Art. LXII.—On the Traces of Ancient Glaciers in Nelson Province.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 4, 1871, Page 336

Art. LXII.—On the Traces of Ancient Glaciers in Nelson Province. By A. D. Dobson, C.E. [Read before the Nelson Association for the Promotion of Science and Industry 3rd May, 1871.] The labours and investigations of scientific explorers, especially those so successfully conducted by Dr. Haast, prove, beyond doubt, that at a very recent period all the higher mountains of New Zealand were covered with perpetual snow. Large glaciers filled every valley, and most of the large rivers were glacial torrents. Since that period the climate has become more genial—the glaciers have receded into the mountain gorges, or altogether disappeared, and only the highest mountains are now covered with perpetual snow; but these earlier glaciers have left traces behind them, showing the magnitude they had attained, and masses of moraine matter lie scattered over the face of the country in every direction. The largest glaciers that existed during what may perhaps be correctly termed the glacial period of New Zealand, lay on the western slopes of the Southern Alps; enormous masses of loose rock lie stretched in rivers and ridges between Hokitika and Jackson Bay, proving that these glaciers extended

from the mountains to what is at present the coast line; and if the relative level of sea and land was the same at that time as it is now, the west coast of the Middle Island must have much resembled the North Polar lands described by Dr. Kane, where the glaciers descending from the mountains reach the sea, and, breaking off in enormous masses, float away, covered with rocks and moraine matter The glaciers, both ancient and modern, of the Southern Alps have been fully explored and reported upon by Dr. Haast, and are only mentioned here to show that, with these enormous traces of former glaciation in Westland and Canterbury, it is to be expected that similar appearances will be found amongst the Nelson mountains. The mountains to the westward of Nelson rise to an altitude of from 5,000 to 6,000 feet above the sea level, and are covered with snow during the winter months; many streams rise amongst them, and at the head of nearly every large stream well defined traces of glacial action exist. I am not aware that these facts have ever been described before, and those unacquainted with the appearance of glacial phenomena, and also ignorant of the extent to which the country was once covered with snow and ice, might easily pass by these indications, or, if noticing indifferently, might perhaps mistake them for the traces of avalanches. The ancient glacier beds are much overgrown with scrub and bush, the striated and polished rocks frequently covered with broken rock and grass, so that there is not often much to catch the eye of a casual observer. The most clearly-defined glacier bed that I have seen west of Nelson is situated at the head of the Boulder River, a tributary of the Aorere, which falls into Golden Bay at Collingwood. The river takes its rise from a lake (Te Warau), which is 3,200 feet above the sea level, and occupies the lower end of a narrow valley formed by Lead Hill on the west, and a ridge which divides the Boulder River from Rocky River and the heads of the Anatoki on the east. Lead Hill is a mass of granite, which rises to a height of 4,450 feet above the sea, and has forced up the slate which forms the opposite ridge to the eastward to a highly inclined angle. The valley in which the lake is situated lies between the granite and the slate, and is about three miles long from the southward; it commences amongst the mountain spurs, ending at the north in a cliff 200 feet high, over which the water from the lake falls in one unbroken sheet. Just above the cliff a dome of granite rises about 100 feet above the lake, its sides furrowed and polished, and every angle rubbed off, giving it the appearance of a huge grey snowball left standing on the hill side. All the surrounding rocks are also scored and polished for a height of about 150 feet above the lake. The height at which the scouring action of the glacier ceased is most distinctly marked,

and here and there on the furrowed rocks large blocks of granite and slate lie strewn, as though placed there but yesterday fresh from the quarry. On the western side of the valley, near the lower end, a number of granite blocks lying upon the slate show the height at which the ice formerly stood, and these blocks must have been brought down from the peaks at the head of the valley, or else from the west side, as no granite occurs on the east side. The glacier, when it attained its fullest size, was about four miles long from the névé saddle to the terminal face, which must have been about 200 feet high, and rested on the top of the cliff above described, over which the glacier would be forced as the daily motion carried it forward. At the foot of the cliff the deep ravine in which the stream runs is partly filled with large blocks of rock, lying as when borne down and thrown there by the glacier. This interesting spot is within a day's walk of Collingwood, and a fair bush track exists for the whole distance, so that it can be easily reacbed; and, apart from its scientific interest, the deep blue lake, with its green and grey setting of birch trees and granite, forms a most pleasing scene, which would well repay the artist or lover of scenery for the labour of the ascent. Several small glaciers existed at the heads of the Anatoki, and most probably one of considerable size at the head of Clarke River, but that locality I have not yet been able to visit. The Mount Arthur range, which attains its greatest height (5,800 feet) in the Mitre Peak of Mount Arthur, gave rise to many glaciers. The most conspicuous remains that I have had the opportunity of visiting I shall attempt to describe. Commencing with the eastern side of the range, the principal one lay just at the foot of Mount Arthur. The terminal moraine occupied the bed of the stream at the point where the basin at the foot of the Mitre Peak contracts and the stream enters a gorge. A broad expanse of rounded rocks, kept bare now by the winter snows from the overhanging peaks, shows the amount of ice action to which the rocks have been subjected. This glacier was triangular in shape, the terminal face occurring at the apex of the triangle, and the ridge of the mountains formed, the base. The area was about two square miles, and the terminal face about 3,000 feet above the sea level. At the head of the Bâton a small glacier occupied an oval basin at the foot of Jones' Saddle, having an area of about 400 acres. The terminal and lateral moraines are very distinct, although much covered in places with detritus from the mountain sides. The lower part of the old bed is a swampy flat, the moraines having formed a dam which retained all the mud gravel brought down the mountains by the rains. It is most probable that this glacier was fed by glaciers of the second order, which existed on the flat sloping mountain tops that surronded it. A more beautiful scene than this must have presented when at its greatest size it would be difficult to con-

