Art. LII.—On the Old Lake System of New Zealand, with some -observations as to the formation of the Canterbury Plains.
[Read before the Wellington Philosophical Society, 29th January, 1876.]
Having succeeded in proving the former connection of the Islands of New Zealand, with its necessary sequence a great Cook Strait river, and a fresh water lake in the Harbour of Port Nicholson, I propose to enlarge the scope of the argument very considerably, for I can perceive that lakes have been very extensive throughout the North Island, and have played a remarkable part in the arrangement of the surface of the land.
When we reach the interior of the North Island, we find immense areas covered by pumice. This has been finally arranged in such a way that it could only have been done by the action of water. We find it spread out in large plains or in gentle undulations, and we also find it terraced. How are we to account for this? The pumice as thrown out by the volcanoes would not arrange itself in this manner. The action of rivers, of running water, is inadmissible. The pumice would be deposited only upon the old banks of rivers. A depression of the land to admit the action of the sea is inadmissible. We do not find in the pumice deposits any marine fossils, and if we were to admit the ocean without due precautions, all the pumice would be carried out to sea.
Circumstances force us to adopt the hypothesis of extensive lakes in the interior. When we have once got hold of the idea, I do not think the proof is very difficult.
For instance, the Upper Waikato forms a basin, the Lower Waikato forms another basin, each surrounded by hills on all sides. Before the river had cut a channel through the hills which separated the Upper from the Lower Waikato, the Upper Waikato must have formed a lake. Before the river had opened a channel through the ranges which separate it from the sea, the Lower Waikato must have formed another lake. Any one who knows the district must see this at a glance.
As the river gradually cut its way through the hills, water-logged *pumice would be left behind in terraces, but previously to this the pumice, by the distributing action of the waters of the lakes, would be spread over the whole area. There is, I think, no other way of accounting for the distribution of the pumice.
In the country skirting Ruapehu to the south, the outcrops of the marine tertiaries rise as a fringe. Before this fringe was broken through by the rivers, lakes must have existed at the base of Ruapehu. On the Ruamata Plains on the western side of that mountain, or perhaps, rather of Ton-gariro, I observed large deposits of pumice, evidently arranged by water, and necessarily by the waters of a lake.
I have not seen the large deposits of pumice to the north and east of Lake Taupo in the direction of the Thames and hot lakes, and shall, therefore, say nothing about them, except that I have no doubt that their distribution has been caused by water, and of necessity by the waters of a lake or lakes. To go beyond the pumice country, a great part of the Forty-Mile Bush, in the Provinces of Wellington and Hawkes Bay, must
[Footnote] * I am in doubt whether pumice will become water-logged. If not, we can easily account for its position by its being gradually left behind, as the river in cutting its way through the rangers, slowly lowered the waters of the lake.
have been a lake while the Manawatu River was engaged in cutting its way through the Gorge, and possibly there may have been a lake of some extent in the Patea country before the tributaries of the Rangitikei, the Hautapu, and the Moahuanga succeeded in excavating the deep canons through which they traverse the marine tertiaries.
I am tempted to mention a point connected with the vicinity of Wellington, and, therefore, with the theory of a considerable former elevation of land in this vicinity, and possibly, with a lake far exceeding in dimensions the present limits of the harbour. We find at considerable elevation rounded water-worn pebbles sparsely embedded in the clay and soil lying over the rock of the hills. How these pebbles got there has to me been a puzzle for a long time. I am inclined to think that a lake standing at an elevation of perhaps hundreds of feet over the present sea level is the only means of accounting for the phenomenon. I should say that these pebbles, as chiefly observed by me on the Peninsula, look as if they had been deposited at some distance from the head of the lake, in fact, where the deposit would be rare. I think further investigation will show that deposits of water-worn material, found along the ranges surrounding Port Nicholson, will be more easily accounted for by a theory of lacustrine origin than by bringing in the agency of glaciers.
