Art. LXVIII.—On the Formation of Lake Wakatipu.
[Read before the Southland Institute, 19th August, 1881.]
The Wakatipu Lake is deepest about half way between its two ends, near the Queenstown bend. The depth is very great, probably the bottom is below sea-level, at any rate it is below the level of the Waimea and Dipton Plains. During occasional visits to this lake, I have been “exercised” to use an old theological expression) to account for this. I know that some have tried to explain it by the theory that the lake bottom had been scooped out to great depth by glacier action. But this was difficult to imagine, Having had occasion during the last twelve months to visit the locality, I carefully examined the country, and came to the conclusion that, whatever may be the case with the neighbouring lakes, the Wakatipu has been formed in much the same way as several of the lakes on the southern side of the Alps in Europe—namely, by the subsidence of the great mountain chain on the side of which it lies.
That the Wakatipu country has been subject to upheaval and subsidence is clear enough, but the mode of the last subsidence is not so clear. At first sight it would appear, to judge from the plains on the east side of the Southern Alps, and from the fiords on the west (these latter being mountain valleys into which the sea has come), that the island had risen gently on one side of the central chain (as an axis), and had subsided heavily on the other, but a closer examination, I think, will show that this is not what happened. It looks rather as if the island—at any rate this end of it—had gradually sunk as from a hinge from the present east coast line, or more probably from a line far to seaward beyond it. That this end of the island did subside in the manner I describe, is proved, I think, by the existence of the fiords and the comparative shallowness of the sea on the west coast, and by the fact that the seaward moss (which is now gradually rising) shows the remains close to the surface of a submerged forest destroyed in a comparatively recent time. A subsidence in this way would be greatest furthest from the hinge, and least near it—the sea would consequently come over a fringe of land, and be shallow on the east, and would cover much land deeply on the west.
Now the alpine range, before the subsidence I am speaking of, undoubtedly stood at a much higher elevation than at present, at least 5,000, more likely 10,000 feet, as the deep fiords on the west side of Fiord County
indicate. When the head of Wakatipu Lake was raised that height and the lower end a corresponding height, the lake must have contained a glacier. At any rate, it did once contain one, as the Kingston moraine testifies. Keeping in mind that the lake country subsided not from the central chain as an axis, but from the sea coast as from a hinge;—that in other words the lake did not rise at the lower end and sink at the upper, but that the whole of the lake sank, more of course at the head and less at the foot;—we can see clearly how the deep part in the middle came to be. This would simply be as follows:—The valley in which the lake lies, and which is about 80 miles long and narrow for its length, was the bed of a glacier. A glacier moves like a river and would doubtless also work down the valley in a parabolic curve (as our president has demonstrated in a paper printed in vol. vi. of the “Transactions” that the principal rivers here do), the shoulder of the curve being towards the upper end of the valley. Now the gradual lowering in height of the glacier (which I am assuming stopped short at Kingston) would have the effect of causing it to melt away at the lower end, and as the foot receded from the moraine, it would continue to deposit debris and make the valley at that part (already stopped up by the moraine) to decline gradually backwards. The future lake would be shallower at that end—more or less as the wearing away of the glacier was quicker or slower. At the same time the sinking of the curve higher up would tend to make the slope up to the moraine steeper. Whilst this was going on, the glacier would slowly melt and in course of time would disappear, water would take its place and the lake (at a much higher level, however, than at present) would come into existence, formed partly by the melted ice of the glacier, and partly by summer rains and melted snow off the ranges. The two rivers which run in at the upper end would gradually deposit a delta (the present Dart Hundred) and the ultimate effect of all these proceedings would be to make the lake deepest at the middle.
From the foregoing considerations, it seems doubtful that the Kingston Valley has ever been the outlet of Lake Wakatipu. It was of the glacier but not of the lake, which, if I am right in theory, must, as I have said, have originated by the sinking of the upper part of the valley below the level of the moraine. Most probably the lake always found an outlet, as at present.
In conclusion, I would remark that I am well aware I have suggested nothing new. But my object in writing this short paper is to excite an interest in a subject (the mode of formation of the large southern lakes) which, as these lakes lie it may be said at our very doors, it naturally falls to the Southland Institute to investigate.