Art. LXIX.—On the Date of the Glacial Period; a, Comparison of Views represented in Papers published in the Transactions of the N.Z. Institute, Vols. V. and VI.
[Read before the Nelson Association, 5th April, 1875.]
The last issued volume of the Transactions of the New Zealand Institute contains several papers on the Glacial Period of New Zealand, which are exceedingly interesting, not only from the descriptions given of geological and geographical phenomena, but also from the different conclusions arrived at by the several writers. I have ventured to think it would be interesting to others as well as myself to see the different views placed side by side, with the object of ascertaining in what they agree, and in what differ; and although Dr. Hector, in his annual address to the Wellington Philosophical Society, very truly remarks, “that much has still to be done before any decision on this point (date of glacial period) can be arrived at,” I am of opinion that an examination of the various arguments as at present advanced will greatly assist in clearing the way, and may, perhaps, put observers upon their guard against fallacious theories. But very few observers are capable of simply recording observations without having some theory on which to adjust them; it therefore becomes the more necessary to examine carefully into every hypothesis, and reject all which are palpably untenable, holding temporarily to the one which seems to explain best the records of bygone times which everywhere surround us.
In volume V. of the Transactions we find an interesting paper on the Date
of the last great Glacial Period of New Zealand, by Captain Hutton, without which the consideration of this subject would be incomplete. The author's views, briefly summarised, are as follows:—That the greatest extension of the glaciers was coincident with the greatest elevation of the land, and occurred in older pliocene times, which was a period of elevation; the newer pliocene period was one of subsidence; followed by elevation in pleistocene times, and that elevation is still going on.
In volume VI., Mr. W. T. L. Travers, in a paper on the Extinct Glaciers of the Middle (South) Island of New Zealand, infers that the extension of glaciers was due to a great elevation of the land in miocene times, which elevation continued into and was greatest during pliocene times, when subsidence commenced and continued into pleistocene times, during which fresh upheaval took place; and that in consequence of the elevation of the land, and a much larger area being above water New Zealand assumed a quasi-continental character.
Mr. J. T. Thomson, F.R.G.S., (Glacial Action in Otago) considers the island must have been at a considerably lower level than at present, the land covered with glaciers, and surrounded by a wintry ocean, the country generally presenting similar features to Victoria Land in the Antarctic Ocean. The author, so far as I can learn, does not touch upon the question of geological age.
In a paper written by myself (Notes on the Glacial Period), the greatest extension of the glaciers was considered to be due to elevation, which commenced towards the close of the pliocene period and continued, the greatest elevation of the land being marked by the greatest glacial extension; subsidence then took place, continued up to the present time, and is still continuing.
So far I have briefly given the views advanced by the writers of the papers before mentioned; but, in addition to the papers, we have the remarks made by Doctors Hector and Haast in the annual addresses published in the Proceedings, which will well repay perusal, and to which I shall allude subsequently, merely premising that, from what I have read of the writings of these gentlemen, if I understand them aright, they neither consider much elevation of the land to have been a necessary accompaniment to the glacial period, referring the change more to other causes—the wearing away of mountains, etc., and possibly to changes in the relative proportion of land and water in the neighbourhood of New Zealand.
Messrs. Hutton, Travers, and myself are agreed in supposing a great elevation of land to have been the primary cause of the extension of the glaciers, and subsequent depression the cause of the extension, but differ materially as to the date of the elevation and subsidence respectively; Captain Hutton considering the old pliocene period one of elevation, the newer pliocene one of subsidence, followed by elevation, which he supposes to be at present
going on. Mr. Travers agrees in the main with Captain Hutton, only supposes the elevation to commence during the miocene period, and continue into pliocene times, subsidence and fresh upheavals occurring during pleistocene; whilst I have considered both the elevation and subsidence to have occurred in postpliocene times, and that no general elevation has occurred since.
Before the former of these theories (Hutton's) can be accepted as tenable, it is necessary to show that the newer pliocene is partly contemporary with, and partly subsequent to, the glacial period, and in this case, as the glaciers disappeared during the pliocene period, it would be impossible to find any pleistocene strata covered by morainic matter. Captain Hutton regards the Moutere Hill drifts, which cover so large a portion of the north central portion of the Nelson Province, as older than, the glaciers, in which case they must be either miocene or older pliocene, but could not possibly be of later origin. If I am not mistaken, the Moutere Hills are composed of the same drift (i.e., are of the same age) as the drift which covers all the lower country from the shores of Blind Bay on the north to the Mikonui Hills on the south—it varies in character, being generally composed of the same materials as the nearest adjoining mountains—for the most part it lies nearly horizontal, but in the Grey Valley and to the southward appears to have been slightly disturbed, dipping somewhat to the northward and eastward. The general structure is much about the same throughout—drift shingle alternating with beds of sand, the shingle in places forming a tough conglomerate with the stones more or less decomposed; in others the shingle hard and loose, and washed down the gullies by every rain. Dr. Haast thus describes a section he obtained in the Grey Valley:—
“Near the junction of the Mawheraiti with the Grey, on the southern side of the main river, I obtained another section. Here the banks are almost vertical, exposing a section of nearly 120 feet. In the river itself we find a large stratum of clay marl, in which are also many pieces of drift wood converted into lignite. These beds, of a bluish colour, are nearly horizontal, and at one place the stump of a tree, fifteen inches in diameter, broken off above the root, stands apparently in sitû, the roots still adhering strongly to the clay marl, so as to lead to the impression that it grew upon the spot. These beds, which rise nine feet above the level of the river, were probably deposited in a shallow estuary; they are divided at unequal intervals by horizontally deposited layers of mica; they change insensibly into loam, which is succeeded by a large accumulation of sand, gravel, and loam, interstratified with layers of boulders, partly angular and partly rounded, and resembling very much the drift formation near Nelson.” *
[Footnote] * Report on a Topographical and Geological Exploration of the Western Districts of the Nelson Province, N.Z., by Dr. Haast, p. 103.
