Art. XLVIII.—Port Nicholson an Ancient Fresh-water Lake.
[Read before the Wellington Philosophical Society, 18th August, 1873.]
A Remark and a question by Dr. Hector have led to the subject of the following observations. The remark was that the peculiar denudation of the Miramar Peninsula was difficult to account for under present conditions, and gave him the idea that it was formerly the summit of a mountain. The question was, whether I had observed any signs of marine remains in this
locality higher than a height of about fifteen feet above the sea. To this I replied in the negative.
Cogitating over these matters, I have come to the conclusion that the probabilities are that the land in this neighbourhood was never, since the older rocks were elevated, at a lower level than about fifteen feet below that at which it stands at present, and that at some time, probably during the depression of the tertiaries, it attained a great elevation, possibly equal to the present extreme altitude of Tararua, viz., 5,000 to 6,000 feet, perhaps higher. At this time it is reasonable to suppose that Cook Strait did not exist, and that the islands were united.
If we follow up the main chains of Tararua and Ruahine to the northward we find a gradual rise of the tertiary beds which rest on the flanks of these mountains, attaining, at the Manawatu, a height of between 400 and 500 feet, and in the vicinity of the Kaimanawa range a height of 2,700 feet.
These rocks extend from eocene to pliocene, and many changes of level no doubt occurred during their deposition, but while in course of formation they must have been beneath the sea.
My supposition is, that previous to the time when they were in course of formation there was an oscillatory movement which depressed the more northern rocks and raised those in this vicinity; while, at the time when these tertiaries were raised above the sea level, the movement was in an opposite direction, the whole of the west coast tertiaries emerged and were raised at their northern limit to a great elevation, while the land in this neighbourhood was depressed until it sunk below its present level, and probably at the same time Cook Strait was formed and the islands separated.
I have formerly remarked on the various terraces which may be observed on the coast towards Terawiti, and have supposed them to mark old beaches, showing lines of rise of the land. As I think no remains of marine origin have been found in them, I now suggest that they mark the banks and various levels of an ancient river, the other bank having disappeared in the waters of the Strait. With a supposed high elevation and greater mass of land, we may suppose a larger river or rivers than any which we now possess in this vicinity, and some things may thus be accounted for which are difficult of explanation otherwise. Thus, if we consider the boulder and gravel formation which forms the isthmus between Evans and Lyall Bays, and on which the sand-hills are a mere excrescence, it is difficult at first sight to perceive where the boulders came from. No boulders or gravel are now washed up on either shore, and it is not perceptible why, if the land stood at a lower level of fifteen feet, boulders should then be thrown up. Still less could they be thrown up if the land stood at a higher elevation than it does at present.
I have come to the conclusion that at a time when the present entrance to the harbour was closed, and when Port Nicholson was a fresh-water lake, the boulders of the isthmus mark a river bar—not a bar at the mouth of the river, but a bar at the foot of a pool or lake and above a rapid—probably accumulated against a ridge of harder rocks. That this bar must have been re-arranged and altered by the sea at the time when the land was depressed some fifteen feet below its present level is sufficiently obvious, and accounts for its present appearance as having been latterly a marine boulder bank, under somewhat similar conditions to that at Napier, although different as lying between two bays.
When we consider the matter fully, it seems a necessity that Port Nicholson must have been formerly a fresh-water lake. The borings taken from the wharf showed the remains of land vegetation at a considerable depth. There are numerous other reasons to suppose that, with the exception of the before-mentioned rise of fifteen feet, there has been a long era of depression, and that consequently the land must formerly have stood at a much higher elevation. One feature is, I think, conclusive as to a certain amount of elevation, viz., the present entrance of the harbour. From the remains of Barrett Reef and other rocks it is evident that this entrance or passage has been excavated chiefly by denudation. Now, it is impossible to suppose this to have been done by the ebb and flow of the tide. The requisite effect is to be easily accounted for by supposing Port Nicholson to have been a fresh-water lake with an outlet in Evans Bay. From the erosion of the coast line the locality of the entrance of the harbour became weakened, the waters took that direction and scoured out the channel, leaving the old Evans Bay passage high and dry. Supposing the present entrance of the harbour closed, Port Nicholson would even now become a fresh-water lake, with an outlet in Evans Bay; and taking the mean elevation of the isthmus at fourteen feet above high-water mark, and supposing a depth of stream of only five feet, the waters of the harbour or lake would be raised so as to submerge a large part of the Hutt Valley.
