Art. III.—Observations on the Evidences of recent Change in the Elevation of the Waikato District.
[Read before the Auckland Institute, 6th December, 1875.]
That rivers are ever scooping their beds to lower levels, and eroding their banks until new channels are established, are matters of common observation. Considering the immense weight of water in a river like the Waikato, its moderately rapid current, and its course, in the lower parts, through alluvial flats composed of materials of the lightest nature, it is at first sight subject for wonder that the changes are not more rapid than they are. It is, however, true that the lower Waikato cannot now cut its channel very much deeper in a practical view, unless the land is raised, relative to sea level, because a certain definite gradient has to be preserved to carry the water off to sea. But if we suppose the land to be elevated, suddenly or otherwise, a great change would soon be observed in the condition of the river. Falls or rapids would be established at its mouth, which, in more or less time, according to the nature of the bed, would reduce the gradient to what it was before. During the time this was being effected, the increased current would have formed a new channel, sometimes coincident with the old one, but often crossing and recrossing it, until, when the normal level and current had again been established, the old river course would be traceable as a series of lagoons or narrow winding swamps, elevated above the new level of the river, by as much as the land had been raised.
If, again, we suppose a bar of harder formation than the rest of the valley to have existed and which had dammed the waters back; to be reduced by the current, the same kind of changes would take place in the channel above, as we have considered would be effected by the raising of the land, and the amount by which the dam had been lowered would be traceable approximately, in viewing the levels of the old and new channels.
In the lower and middle Waikato, the features of the country indicate that possibly both these causes have contributed to the changes of the position and levels of the river. At Mangatawhiri the river leaves the sandy alluvial flats and takes westward through the Tuakau Gorge to the sea. If we view the country upwards from thence, we can observe some of the more salient features of the scene. And we find on passing each successive gorge through which the river has, in by-gone ages, cut its way, that the banks are higher than they are below, presenting in the higher reaches three, five, and even seven terraces, each indicative of a level for the time being of the river, or lake-like stream, before these natural dams in the gorges were lowered.
The evidence also that at these ancient river levels, the waters found other routes to the sea, nearly amounts to demonstration. So much for the lowering of the river through the formation of the land. Regarding the country having risen, it is, almost equally beyond doubt, that the sea once washed the bases of the inland hills in the Thames and Waikato valleys. In Vol. III. Art. 25, “Trans. N.Z. Inst.,” Mr. Kirk gives a list of littoral plants, some of which he found established one hundred miles inland of the present tidal influence. The natural inference that these plants were left in the salt marshes, formerly at the base of the now far inland hills, by the sea which receded as the land arose, supports the opinion expressed by Dr. Hochstetter, and points to a comparatively recent elevation of the land. It seems clear enough, then, that to both erosion of natural dams, and to an elevation of the country, are to be traced the causes of the river now flowing at from ten to more than one hundred feet (speaking well within the mark) lower than it has formerly done. These estimations being from the sea to the Maungatautari Gorge, to which division the present notes are mainly confined.
The speculative thought to which we are invited by these considerations abounds in interest; but when we find in the midst of alluvial sands, occupying the place where once the ocean rolled, indubitable evidence of the previous subsidence of the whole country, we find the subject increased in complexity, and leading to fields of vast speculative study, which topographers will not soon exhaust. The waters of the river have lowered, or the land has been raised, or both combined. The extent of this we can
see and measure. The land has been submerged, but to what extent we cannot tell. The proofs of this subsidence we at present adduce are two. The first lies in the “sunken forest” of the lower Waikato. This we find at a distance of forty-five or fifty miles from the sea, and there are the remains of an ancient forest, the trunks of whose trees are standing as they grew. The tops of the stumps reach to about mean summer water, and stand about two feet above its lowest level. Hard and dangerous the river captains find them, and much labour has been expended in cutting and mending channels for the navigation. These trees never grew in the water. They are of kinds well-known to us as the general forest trees of the present day. The specimen shown is from a “snag” which lately sunk the steamer “Waipa.” It is kauri, but nearly all the larger trees now known are also to be found as snags or in positions where their roots are of a certainty far below the level of high water in the ocean. And this forest may have been on an upland plateau, may have crowned the summit of a hill; nothing can with certainty indicate, but the fact remains that this forest was submerged, cut off by fire or decay at the level of the water, and buried under about six feet of pumice sand, through which the broad river now flows. It has at present a tendency to cut into the eastern bank, and in so doing continually unearths other stumps in every way alike to those now standing in the river.
The other proof of subsidence now submitted was discovered only a few weeks ago in sinking cylinders forming the piers of the Waikato Bridge at Ngaruawahia, sixty miles from the sea. The bed of the river there is pumice sand and gravel. A stratum of hard sandy clay underlies this, dipping to the south. Below this is a hard and compact bed of shingle and coarse green-sand without a trace of pumice. The cylinders were sunk into this shingle by the pneumatic system, and reach several feet below extreme low water in Auckland Harbour. It was found to be composed of fragments of clay-slate rock waterworn, but only to the extent of smoothing and well-rounding the corners. On examining Dr. Hochstetter's geological map, we find in the Waikato Valley above this place no indication of such rock nearer than the Kaimanawa Range south of Taupo. But much more adjacent, in the Waipa Valley, Hakarimata Range, lying parallel to and westward of the river, is marked as composed of clay-slates, and it is possible that these stones were torn from the ravines of that range and deposited in the river-bed, which must then have been far above the sea-level. To what depth this shingle deposit extends is not likely to be soon known. It was explored only so far as was necessary to determine its suitability, in nature and position, to carry the bridge. It may have been a mountain torrent high up in a continental range. The subsidence may
be fifty feet or it may be five hundred. The nature and condition of this old river-bed indicates that a strong current and clear water prevailed. When the country sank it must have continued until the sea flowed over the present Thames and Waikato Valleys, and since which time it has receded to its present coast by the re-elevation of the country.
At one time, too, the waters of the middle Waikato rose above the level of the so-called delta. Then were deposited these vast beds of pumice, gravel and sand, bearing evidence of subsequent disturbing upheaval. The present argillaceous ranges were then so many islands, the tops of submerged mountains or hills. In a ravine which, a few years ago, the waters of a shallow swamp in Ngaruawahia worked out of the pumice of that township, are seen the stumps of trees, fifteen feet below the present surface, standing on the rich soil in which they grew. Next we trace a level of the river intermediate between that lacustrine era and the present, when the flats of the delta were left dry, and a new channel, yet clearly traceable, carried the waters for a time, and then were formed most probably the alluvial clay flats of Taupiri Gorge. The current was then too rapid for the deposit of pumice sand, for we only find it in isolated pockets as if deposited by eddies. In the wide-spreading valley below, however, the light and all but floating sand was laid over the whole low country, covering the “sunken forest,” and leaving it much as we find it. Another slight rise in the country, and the present aspect of affairs was presented—the river has cut lower through the sands, leaving the swamps of the delta far above its level, and again exposing to our view the “forest.”
We have thus attempted to sketch a very shadowy outline of some very momentous changes which have occurred in this part of our island. The details will yet. we hope, be filled in by more able hands, guided by scientific geological knowledge and research.