Art. XLIX.—Notes on the Glacial Period.
[Read before the Wellington Philosophical Society, 22nd September, 1873.]
In a former paper, * which was merely a description of glacial remains in the Nelson Province, I suggested that the former extension of glaciers was due either to the much greater elevation of the land above the sea level at that period, or to the existence of land adjoining to the southward. Since writing that paper I have come to the conclusion that the last greatest glacial extension was due to a greater elevation of the land, and that, although other agencies—
[Footnote] * Trans. N.Z. Inst., Vol. IV., p. 336.
such as the wearing down of the mountains by glacial and other action, and the destruction of land by the sea—have all tended to render the climate less rigorous, the subsidence of the land has to be looked to as the chief cause of the termination of what may be called, for the sake of convenience, the glacial period.
On the west coast of the Middle Island the effects of glacial action, as exhibited by masses of moraine matter covering large areas, are so striking as to arrest the attention of even the most superficial observer. Struck by the recent appearance of many of the moraines, and the manner in which glacial drift caps the general drift of the country, I have ventured to collect the numerous notes I have made during the last ten years, and from them deduce what I conceive to have been the changes which have taken place during the latest geological periods.
At the end of the pliocene period I consider that but little of the Middle Island stood above water; the main back-bone of the island probably constituted a series of rocky islands, the sea level being about 2,000 feet higher during this period, which must have been of great duration; the gravel drifts were formed which cover the greater portion of the level lands, and cap all the older formations from Nelson to Hokitika. During this period were formed the Moutere Hills, and all that great face of drift hills which occupies the whole of the depression between the east and west ranges, from the southern shores of Blind Bay to Lake Rotoiti. Here the drift formation is interrupted by the mountains which divide the Lakes Rotoiti and Rotoroa, and also by the mountains forming the watershed between Rotoroa and the westward streams. Again, the drift formation occurs in the valleys of the Matakitaki and Maruia, and then continues on almost uninterruptedly, capping the older rocks throughout the flat country right down to the Mikonui River. Following the coast northwards from the Grey, the drift again occurs in all available places for deposition at heights varying from 10 to 500 feet above the present sea level. The reason for the drift attaining such a much greater height at the head waters of the Buller and Grey was, that there the drift was free from the destructive effects of stormy seas, and was deposited by streams flowing into a quiet strait protected by high land on the east and west, and was subjected only to the settling action of marine currents running through the strait; whereas on the coast line the heavy westerly swell from the Pacific, aided by the strong littoral currents, prevented the deposition of drift except in sheltered places.
It was during this period that the formation of the great gravel drift of the Canterbury plains began. The country there began to rise, and although the formation of gravel drifts continued on the lower levels, the waters of precipitation began to carve them on the higher levels into the forms
they now present. The elevating forces continued in action until the land was very considerably higher than at present; and, as the mountains were thrust higher into those colder regions of the air where snow does not melt, glaciers were formed in the deepest valleys, and, extending as the land rose higher, they reached those limits where we now find the enormous masses of moraine matter.
The period of the greatest extension of the glaciers marks the time of greatest elevation. After this subsidence commenced, and continued with slight local interruptions up to the present time, and is, in my opinion, still continuing. Moraine accumulations occur on the east side of the Southern Alps, chiefly at the lower extremities of all the large lakes, and form dams by which the lakes are partly formed. These are, by no means, the only places; I mention them as being those where moraine accumulations occur in the greatest mass, and are the most striking to an observer.
On the west coast the moraine matter occupies far more ground, and attains a much greater thickness, than is observable on the east side. From Bold Head southwards, as far as Jackson Bay, numerous cliffs form the coast line, which are the moraines of ancient glaciers. Lateral moraines run landwards, narrowing as the hills are reached, whilst in many places the final meeting of moraines has left low, irregular hills, entirely composed of loose masses of rock now covered with dense vegetation. Receding from the coast line, and examining the drift at the head of the main tributaries of the Grey, Teremakau, and Hokitika, the moraines are everywhere to be found, though very much smaller in size. This is due to the fact of the mountains being very much lower than they are to the southward. The Southern Alps culminate in Mount Cook, the range lessening in height to the south and north, besides being much more narrow, thus allowing less room for snow-fields. The moraine matter whenever found overlies the shingle drift, and in all the places which I have examined it shows no signs of having been under water or subject in any way to marine action, which could not possibly be the case if the sea had stood at any considerably higher level than it does at present, since the glacial period. The loose moraine mounds forming the cliffs near Abut Head and near the Poerua would be quickly levelled if subjected to the action of the sea. It is this undisturbed appearance which leads me to conclude that there can have been no general elevation of land since the glacial period. There is a marked absence of raised beaches; the few that do exist on the West Coast were, I consider, formed during the period of elevation, as they occur in several places at heights varying from 50 to 200 and 300 feet above sea level. They only occur in sheltered spots, and have been subject in many places to much denudation.
Briefly stated, my hypothesis is this: That the glacial period commenced
during a period of elevation, during which many of the existing raised beaches and auriferous leads were formed; that continued subsidence followed the close of the glacial period, and that subsidence is still continuing.
If this is correct we must expect to find traces of ancient beaches overlaid by glacial drift, and glacial drift at far lower levels than many of the beaches. Of the latter there are abundant examples south of Hokitika, where the morainic accumulations cover a great portion of the level country, which I have already described. Of the former there is an excellent example north of the Buller River, where the auriferous beach drifts on the slopes of Mount Rochfort are covered by a large mass of sandstone boulders derived from the Mount Rochfort sandstones, and evidently transported to their present position by glacial action.
In the foregoing the term “glacial period” means the last period of great glacial extension.