Morainic Mounds on the Waimarino Plain near Ruapehu.
[Read before the Otago Institute, 11th November, 1924; received by Editor, 19th December, 1924; issued separately, 31st March, 1926.]
On Section 14 of the Waimarino Military Reserve, eight miles as the crow flies from the summit of Ruapehu and about six miles from Waimarino, there are two groups of debris-hummocks that rise above the general level of the plain. They lie between the Whakapapanui and Tawhai Streams, both of which drain the north-west slopes of Ruapehu. One group, comprising scores of cones, extends from near the well-known Haunted Hut to the Rotoaira main road, while the other, a mere string of isolated mounds, begins at the main road near the Tawhai, and dots the plain to the north-west, ending about half a mile from the Whakapapaiti. After crossing the Whakapapanui, as one travels from Rotoaira towards Waimarino Railway-station the hummocks at once catch the eye, those to the right of the road near the Tawhai being especially prominent objects.
Generally the hummocks range from 20 ft. to 50 ft. high, but some are as low as 10 ft., and a few reach a height of 60 ft. Where they have been cut through along the road-line they are found to be composed of clayey matter and silts mingled with angular fragments of andesite ranging from small particles to blocks 6 ft. or more in greatest dimension. Most of the mounds have a circular or oval base. In places where two mounds unite the base is an elongated oval. Many of the mounds are cone-shaped. The Waimarino Plain, on which the hummocks stand, ranges from 2,600 ft. above sea-level at the lower end to 3,000 ft. along the foot of Ruapehu.
When we come to consider the origin of these debris-cones two explanations present themselves: they are either glacial, or the relics of a dissected flood-plain.
The Waimarino Plain is not a river or valley plain formed by ordinary fluviatile action, but an elevated platform evidently of fluvio-glacial origin built up around the base of Ruapehu. Where the plain is trenched by the consequent streams descending from the slopes of that mountain it is seen to be composed of clayey matter and semi-plastic silts in which are embedded angular rock-fragments in great abundance. In places there are present beds of well-worn gravels. The coarse, bouldery gravels exposed along the courses of the streams that now cross the plain are in the main, if not entirely, composed of re-sorted material.
The debris cones and mounds lying between the Haunted Hut and the Rotoaira Road are dotted irregularly over the plain, in places for a width of half a mile. They generally stand close to one another, and there is no evidence that drainage-channels or streams ever existed between them; on the contrary, undrained hollows lie between many of them.
I have already mentioned that the mounds are in part composed of large angular blocks of andesite. If the mounds were the residuals of stream-action one would expect to find the hollows between them piled with blocks derived from the excavated ground; but no such piles of blocks exist.
In general outline the Waimarino debris-cones resemble the morainic mounds that rise from the upper Waitaki Plain between Omarama and Lake Ohau. Frequent debris-cones have been described by Griffith Taylor * as occurring at McMurdo Sound and Granite Harbour, in South Victoria Land. I myself have seen similar glacial mounds in the Box River region of Canada; also along the front of the Malaspina piedmont glacier, and at many places on the shores of Puget Sound.
Taylor made a close examination of some of the Antarctic debris-cones, and finally concluded that they were composed of material dropped from notches on the edge of the glacier front.
The crescent-shaped morainic ridges so often seen in Switzerland, New Zealand, and other glaciated regions were obviously terminal moraines piled up at periods when the rate of melting at the ice front equalled the rate of ice-flow. The morainic ridge to the west of the Waimarino Railway-station, described by me in 1909, evidently marked a one-time terminal front of the Pleistocene Waimarino glacier.† The Waimarino mounds were formed during the final retreat of the glacier, the material being piled up in ice-caves and notches along the edge of the ice-front.
The Waimarino glacier at one time filled the Mangahuia depression between Ruapehu and Hauhungatahi, and debouched over the plain, whence it was diverted by the high ridges to the west of Whakapapaiti towards the Retaruke country, over which much glacial detritus is spread. At the same time the Waiouru glacier, descending from the south-east flank of Ruapehu, sent a tongue of ice down the Hautapu Valley, scattering a trail of clay, silt, and rock-fragments as far south as Mangaweka, where many large erratic blocks of andesite derived from Ruapehu occur high up on the valley-slopes.
[Footnote] * Griffith Taylor, The Physiography of the McMurdo and Granite Harbour Region. British Antarctic (“Terra Nova”) Expedition, pp. 69–73. London, 1922.
[Footnote] † J. Park, Trans. N.Z. Inst, vol. 43, 1910, p. 580.