Art. LVIII.—On the Glacial Till in Hautapu Valley, Rangitikei, Wellington.
[Read before the Otago Institute, 10th August, 1909.]
I Wish to bring under the notice of the Society some important evidence of ancient glaciation I have recently discovered in the western portion of the Province of Wellington. The general geological structure and physiographical features of this region are as follows:—
The triangular area lying between the Manawatu River and Wanganui is occupied by a gravel drift 400 ft. or 500 ft. thick. Along its inland border this drift rests against a series of marine clays of Pliocene age that are horizontal or dip gently towards the sea. These clays, which are commonly known by the Maori name “papa,” extend northward to the flanks of the Kaimanawa Mountains, along their northern limit skirting the upland plains that wrap around Ruapehu. In a few places they are intercalated with thin beds of shelly limestone, or contain irregular layers of hard calcareous nodules.
The gravel drift forms the coastal plain, which is now highly cultivated; but north of this the papa country is broken into hills and ridges, which are covered with a dense forest. Inland of the forest-belt, which is forty or fifty miles wide and runs parallel with the coast-line, the country is undulating grass land.
The Kaimanawa Mountains, from 4,500 ft. to 6,000 ft. high, are drained by the Rangitikei River and its tributary the Maowhango; and the papa country, going westward, by the Hautapu, Turakina, and Wangaehu, the first a branch of the Rangitikei. These rivers rise in the inland treeless country, traverse the forest-covered papa lands, and all, except the Hautapu and Moawhango, flow across the coastal plain, in which they have excavated deep channels with vertical banks. The Wangaehu River drains the south-east slopes of Ruapehu, from 7,500 ft. to 9,000 ft. high, rising in the glacier occupying the crater of that mountain.
Beginning at Waiouru, situated on the inland grass lands, and thence proceeding eastward across the ridges dividing the Wangaehu from the Hautapu, the hills are found to present the smooth-flowing outlines, truncated crests, and terraced slopes characteristic of glacial erosion. Many of the higher hills are domed, while the low hills are hummocky and whalebacked in form. At Taihape the hills are beautifully rounded, coned, and domed, and at Mataroa there is a fine example of a U-shaped valley. The evidences of ice erosion are also conspicuous both in the upper and lower Rangitikei.
During a recent examination of this region I also found that a large stretch of country, beginning at Karioi, near the foot of Ruapehu, and extending across the dividing-ridges into the Hautapu Valley, was covered with a sheet of glacial boulder-clay or till.
The till consists of clays, or of clays mixed with andesite blocks, or of andesite blocks containing little or no clay. It is generally unstratified, but in a few places layers of boulders are interbedded with bands of clay. The boulders range from small blocks up to masses 6 ft. and 8 ft. in diameter. They are always angular or semiangular, the latter being the result of
weathering. Water-worn material is present in some places, but never comprises more than a small proportion of the material. The number of very large blocks of andesite increases in proceeding down the valley, masses over 6 ft. in diameter being more plentiful between Mataroa and Taihape than elsewhere.
The thickness of the till varies from 0 ft. to 60 ft., so far as can be seen in the road and railway cuttings, but it may possibly exceed 60 ft. In many places the till rests on the summit of hills 300 ft. and 400 ft. above the floor of the old glacial valley, and in these situations it is impossible to determine the actual thickness.
Fig. 1.—Section from Ruapehu across Karioi Basin, drained by Wangaehu River, to Rangitikei Valley, showing Distribution of Glacial Till.
On the Karioi side of the divide the till is mainly composed of clays; but on the Waiouru plateau, at a height of 2,660 ft. above the sea, andesite blocks are in places present in large numbers.
From the 244-mile mark onward, passing down the Hautapu Valley, blocks become more and more abundant until Taihape is reached, beyond which the clays predominate.
As measured along the railway-line, the till extends down the Hautapu Valley for a distance of some twenty-six miles, ending near Utiku at a height of 1,220 ft. above the sea. Altogether the andesite blocks at the southern limit of the till have travelled some forty-five miles from their source at Ruapehu. The width of ground over which they are spread, so far as the clearing of the forest will permit examination, is found to vary from two to five miles.
Good sections of the till are exposed in the shallow railway-cuttings a short distance west of Waiouru Railway-station, and in the deep cuttings between Turanga-a-rere and Utiku, more particularly at 1 mile 60 chains south of Turanga-a-rere, and also at four and five miles south of that place.
The Hautapu Valley is divided from the Wangaehu River, which drains the south-east slopes of Ruapehu, by a ridge of hills that rises from 400 ft. to 900 ft. above the Karioi Flat. It is over this divide that the andesitic material has been transported by the ice radiating from Ruapehu.
There was, it would appear, a differential movement in the Ruapehu glacier, the bottom stream of ice flowing south through the Karioi Basin and thence along the Wangaehu Valley towards the sea, the upper and greater stream flowing south-east across the divide into the Hautapu Valley, carrying a load of andesitic débris which it spread over an area of 200 or 300 square miles. At all points the till rests on a deeply eroded surface of the marine Pliocene clays, covering hills and hollows alike along the course of the old glacial valley, the direction of which can easily be traced from any high point of view.