ceive—the main glaciers lying in an oval valley surrounded on three sides by mountains rising 1,500 feet above it, full of cliffs and rugged pinnacles capped with large snow fields, must have been extremely grand. Near the head of the Wangapeka, at the foot of Mount Target, is a small glacier, about one mile long and about 200 yards across in. the widest part, in whose centre occurred an ice cascade 150 feet high, and the ice was here contracted to about sixty yards in width. The lower part of the bed is at present a swampy flat covered with grass, the moraines having dammed back all débris brought down by the mountain streams, the old bed has been filled up nearly level with the top of the terminal moraine. The terminal face is about 3,600 feet above sea level. Much larger glaciers existed on the west side of the range than on the east; these for the most part poured their waters into the branches of the Crow River, a tributary of the Karamea. The two largest occupied the north and south branches of that stream; the northern one was about four miles long, and from half to a quarter of a mile wide. The terminal moraines being about 2,700 feet above the sea level, a large snow field supplied this glacier— the whole of the west slope of the main range from the peaks near Jones' Saddle to Hough's Saddle, and all the east slopes of the mountains forming the west side of the basin—containing in the aggregate about nine square miles. The old glacier bed has been very much filled with detritius from the mountains, which has buried the moraines, so that it is difficult to find unmistakable signs of ice action without careful search. The terminal moraine has been almost completely hidden, but may be found where the present stream has cut its way through it. The glacier which occupied the southern branch descended from the northeast slopes of Mount Target, and is of considerably less extent than the one last described. I was unable when in that district to spare the time to search for signs of its terminal face, but I should think it must have been about 3,000 feet above sea level. These glacial remains, which I have attempted to describe, are only a few of the principal ones of a great number. At the head of every gully near the main peaks small flats are visible covered with grass, which undoubtedly have been formed by the filling up of old beds, the moraines acting as dams to retain the material washed down the mountains. The largest glaciers in all cases occurred near the highest peaks. During the period the line of perpetual snow must have been very much lower down the mountains than it is at present. I should be inclined to think that it was about on a level which is now only about 4,500 feet above the sea, and from the very distinct manner in which many of the old beds are still marked, and the unburied state of many of the moraines, I think the disappearance of the ice must have been at a very recent date. No one can behold Lake Te Warau

or the head of the Bâton without being struck by the freshness of all the indications of ice action existing there. At Lake Rotoiti also are extensive glacial remains; though these have been frequently described before, it may not be out of place here to say a few words respecting them. The lake occupies an old glacier bed, and is about 2,060 feet above the sea level; the glacier descended from Mount Travers, and is about twelve miles in length and two miles wide, near the lower end. The terminal moraine now forms a series of low hills encircling the northern end of the lake, and the bed of the Rotoiti for several miles from the outlet of the lake is filled with ice borne rocks. The Black Valley, which lies at the northern end of the St. Arnaud range, was also filled with ice, and the road which leads from the Rundell to the lake passes along on a lateral moraine for some distance; large blocks of stone lie abot the surface, the sharpness of their angles and the absence of any signs of water-wear clearly proving that their appearance there is due entirely to glacial agency. Both the Spencer mountains and the St. Arnaud range must be very rich in glacial remains, and have been but little explored in their higher parts, being covered with dense bush, and lying back some distance from roads and inhabited country; their thorough exploration is a matter involving very considerable time, labour, and expense, and can only be undertaken by those whose duty leads them into such country, or those who have ample time and means at their command. The examination of these phenomena clearly proving a wonderful alteration of temperature within a very late period, naturally induces the mind to seek an explanation of the change, and two hypotheses offer, I think, a reasonable solution of the difficulty. 1. That New Zealand has sunk during late periods, bringing the former snow line much nearer sea level. 2. That there formerly existed land to the southward, perhaps forming part of a southern continent, the proximity of which would naturally be attended by a great increase of cold in New Zealand, and its disappearance would be accompanied by a complete change in the climate of all neighbouring lands. Which of these solutions is the right one I cannot attempt to say, and only offer them as suggestions, which must be taken for what they are worth. I feel most inclined to favour the former. There is, I think, but little doubt that these islands have been subjected to many, and very considerable alterations of level during recent periods, and a subsidence of a few hundred feet would necessarily make a very marked difference in islands possessing the physical configuration of New Zealand. But these speculations lead much further than can be followed in this paper, the object of which is simply to describe a few of the traces of the glacial period which exist in our immediate

neighbourhood, in the hope that by the collection of a few observations (however erroneously interpreted) it may to some extent assist and lighten the labour of those whose wisdom and experience enables them to interpret correctly those signs which record the geological history of the later periods.