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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

The Rakaia Glacier

The Rakaia Glacier.

The glacier next in size has doubtless been the Rakaia glacier, having been at least of a length of 58 miles. The farthest point down the Canterbury plains on which morainic accumulations are visible is the so-called Woolshed Hill, seven to eight miles below the Rakaia Gorge. This hill rises about 100 feet above the plains, it has a triangular shape, and consists of shingle and sand, from which in many places angular blocks of sandstone, felstone and slates appear above the surface. If we consider that the gigantic torrential rivers issuing from the post-pliocene glaciers had the power of destroying easily beds of such incoherent nature as the morainie accumulations, which barred their passage, it is natural that only in very favourable cases such deposits could have been preserved. It is thus evident that the lowest points to which the post-pliocene glaciers reached are not always traceable, but the occurrence ot such isolated beds as the Woolshed Hill so far away from the principal morainic accumulations shows us that we can scarcely fix the limits to which the huge ice-streams of the page 387New Zealand Glacier period readied. These Woolshed Hill beds having, doubtless, been formed by the Great Rakaia glacier, it is evident that the latter must have had truly enormons dimensions at some time. We can conclude from the characteristic features of the valley of the Rakaia, that during the extension of the glacier it was filled with ice from side to side, and that it was hollowed or scooped out in a very striking manner, principally above those localities where beds of peculiar hardness crossed it. The occurrence of these very hard rocks is the principal cause of the formation of Lake Coleridge. Where these beds occur we find that their remaining portions stand out in very bold characteristic forms, as roches moutonnées, having given rise to such names as Gibraltar, Sebastopol, &c. (See section No. 2 on plate 7.)

The main glacier coming down by the valley of the Rakaia, of which we can trace the limits with certainty, reached into the plains a few miles below Rockwood, and crossing them in a semicircular band abutted against the south-eastern slopes of Mount Hutt. It is also evident that the hard melaphyres, porphyries, and porphyry conglomerates stretching across the valley where at present the gorge of the Rakaia is situated, formed a great barrier to the advance of the glacier, until these rocks were sufficiently ground down or removed to allow the glacier to advance beyond. There is also evidence that when the post-pliocene glacier retreated, a, large lake was for a long time formed above the gorge, which in course of time was filled up with glacier mud (silt), and at the same time with shingle deposits brought down by the glaciers and rivers. These shingle-beds cover invariably the beds of silt. The formation of the gorge is of still younger origin. Morainic shelves occur also at an altitude of 500 to 600 feet on the northern and 200 to 300 feet on the southern side of the Snowy Peak range, thus showing the height of the glacier above the valley and its general slope. The great Rakaia glacier, besides its principal outlet by the main valley, had another branch descending to the plains by the valley of the Selwyn, having probably had at an early period of its existence very large dimensions before the barrier across the main valley, formed by the porphyritic rocks, had been scooped out sufficiently by that great ice-stream. Another important branch of the Rakaia glacier passed along the broad opening now occupied by the Cameron river, uniting with the glaciers from the Cameron and Ashburton valleys, and further on with a branch of the great Rangitata glacier. The united ice-stream descended the valley of the Ashburton, and reached some page 388distance beyond the front ranges, where a few portions of the frontal moraines are still preserved about two miles from the eastern foot of the limestone hills near Mount Somers; total length about sixty miles. In the second period of its existence, when the large lake above the gorge of the Rakaia was formed, this branch after its retreat produced in its turn Lake Heron. The frontal moraines on the southern side of that lake are still well preserved, and the bed of its former outlet towards the Ashburton is clearly defined. The present outlet of the lake, however, uniting with the River Cameron, has now a reverse flow to the upper Rakaia.