Dr. Haast makes no mention of finding fossils in these strata, but suggests the age as “younger tertiary (pliocene?).” In going up the Ahaura River excellent sections of these beds are obtained at the Big Gully, ten miles from Ahaura township. The river has cut down through 400 feet, and there is no sign of it having yet reached the bottom of the series. Going further up the river the strata dip more to the eastward. Just before reaching Starvation Point the stones get much larger, having much the appearance of an ancient river bed, then suddenly the drift changes to loose sand, covered with morainic matter—loose angular blocks many tons in weight lie scattered over the surface.
These drifts, both in the Grey Valley and the Upper Buller, rise nearly 2000 feet above the sea level. It is in the rewashes of this drift that the principal alluvial workings in the Grey Valley occur.
In the Upper Buller, near Lake Rotoiti, the drift is for the most part very loose, but proceeding to the northward it becomes more clayey, the stones being much decomposed, forming a softish conglomerate, with intercalated beds of clay and sandstone containing streaks of lignite. Although I have taken every opportunity of searching for fossils in these beds, I have never yet succeeded in finding any, nor have I ever heard of any being found. Judging from position and general appearance, I consider the drifts must be much younger than the Nelson tertiaries at the Port Hills and Jenkins' coal mine; and, without very strong evidence to the contrary, I see no reason for assigning to them the great age that would be necessary, adopting the hypothesis of either Captain Hutton or Mr. Travers; for whether we consider the glacial period to have been in older pliocene, or to have begun in miocene, times, the drift being older than the glaciers must be at least miocene. I have discussed this question at some length, as I believe the careful examination of the drift formations of New Zealand generally will throw a great deal of light upon many points at present unsettled, and that until the age of the drift is satisfactorily settled we may speculate in vain upon the age of the glacial period.
The next question that claims attention is the general question of elevation and subsequent depression, which, apart from the question of age, is common to the hypotheses of Captain Hutton, Mr. Travers, and myself. A glacial epoch is, no doubt, easily accounted for, by supposing an elevation of land into the snow line, and the gradual extinction of the glaciers must necessarily follow a subsidence; but before this can be accepted as anything more than a provisional hypothesis much more proof is required than has been put forward as yet. I am inclined to consider the great drift to have been partially caused by marine action, but have no evidence to bring forward in support of my views, further than the similarity of the drift and gold in many of the inland
leads, now lying at high levels, to the drift and gold of the leads lying along the coast above the present level of the sea, which so exactly resemble the beach leads at present forming that I cannot doubt but they have been formed in a similar manner.
I may here mention that an elevation theory is not by any means generally accepted as the chief cause of glacial extension. Dr. Haast, who has studied glacial phenomena more than any other geologist in New Zealand, from what I can learn from his writings, does not appear to consider former glaciation due to greater elevation; and Dr. Hector, in his address published in the sixth volume of the Transactions, suggests a larger area of ground both above high water mark and above the snow line, and alterations in the surroundings of New Zealand.*
Recent upheaval is the last phase to be considered, and in this I do not agree with either Captain Hutton or Mr. Travers; and I cannot see that any evidence in favour of recent elevation is given by either of the above named gentlemen; had any recent elevation taken place anywhere round the shores of Blind Bay, traces should certainly be existing at present. I am well acquainted with the north and west coast of the Nelson Province, and the east and west coast of the Canterbury Province, but know of no raised beaches which I should consider of late date. It is true, I conceive the auriferous drifts which are worked for gold on the coast, at levels varying from fifty to 400 feet above sea level, owe their shape to the combined action of sea and river currents; but I believe these leads or beaches to have been deposited prior to the glacial period, as in several places they are covered with moraine matter.