To compare small things with great, it has been held that the Black Sea and the Mediterranean once formed a succession of lakes. Before the waters of the Black Sea had opened the channels of the Bosphorus and the Dardanelles, that sea would have formed the upper lake, with a subsidiary small one in the Sea of Marmora.
The soundings near Malta and Sicily show that, allowing for a former greater height of the sea bottom, a barrier probably existed there which enclosed the Eastern Mediterranean, and formed it and the Levant into a second lake; Malta being then connected with Africa. Indeed, the soundings show a channel which looks as if it had been excavated by the waters of a large river. The third lake would extend from the Channel of Malta to the Pillars of Hercules. It does not follow that these lakes were fresh water. Large basins of salt water may have been left during geological changes, and the influx of river water may not have been more than sufficient to balance the great evaporation of those regions, combined with the outpour into the Atlantic, which gradually removed the barriers. We may compare Lake Taupo to the Black Sea, the Upper Waikato to the Eastern Mediterranean Lake, and the Lower Waikato to the Western Lake.
Having prepared the way by the establishment of large lakes on the North Island for something similar with regard to the South Island, I now
propose to consider the much vexed question of the formation of the Canterbury Plains. I think the subject requires more discussion.
The point to decide is, by what agency the gravels which cover the plains have been spread evenly round the contours of the skirting mountains?
There is little or no difficulty in accounting for the gravels. The rivers traverse ranges of mountains of which the rocks are peculiarly favourable for the formation of gravel, and, given sufficient time, possibly the agencies at present in operation would be sufficient for the production of all the deposit, without calling in the help of more ice. But the distribution of the gravel is the difficulty. Captain Hutton would submerge the whole area, and use the waters of the ocean as the distributing power. If a single marine fossil could be found in the gravels of the plains, this theory might be tenable, but the absence of such evidence in any part of the large area under discussion must, I think, be held to be absolutely fatal to the argument.
That the gravels have been distributed by water there can be no doubt, but rivers will not carry gravel up hill, or create a level beach-mark of gravel round the contours of the bounding hills. Having therefore excluded the action of the sea and of rivers nothing would appear to remain to perform the work but the waters of a lake.
To form the Canterbury Plains into a lake it is necessary to suppose a barrier of high land to the eastward. This is a serious mechanical operation, but not more difficult than it would be to submerge the plains beneath the sea. Let us see what arguments we can find in favour of the proposition.
It is considered by many to be an axiom, that where volcanic eruptions have broken out the district is an area of subsidence, and in this part of the world we have strong evidence of the correctness of this view.
We have the numerous volcanic islands of the Pacific with their fringing coral reefs, and we have every reason to suppose that the volcanic districts of New Zealand shew a subsidence from a former much higher level. Thus it is probable that when the eruptions of Mount Egmont first commenced the islands were, or had been shortly before, united, and that a gradual subsidence had been going on previous to the outbursts of this and other volcanoes to the northward. Apply this reasoning to the east coast of the South Island. We there find a line of volcanic eruptions, extending from Banks Peninsula to Dunedin, and shewing at Timaru, Moeraki, Waikouaiti, besides the before-named places.
If this line shews an area of subsidence at or about the time of eruption it pre-supposes a former greater height of land in that direction, and thus
we get the conditions necessary to form the Canterbury Plains into a lake.
I would suggest that at the period when the islands were united it is reasonable to suppose a greater extension of land generally, and perhaps particularly to the north-west and south-east of Cook Strait, and the east of the South Island.
It will be seen that I agree with Dr. Haast's view of the lacustrine distribution of the gravels of the plains. I hardly think, however, that there is a particle of evidence of the former glaciation of New Zealand. Glaciation is one thing, glaciers something totally different, and not inconsistent with a climate similar to what now exists. There is any amount of evidence of the former further extension of the glaciers of New Zealand; but this may have been owing to a greater extension of high, and particularly of plateau land, and to depression of the interior of Australia thereby extinguishing the cause of heated winds, without calling in the very serious change of climate involved in the term “glaciation.”
