Notes on the Glaciation of Ruapehu.
Read before the Otago Institute, 8th December, 1925; received by Editor, 30th December, 1925; issued separately, 7th December, 1926.]
In January, 1923, I spent a few days on the slopes of Mt. Ruapehu, which we climbed on the 17th. I have been asked to write some account of my impressions of the glacial features noticed on our ascent.
Ruapehu rises “from a circular base of some forty miles in circumference to a summit [of 9175 feet] truncated by a crater nearly a mile across.… The summit-crater is almost filled with ice, which has used the hollow as a ‘collecting-ground,’ the excess overflowing the low parts of the crater-ring chiefly towards the east, while a part (of the ice) moves in the direction of a small hot lake in the middle of the crater, formed by the action of the escaping steam on the ice, the supply of water being constantly replenished by the melting of small icebergs which break away from the ice-front as it reaches the hot water.”*
In the diagrammatic sketch (fig. 1), it will be seen that the cone rises rather rapidly from extensive tussock-plains situated at a level of about 3,500 feet above the sea.
A word or two as to the zones traversed in the ascent may be of interest. The first belt consists of forest growth, largely mountain beech, and is about one mile wide. Then a zone of rising moorland is crossed with heather and grasses, the path being marked by small cairns. Deep gullies are carved out of the soft volcanic debris on which flourished a few flowers.
At an elevation about 1,000 feet above the hut we came on an isolated block of lava about eight feet across, which stood out markedly on the relatively smooth slopes thereabouts. I show its approximate position on the sketch. It reminded me of many erratics I had seen in other regions, but no definite proof other than shape and position can be adduced. The absence of striae means little. In the Antarctic in 1911 we traversed 100 miles of country where ground-moraine was present for miles, and saw only one striated block. The same type of rock—a rather soft and coarse eruptive rock—is found in both regions.
About 5000 feet up great plains of pumice and silt show how intense is the erosion of the steep soft ashy material. These silts are especially noticeable above the great dyke called “Mead's Wall.” The track vanishes now, as a belt of craggy lava and scoria
[Footnote] *R. Speight, “The Tongariro National Park,” in New Zealand Nature Notes, a Handbook issued for the Wellington meeting of the Australasian Association for the Advancement of Science, January, 1923, p. 32.
is traversed which leads one up to the snowline—about 6,500 feet high in summer.
At this level conditions remind one of those at Kosciusko in New South Wales. Large snow-drifts occupy sheltered hollows, and strong streams descend from hollows at their lower ends. Here is a very interesting zone, where in my opinion undoubted glacial evidence exists. I was unable to obtain accurate levels, but I estimate the snout of the Whakapapa Glacier is at about 7,500 feet elevation. The slopes here consist of solid masses of basic tuff, or some similar rock, and one flattish area just to the north of the stream some fifty yards across, as far as I can remember, has a “plucked” appearance, as if a glacier had removed all loose material but had not proceeded so far as to smooth the surface. Hereabouts were a number of shallow grooves and many striae. We noted a rock seven feet across, with some striae on its faces, but I was not able to give up the time necessary for a complete examination.
The glacier snout is soon reached. It lies in a small valley about two hundred yards across, with lava walls rising forty or fifty feet above the present glacier. The cross section is not unlike the typical “catenary curve” of the glacial valley. The glacier, however, so far as we could see it, was covered deep with snow in its lower portion. In the upper portion I saw some areas of loose rounded agglomerates of a mixture of snow and ice, which approximated to névé. Of solid ice none was visible on our visit to the glacier proper. At the top, above the hot lake, there was a fine bergschrund below Paretetaitonga, and numerous crevasses around the lake.
Noteworthy features to the north of the lower glacier were several cirques (See fig. 1). In all probability these are being excavated by “thaw and freeze” processes at the present time. I do not think they indicate greater glaciation in the past, for their present site where, I imagine, a temperature of 32°F. is often experienced, is probably the optimum for nivation. This aspect is discussed in the writer's account of the cirques on Mt. Field.*
The whole glacial environment is to be compared with that of Mount Kosciusko, but here probably only intermittent cirque-cutting is proceeding, for there are no glaciers, and it is rarely that the drifts last the whole twelve months.
Keeping in mind the character of the rocks, one could not expect clear traces of ancient glaciation, for the later subaerial erosion is too intense. It would, however, be very surprising if the North Island had not experienced the general Pleistocene cooling. (One does not know, of course, how old is the cone of Ruapehu.) It is possible that on close examination glaciation may be found down to 4,500 feet. There is little doubt in my mind that not far back in geological time the glaciers extended a thousand feet below their present level.
[Footnote] *T. G. Taylor, “The Glaciation of Mount Field, Tasmania,” Proc. Royal Society, Tasmania, 1922, pp. 193 ff.