The Canterbury plains show no signs of raised beaches, and Dr. Haast's report upon the formation proves beyond all doubt their fluviatile origin. In a paper on the Glacial Period, published in the last volume of the Transactions of the New Zealand Institute, I stated that the drift formation of the Canterbury plains began during the pliocene period, when I considered the greater part of the lower lands of the South Island were submerged beneath the sea. This statement requires qualification; if I am correct in imagining a great submergence to have taken place previously to the glacial epoch, marine deposits and gravel drift would undoubtedly have been formed, but the enormous amount of detritus brought down by the rivers during the glacial period would be sufficient to entirely bury all evidence of marine action throughout the area now occupied by the Canterbury plains. This want of evidence of marine action is the greatest difficulty to my mind in the way of the elevation theory, without we assume the elevation to have been chiefly confined to the western side of the island, which rose and fell without affecting the general level of the east coast.
[Footnote] * Trans. and Proc. N.Z. Inst., VI., pp. 374 and 385.
This is not unreasonable, for without proof I see no reason why we should imagine elevation or subsidence to occur horizontally over any large area; movement confined chiefly to the west side of the island would account for several apparently irreconcilable difficulties. It must be understood that I mean movement greatest near the west coast, and lessening eastward, becoming nothing at the east coast; it would fully account for raised beaches on the west coast, and none on the east. As there is some doubt as to whether the auriferous leads on the coast are raised beaches, a short description will not be out of place here. From Hokitika on the south to Karamea on the north, drift deposits occur, which are all more or less auriferous. These deposits are composed of gravel and sand; the gravel being granite, indurated sandstone, quartz, and hard schistose rocks, all well water-worn, and having the circular disk-like form always met with in beach shingle.
Confining description to the Nelson Province, a lead of this description occurs at Point Elizabeth, four miles north of the river Grey; it is about fifty feet above the present sea level, and was most probably deposited by the combined action of the sea and river on what was then a beach at the mouth of the Grey, the river Grey having once had its outlet to the northward of Point Elizabeth, before it broke through the limestone range and formed its present channel. The drift either lodged only in this spot, or, if there was a greater extension, all trace has been washed away, for the deposit which proved so very rich in the early days of the Grey diggings extended for a distance not exceeding thirty chains in length, nearly parallel to the existing beach, and not more than one or two chains in width.
Proceeding northwards, patches of gravel occur at a height of about sixty feet above sea level at various places along the hill sides, but none have yielded much gold, until we arrive at Brighton, where the drift extends over a considerable area of country. The gold leads in this district exactly resemble beaches, and are parallel to the present water line.
From Brighton northward to the Waimangaroa the drift extends, bearing numerous gold leads, the highest being at Dawson's Terrace, about 400 feet above the present sea level. Near Charleston leads were worked on six different levels, all of which bore the same littoral character. North of the Buller the leads rest on a bottom of fine sandstone, are overlaid by a drift of heavy granite boulders, which is in its turn capped by a mass of large sandstone blocks, which I believe have been brought down from Mount Rochfort by glacial action.
It is impossible to give an adequate description of these gold leads in a few words, or to convey any useful information unless accompanied by plan and section, but I hope at some future time to be able to write a description accompanied by such plans and sections that will have some value apart from any hypothesis as to their date and origin.
Southward of Hokitika enormous glaciers covered the face of the country, leaving mountains of moraine matter to indicate their former dimensions; lateral moraines run from the foot of the mountains to the sea, but nowhere could I observe any trace of marine action posterior to the glacial times.
In a report on the West Coast Gold Fields, made by Dr. Haast for the Canterbury Government in 1865, after a careful examination of the country and the gold workings, the auriferous drift covering the face of the country from the Grey to the Mikonui—and which is no doubt of the same age as that occurring throughout the Grey Valley—is attributed to the pliocene period. Throughout the report he speaks of the loose nature of this drift, and attributes its preservation to favourable circumstances, it having been destroyed in many places by glacial action. He describes the drift overlaid by moraine matter at Kanieri, near Hokitika, and in all cases, so far as I can learn, he makes no mention of any recent raised beaches, or any trace of marine action on the great moraines—matters which could scarcely have escaped his observation.
I think, from the consideration of the foregoing, it may be safely assumed that no general elevation has occurred since the glacial period, and that if subsidence was the cause of the extinction of the glaciers, subsidence may be considered as the latest movement. The age of the youngest member of the “great gold drift” series, to meet the hypothesis of Messrs. Hutton and Travers, must not be later than miocene, and, as I have previously remarked, there is no evidence advanced to support the belief in such an age.
The elevation and subsequent subsidence hypothesis, as accounting for the former great extension and after extinction of the glaciers, may be considered a good provisional one, as readily accounting for a great variety of phenomena. But, having already given my own views, I need not repeat them, and shall now leave the subject, trusting that I have discussed the matter clearly, and have given fairly the views of the authors of the papers under discussion.
Before concluding, I may remark that the evidences of glacial action so widely distributed over the South Island of New Zealand, and occurring on so grand a scale as to attract attention even from the most uncultivated observer, form one of the most interesting of the geological records which the lovers of science are called upon to decipher.