The era in which the Canterbury Plains formed a lake must have preceded that of the eruption of Banks Peninsula, as Dr. Haast states that the latter shows none of the gravels of the plains on its surface.
One point of geological evidence often leads up to another. I have previously proved the connection of the islands, and this involves not only an elevation, but probably a large extension of land.
The elevation and extension were probably in two directions. To the westward and to the northward of the centre of Cook Strait; to the eastward of the South, and perhaps also a part of the North Island.
During tertiary times New Zealand must have been an archipelago. After the deposition of the marine tertiaries the country rose, and in its rise appears to have left barriers allowing the formation of large lakes in both islands. Subsequent depressions on both sides of the islands at the time of the volcanic outbreaks tended to obliterate the barriers of some of the lakes, but in the North Island these lakes remained until after the period of the great ejection of pumice from the central volcanoes of the Tongariri group, which, be it remembered have been the great pumice producers among the New Zealand volcanoes.
Thus my theory of the connection of the islands, of a Cook Strait river, and Port Nicholson having been a fresh water lake, has led in regular sequence of argument to a greater elevation of land in Pleistocene times, to great extension of lakes in the North Island, and to the conversion of the Canterbury Plains into a fresh water lake. Many subsequent changes have taken place upon these plains; but the distribution of the horizontal margin of the gravel must, I think, be held to have been effected by the waves and and movement of the waters of a large lake.
I am further tempted to suppose a former large lake in Cook Strait, through which the waters of the great Cook Strait river ran. My reasons for this are the configuration of the land, and the absence of soundings, denoting a former river channel in the broad part of the Strait. The distribution of gravel and clay, also, in what we may call the fringe of the west coast of the Province of Wellington, extending into Taranaki, being the belt of open and more level country than what exists further inland, shows, I think, the action either of a lake or of the sea in levelling the surface and distributing the deposition of the gravels and clays. But in this district there are found in places undoubted deposits of marine fossils, such as in the cliffs at Rangitikei on descending to Mr. Fox's house, and at the crossing of the Parewa, so that the question requires much further observation and investigation.
I am inclined to think that the large deposits of gravel and clay between the Manawatu and the Rangitikei, between the Rangitikei and Turakina, or we may say Whanganui, do not show any signs of marine fossils, except in one or two places, and as local changes of level may not have been unfrequent in these districts it is quite possible that the bulk of the gravels and clays may have been lake deposits, while a local depression may have allowed some intermixture of marine fossils, either in Pleistocene or in recent times.
There is abundant evidence that in the Province of Otago the lake system was formerly on a very extensive scale. The Clutha, the Taieri, and other rivers cut through barriers of hills or mountains, which previously must have remained as lakes, the waters of which poured into the upper basins.
Altogether it must, I think, be admitted that lakes during Pleistocene and recent times have been remarkably numerous in New Zealand, and have performed a great amount of work in levelling and distributing the superficial strata.
When we read of the small retrogression caused by the enormous rush of water at the falls of Niagara, and the estimates formed of the time which this force will take to excavate the river valley to the lake above, we may form some idea of the long periods of work which many New Zealand rivers must have had to excavate hard rock before the upper waters were released.
I am inclined to compare many New Zealand lake basins to the plains of Thessaly. These, with the mountains of Macedonia on the north, the Pindus Ranges on the west, Mount œtna, or Othrys, on the south, present also a mountain barrier to the east. It is more than forty years since I have ridden over these plains; but when I think of the conformation of the
surface, it strikes me that a large lake must have existed there before the days of Hercules, or of Agamemnon and the siege of Troy, and before the Peneus broke through the high ground between Olympus and Ossa. I may be mistaken in this idea, because I had not the time to visit the Vale of Tempe; but it strikes me that the remarkable rocks on the summit of which the Monasteries of Meteores are built are probably the wrecks of a former denudation effected while the lake was draining off.
The resemblance above pointed out is certainly greater than that “between Monmouth and Macedon.”