swamps and streams of the country generally caused the difference; but, considering that the flood was at its highest at Mercer for several days, and the enormous difference in supply which 5ft. of depth in the Waikato and Waipa would make, this seems scarcely probable.
[Footnote] * Trans. N.Z. Inst., vol. xiii., p. 398.