In the river bed of all the Rakaia branches, remnants of morainic accumulations mostly of a frontal character are met with, showing that the glaciers of which the trunk glacier was formed when retreating remained either stationary for some time, or what is still more probable, may be taken as the boundary of the new advance of the glaciers, after a temporary retreat to higher regions. Although the space at my command will scarcely allow me to go into details, I think that I should give here one striking example, proving that in some cases the latter theory is supported by most remarkable facts. The Rakaia, opposite Fighting Hill, makes a sharp bend towards the south before it enters the gorge proper. Here the banks consist first of 70 to 80 feet of lacustrine deposits (silt), above which about 400 feet of river shingle repose. The whole section shows most instructively how the river slowly advancing with its delta, has gradually filled up the large lake once existing here after the retreat of the glacier. At this bend begins a remarkable channel running with an average breadth of 200 feet for about three miles in a straight line. After ascending four well marked river terraces in its course, it abuts against a small roche moutonnée of quartziferous porphyry standing in the centre of the valley, not far from the ferry. It was doubtless formed after the river channel A (section No. 7 on plate No. 7) had been excavated by the Rakaia, but before any lower ones had been hollowed out; because if we continue its course in a N.W. direction, it is parallel with the course of the river now flowing here, 500 to 600 feet below it. It is evident that when this small glacier advanced in such a striking manner, the great Rakaia lake had not only entirely been filled up by river deposits, but the Rakaia had already begun to excavate its bed into these deposits, by which four well defined terraces had been produced.

This remarkable channel is so striking a feature in the country that page 389it had not escaped the notice of the earliest settlers, by whom it was generally known as "The Railroad,"' and a number of theories were propounded to account for its formation. One of them, believing that a set of faults was running here parallel to each other, was sure that there was a coal-field below; another, by no means wanting in intelligence, was certain that it was made by the hand of man, as an artificial cover for an army. It was also described to me as a riverbed, partly uplifted or sunk across its course, and only after repeated careful examination was I able to assign to it a purely glacier origin. This glacier channel, as before observed, ascends four river terraces, crossing them diagonally as they gradually approach the lower gorge. Here, where the river has cut through a bar of quartzif erous porphyry, they all unite, the river having cut almost straight down for six to 700 feet, leaving nearly perpendicular walls on both sides. If I can trust to some Aneroid observations made at both extremities, the frontal moraine lies about 20 feet above the starting point on the banks of the river. Here, not far from the starting point, a small but well marked frontal moraine crosses the glacier channel (section No. 8). At (B) it crosses a deep channel, in which a branch of the Rakaia was once flowing; the lateral remains are here very low, being scarcely five feet above the floor. The glacier then ascended the slope of the first terrace, having about 30deg. inclination, and possessing an altitude of about 20 feet. On this slope the lateral moraine is well developed on both sides. From the summit of the terrace the glacier channel continues in the same direction, with well defined banks on both sides, rising about 18 feet above its bed, but standing only six to eight feet above the fluviatile deposits of which the terrace is formed. The glacier has thus been able to lay its bed to that extent lower— some 10 to 12 feet. After half a mile, another small frontal moraine crosses the channel at (C). Having followed the first terrace for about a mile, the second terrace, about 20 feet high, is reached, with a small lagoon at its foot (D). After ascending also this terrace the lateral moraine gradually gets lower, now being only ten feet high, and another small terminal moraine (E) has been thrown down. Two more terraces are afterwards ascended, 10 to 12 feet high. Having reached the highest level of the old river bed, the channel then ascends gradually towards the roche moutonnée lying in front of it. The ground assumes now a more hummocky surface, and the lateral moraines are less marked. Numerous erratic blocks lie about, and several small channels for the outlet of water are well defined. At two spots only, before the glacier channel reached the highest level of page 390the valley, small outlets have been in existence. They are, however, very faint, and the lateral moraine has not been removed. It appears, therefore, that the water melting during the advance of the ice, has been leaking through the incoherent shingle bed forming the floor of the glacier, otherwise it would have certainly cut through the lateral moraine, consisting all along of silt only, with rarely any boulder or erratic blocks amongst it. The glacier channel is always stony, the stones being either rounded or sub-angular, but in several instances quite angular. Near the termination of the glacier channel none of the blocks have lost their sharp angles. In a word, the whole forms one of the most interesting geological phenomena I ever met with—a glacier channel nearly three miles long and only about two hundred feet broad, paassing straight over a plain, ascending four terraces together about 80 feet high, without furrowing to any depth the alluvial shingle beds of the broad valley, and being bounded at the same time by banks of silt. It is evident that a very large quantity of ice must have existed higher up the valley from which this narrow strip received the necessary vis a tergo to overcome all obstacles in its way, the larger glacier after the separation of this branch following without doubt the broad river bed where now the gorge of the Rakaia is situated. (See No. 9, ground plan of Railroad glacier on Plate No. 7.)