The Rangitikei glacier flowed southward from the Kaimanawa Mountains and Ruahine Range, and by its superior mass deflected the Hautapu
glacier westward towards the Turakina. The Kaimanawas and Ruahine Ranges are composed of argillite and greywacke; hence the Rangitikei glacier, until it met the Hautapu glacier near Utiku, carried only argillite and greywacke boulders.
In the Manawatu district, the fluvio-glacial coastal drift is composed mainly of greywacke; and in the Rangitíkei area, of greywacke with a sprinkling of andesitic material. Going westward, the andesitic material becomes more and more abundant, until at Wanganui it forms the bulk of the drift.
The andesitic glacial drift in the Hautapu Valley was first discovered by me during the progress of my geological reconnaissance* of the Main Trunk Railway route in –87; but on account of the dense forest I was unable at that time to determine whether the boulders were connected with some local volcanic centre or derived from Ruapehu.
The construction of roads and railways has now enabled me to determine the true character of the deposit; while the clearing of the forest has revealed the glaciated outlines that were then hidden from view under a mantle of almost impenetrable vegetation.
Ruapehu lies in latitude 39° 15′ S.—that is, 1° 30′, or 100 miles, further north than Boulder Lake, in Collingwood, the most northerly point at which evidence of ancient glaciation is known in the South Island. The latitude of Boulder Lake is about 40° 45' S., and of Wellington Harbour 41° 7′ S.
The manner in which glacial drifts were deposited or formed by glaciers is always difficult to explain, on account of the extremely variable character of the deposits. The most satisfactory explanations are always open to doubt. The Hautapu drift is too widespread and variable to be described as a terminal or lateral moraine, and I am inclined to think that it must be regarded as a boulder-clay or till formed mainly of interglacial débris deposited by the Hautapu glacier as it retreated to its centre of movement at Ruapehu. Such material would become in a measure infraglacial when the upper stream in which it was imbedded flowed over the divide into the Hautapu Valley. Besides andesite blocks, the drift in places contains angular masses of shelly limestone, sandstone, and calcareous nodules derived from the Pliocene series over which the ice crept.
The marine-clay series on the denuded surface of which the till rests has always been grouped in the Wanganui system of Pliocene age, which therefore limits the New Zealand glacial period to the Pleistocene—that is, the New Zealand ice age was synchronous with that of the Northern Hemisphere.
Ruapehu attains a height of nearly 9,200 ft. It carries permanent snowfields, and its great crater-basin is filled with what may be termed a summit glacier, or icefield. In the Pleistocene this mountain was a centre of dispersion from which glaciers radiated into the Rangitikei, Wangaehu, Upper Wanganui, and Upper Waikato, the glacier in the last flowing into the Taupo Basin, through the Rangipo Desert.
With respect to the glaciation of the North Island generally, I think it probable that further research will lead to the discovery of evidence in many other places—notably at Newtown, Karori, and Johnsonville, in the vicinity of Wellington; in the Wairarapa, near the Tararuas; and on the east side of the Ruahine Range—notably in the neighbourhood of Woodville, and east of that towards the sea.
[Footnote] * J. Park: Reports of Geol. Expl., –87, p. 69.
Note.—Early in March, 1910, Mr. A. Hamilton reported the occurrence of a great andesitic-boulder deposit between the Waimarino Plain and Raurimu, on the Main TrunkRailway. Later in the same month I examined this area, and found that the western limits of the Waimarino Plain were occupied or fringed by a crescent-shaped chain of morainic hills and ridges rising from 50 ft. to 200 ft. above the level of the plain. This morainic mound stretches from the Maunganui-a-te-ao, on the west side of Ruapehu, to the Upper Wanganui, a distance of over twenty miles. Its breadth varies from two to four miles. On the railway-route it ends near Raurimu, where it is covered with a heavy drift of pumice. The Main Trunk Railway, where it crosses the Waimarino Plain, runs along the foot of the moraine for some miles. Further north the railway traverses an andesitic drift between Oio and Owhango.
North of Waimarino Station the railway passes through the morainic chain for several miles, descending to Raurimu by a series of sharp loops which are known as the “Spiral.” In this portion of the line the structure of the moraine is beautifully exposed in miles of deep cuttings. The material is seen to consist of a tumbled mass of large and small angular andesite blocks mingled with reddish-brown clay and rock-rubble.
Water-worn material is absent or rare in the morainic hills traversed by the railway between Waimarino and Raurimu; and it is only when the line passes into the course of the old glacial valley of the Upper Wanganui, south of Oio, that the andesitic-boulder formation as exposed in the road and railway cuttings is found to contain a proportion of water-worn gravel and boulders.
The Waimarino moraine is a typical example of a terminal moraine. It was obviously formed by the confluent glaciers descending from the west side of Ruapehu, Ngauruhoe, and Tongariro.