Art. XV—On the Occurrence of a Striated Erratic Block of Andesite in the Rangitikei Valley, North Island, New Zealand.
[Read before the Otago Institute, 7th December, 1915.]
In 1909 I described* the occurrence of several massive piles of andesitic blocks in the middle and lower portions of the Hautapu Valley. The source of this andesitic material is Mount Ruapehu; and the significance of the occurrence lies in the fact that the Hautapu River does not reach within
many miles of Ruapehu, but is separated from that mountain by the Wangaehu Valley. The Wangaehu rises on the eastern slopes of Ruapehu, and flows southward through a wide valley fringed on both sides with high terraces.
[Footnote] * J. Park, “Glaciation of Hautapu Valley,” Trans. N.Z. Inst., vol. 42, p. 577.
There is no evidence that the Hautapu did at any time drain the slopes of Ruapehu; but if it did, then obviously its upper sources have been pirated by the Wangaehu—possibly in the late Pleistocene.
The Hautapu is a tributary of the Rangitikei River. The latter rises in the Kaimanawa Range, composed of Lower Mesozoic greywacke and slaty shale, but in its middle and, lower course the river passes over Younger Tertiary marine clays which wrap around the older rocks, and in places rise on the flanks of the range to a height of 4,000 ft. above the sea. The Tertiary formation extends southward to the sea in the form of a great gently sloping plain, now deeply dissected by many rivers and their numerous tributary streams. The slope of this plain coincides approximately with the dip of the strata.
The clays are intercalated at intervals with wedge-shaped beds of impure shelly limestone that attain their greatest thickness in the neighbourhood of the old shore-line.
The Hautapu Stream throughout its whole length lies within the area occupied by the Tertiary clays. Near the sources of the stream the clays are intercalated with a few irregular beds of shelly limestone, seldom more than a few yards thick.
Going southward, following down the Hautapu Valley, the first great pile of andesitic blocks occurs at Turangarere, twenty miles as the crow flies from Ruapehu. Still greater piles occur at Mataroa, thirty miles from Ruapehu, and at Taihape, six miles south of Mataroa. The piles at Mataroa and Taihape are linked up by many isolated blocks, some of great size, perched on the ridges on the north side of the valley.
It is noticeable that the andesitic piles themselves and their constituent blocks become larger and larger as the distance from Ruapehu increases till Taihape is reached.
A small pile of andesitic blocks occurs at Utiku, a few miles south of Taihape, at a point about half a mile from the place where the Hautapu joins the Rangitikei River. This pile lies 1,225 ft. above the sea. Till the present year there was no knowledge of andesitic blocks beyond Utiku, which is thirty-nine miles from Ruapehu.
In February, 1915, I discovered a solitary andesitic erratic in the Rangitikei Valley, at the south end of the Mangaweka Railway tunnel, at a height of 1,070 ft. above the sea. It lies against the foot of a steep ridge of Tertiary clays, 16 ft. above the lower terrace of the Rangitikei, the surface of which is 175 ft. above the bed of the river. (Plate XIV, Fig. 1.) This erratic measures 14 ft. long, 6 ft. wide, and 5.5 ft high, and its weight is approximately 37 tons.
The whole of the underside of the block is smoothed and rounded, and scored with deep grooves and striae, the majority of which run parallel with the longer axis of the mass. At one end the striae extend up the sides for about 2 ft. from the bottom. The diagonal striae are numerous, but, as a rule, not so deep as the longitudinal ones. (Plate XIV, Fig 2.)
This great mass of striated andesite is larger than any block known to occur in the Hautapu Valley. It lies forty-eight miles from its source at Ruapehu, and appears to mark the southern limit reached by the ice-borne blocks.*
[Footnote] * Waterworn boulders of andesite derived from the destruction of the Hautapu moraines are plentiful among the existing river-bed gravels of the Rangitikei River.
In 1909 I expressed the opinion that the piles of andesite in the middle and lower portions of the Hautapu could have been transported and piled in their present places by no known agency but glacier-ice descending from Mount Ruapehu. This mountain is a massive volcano, rising to a height of 9,000 ft. above the sea. The glacier* at present existing on it lies in a basin encircled by peaks ranging from 7,000 ft. to 9,000 ft. high.
Mount Ruapehu is the highest mountain mass in the North Island of New Zealand. It lies in latitude 39° 15′ south, and even now possesses a glacier nearly two square miles in extent. The andesitic erratic in the Rangitikei Valley lies in latitude 39° 45′ south.
The presence of the morainic piles of andesitic material in the Hautapu Valley and the occurrence of the Mangaweka striated erratic in the Rangitikei Valley show that in the Pleistocene period a valley glacier threaded its way down the Hautapu Valley till it reached the Rangitikei Valley. Both the Rangitikei and Hautapu Rivers run in deep rectangular troughs excavated in the floor of the old glacial valley.
In the Pleistocene, glaciers covered much of the highlands of Tasmania, and, according to Professor J. W. Gregory, F.R.S., descended within 400 ft. of the sea. This view has been confirmed by Professor Edgeworth David, F.R.S., who has stated that the glacier-ice came to within a few hundred feet of sea-level, if not down to sea-level itself.
After all, it does not seem very remarkable that a glacier should have descended to within 1,000 ft. of the sea in the southern end of the North Island during the great Pleistocene glaciation of New Zealand.
[Footnote] * J Park, “Geology of New Zealand,” 1910, p. 203. Fig. 94 shows a portion of the existing Ruapehu Glacier.