On the other hand, taking the mean depth of the harbour and entrance at ten fathoms, if we suppose a rise of the land of sixty to seventy feet, we should have a fresh-water lake, although of diminished area, even with the present entrance open. I think it will be found that the barrier was originally sufficiently high to form a lake extending up the valleys of the Lower and Upper Hutt, and that the deposits which have filled these valleys are from the talus of the river drifts falling into a lake. There appear, in the Hutt Valley, to be deposits of heavy boulders succeeded by gravel and clays, and finally by fine alluvium, the latter lately covered by a magnificent forest, now almost entirely destroyed by man.
No strata of marine origin appear to be found in the Hutt Valley. Had
the gravels and clays which fill the basin of this valley been deposited in the waters of the harbour, which, on that supposition, would then have flowed up the valley, remains of marine origin must have been found.
The formation of Thorndon Flat, although difficult to account for without the assistance of the above theory, becomes comparatively easy when supposed to be a gentle deposit in a fresh-water lake. This I suppose it to be. The more I consider the question the more it appears to me impossible to suppose that Port Nicholson could have been for ages anything else than a fresh-water lake.
The question still remains for consideration how the basin of the lake and valley was originally excavated, for excavated it must have been.
Although unwilling, without due cause, to drag in the much-abused agency of ice, yet I must say that I think the most reasonable theory we can form on the subject is, that the great work of excavation was at least commenced, and in great part executed, by the agency of a glacier. If we suppose a great elevation of land in the neighbourhood, possibly including the whole of Cook Strait to Taranaki and Cape Farewell, and the still, at that time, undenuded state of the higher parts of Tararua, it is easy to conceive, or possibly difficult to resist, the inference that a névé crowned the higher plateau, and a glacier once filled the valley of the Hutt and the harbour of Port Nicholson, and so far excavated the valley as to prepare a basin for a lake and subsequent harbour. This would appear to me to be the simplest explanation of the changes which have taken place. It will be seen that the chief foundations for the theory are the peculiar denudation of the district and the boulder-bank in Evans Bay. With regard to the latter it is not necessary to suppose that the boulders were brought from a distance, for they might have been derived from the remains of the Evans Bay denudation. Some few boulders of granites and schists, which I have found on the isthmus, arrived there, no doubt, at a comparatively recent period, and since the re-arrangement of the boulder bank by the sea.
It may seem absurd to notice a curious idea of which many people seem to have got hold, from what information it would be difficult to determine, viz., that Captain Cook sailed into Port Nicholson through the passage of Evans Bay. The best authority on the subject ought to be Capt. Cook himself, and, as he does not mention his visit to this port, it is reasonable to suppose that his ships never entered it. But I think I have shown conclusively that he could not have entered by Evans Bay, for, even supposing that the land at the time of his visit to New Zealand stood at a lower level of fifteen feet—which supposition would require an extraordinary stretch of imagination—that depression would only allow about a foot or two of water at high tide for the passage of his ships, and, notwithstanding the smaller size of the vessels in those days, that depth of water was clearly insufficient.
No further depression of the land is permissible with the evidence at our disposal.
In the above theory I have been obliged to allow for vast changes in the elevation and depression of land, but if we consider that since the deposition of the tertiary strata a large part of Europe has emerged from the ocean, and that tertiary rocks have been elevated to a height of 5,000 feet in Sicily, and, if I remember right, also on the flanks of the Alps, it is surely not unreasonable to suppose that changes of equal magnitude may have taken place in the Southern Hemisphere. There can be no dispute that the tertiaries extending from Cook Strait towards Ruapehu and the Kaimanawa range attain an elevation in the interior of the island of 2,700 feet; and the tertiaries on the eastern side of Tararua reach, in the Puketoi range, a height of fully 2,000 feet.
If the above theory is a sound one it will be interesting for other observers to link the supposed changes of level with those which may have taken place on the southern side of the Strait.
The series of ancient valleys on the opposite shore of the South Island, which now form harbours and sounds from Cloudy Bay to the French Pass, are at once suggestive of depression, and certainly give the idea that Cook Strait is now the base of a synclinal curve, which, on the supposition of the correctness of my theory, formerly formed a horizontal line, or possibly an anticlinal curve.
It will be seen that I have given a liberal allowance of time, for the elevation I suppose must have been previous to the deposition of the older tertiaries.
That Port Nicholson was formerly a fresh-water lake can be proved without the necessity for much elevation, but the peculiar form and denudation of the land, together with the preparation of the valley basin, requires, I think, that we must assume a high elevation in former times.