Art. LX.—Some Evidences of Glaciation on the Shores of Cook Strait and Golden Bay.
[Read before the Otago Institute, 7th December, 1909.]
Somes Islnad, in Wellington Harbour, when viewed from the narrows at the Heads, is seen to possess the lion form of contour that is well known to be characteristic of ice erosion in all glaciated regions.
At Pencarrow Head, on the north side of the entrance to Wellington Harbour, the land on the seaward side is seen to present rock shelves and fine smooth-flowing contours; and a rock shelf or terrace runs parallel with the coast, beginning a short distance past the beacon and running south to Pencarrow Head. This shelf is not horizontal, but rises with a long gentle gradient towards the headland.
Proceeding along the coast towards Cape Terawhiti, in Cook Strait, the shore is seen to be fringed with the remains of a rock shelf or platform that can be traced almost continuously from a point half a mile on the north-west side of Terawhiti Stream to Macmenamen's homestead, a distance of four miles. A small remnant of this bench can still be traced at Cape Terawhiti when viewed in profile.
The rock shelf fringing the coast at Macmenamen's and Karori Stream is so broad and conspicuous that it can be clearly observed when some miles offshore.
This platform varies from 90 ft. to 300 ft. high. It is excavated in the claystones and greywacke which compose the mainland, and is in many places covered with a skin of gravelly material. It passes around the coastline without reference to the adjacent contours, and is deeply notched by the streams that enter the sea from the neighbouring ranges.
From Terawhiti Stream the shelf extends around the coast to Macmena-men's homestead, near which the ill-fated s.s. “Penguin” came to grief in 1909.
I made a careful examination of this coastal shelf in 1887 and again in 1888, and on these occasions satisfied myself that it was not an uplifted marine platform of erosion.
Maritime platforms are distinguished by their uniform height and a uniform shelving gradient towards the sea. The Terawhiti shelf varies greatly in height, and its surface is undulating.
At the time of my earlier examinations I was unablé to find a satisfactory solution of the origin of this coastal terrace; but I now think that an ice-sheet or glacier flowing through the depression occupied by Cook Strait would be able to excavate the fringing platform we now see.
The Terawhiti shelf has a singular resemblance to the Crown Terrace, a glacier platform that fringes the north side of the Arrow Basin, in the Wakatipu region.
After passing Cape Terawhiti, Mana Island is seen lying at the entrance to Porirua Harbour. This island is flattopped, its shape being almost identical with that of the flat ice-shorn islands in Lake Wakatipu.
Passing over to Queen Charlotte Sound, we find that the long, narrow, descending spurs that separate the numerous arms or inlets that open out of the main sound are worn down into long, tapering wedges, the crest of which, at their lower end, is frequently truncated and terraced.
These wedge-shaped spurs are quite a conspicuous feature in the land forms. They have a curious resemblance to the tapering spur lying between the two arms of Moke Lake, in the Wakatipu district.
Proceeding through the sounds towards Cape Jackson, we pass Jackson's Hump, which, whether viewed from the east or the west side, is seen to be a perfect dome. This well-known landmark is situated on the narrow peninsula on the extreme end of which lies Cape Jackson.
Many of the spurs that project into the sounds are sharply truncated in the manner shown in fig. 7, below.
Besides being truncated on the crest, the ends of the spurs are sometimes terraced with a succession of rock shelves. A good example of this is seen at the north-east end of Chatwood Island, which is passed before reaching the French Pass.
Between the French Pass and Nelson there is the Lion Island, which presents the characteristic appearance of an ice-worm surface.
To the south-east of Nelson we have Saddle Hill, with its twin domes, Sugar Cone, and other hills nearer the town, the smooth outlines of which are strongly suggestive of ice erosion.
At the Port Hills, Nelson, there is a thick deposit of gravel, sand, and clay, rudely stratified. This deposit rests on a highly denuded surface of the Miocene Tertiaries, forming the front of the Port Hills, and is faulted and tilted at various angles. It is mainly composed of greywacke and granite, both derived from the neighbourhood of Mackie's Bluff. It is quite clear that this drift at one time extended continuously from Nelson across the mud-flat to Wakapuaka. Its removal from this area is the work of the Maitai River.
The Nelson Boulder-bank is mainly composed of greywacke and granite boulders which have been derived from the great Wakapuaka drift, a portion of which, as we have seen, still remains at the Port Hills. In other words, the Boulder-bank is composed of the re-sorted material of the great drift being piled up in its present form by the action of the sea-currents on the one side and by the Maitai and other streams that enter the bay on the other side. That this drift came from the north-east is proved by the character of the material. The material is fluviatile; but the existing rivers run transversely to the course of the river that must have transported it to its present place. This ancient river must have flowed from the direction of the French Pass. The question is, Was this ancient river connected with the Cook Strait glacier?
The Moutere gravels are composed of greywacke, and can be traced continuously from the shores of Golden Bay to the Hope Saddle. They
are obviously a fluvio-glacial drift, carried forward by the river that drained Rotoiti glacier.
A great fluvio-glacial drift covers the terrace land and mountain-slopes in the Aorere district, Collingwood. This drift can be traced from sea-level on to the crest of the ridges fronting Golden Bay, being found as a thin covering up to a height of 3,000ft. above the sea. The fluvio-glacial origin of this drift was first recognised by Dr. Bell during his survey of the Parapara district in 1907.
The roches moutonnées, rock-basins, and moraines at Boulder Lake, Collingwood, have already been described by Dobson, Bell, and myself, and require no further referrence here.
The crest of the Whakamarama Range, near Collingwood, has been ice-shorn into many beautiful roches moutonnées and domes, and in the drift at the foot of the range there are many striated and polished boulders of grey-wacke. Granite boulders are plentiful, but none of them show ice-striæ.
The evidence showing that glaciers descended to what is now sea-level in the vicinity of Cook Strait is very strong, and when taken in conjunction with the glaciation of the Rangitikei Valley would seem to prove that the greater portion of the South Island and a large portion of the North Island was glaciated in the Pleistocene. A single bit of evidence might be open to doubt, but when all the evidences are strung together they become culminative. The evidence obtainable in Otago, Southland, Canterbury, Nelson, and Wellington would seem to afford conclusive proof that the Pleistocene glaciation attained a greater magnitude than hitherto supposed.
Note.—Since the above was written I have found that great piles of morainic débris occur near Cape Campbell, at the mouth of Kekerangu River, and near the mouth of Shades Creek, all in Marlborough. The material consists mainly of silts, clays, sands, gravels, and boulders, mingled with huge angular masses of Saurian sandstone, Amuri limestone, and enormous masses of fossiliferous sandstone and clay rock derived from the Awatere formation. These moraines are fully described elsewhere. It may, however, be interesting to note here that the masses of fossiliferous clays, sandstones, and silts belonging to the Awatere formation are altogether foreign to the Kekerangu and Shades Creek areas. Where the parent rock was situated is not known.