Art. XXXIV. — The Discovery and Extent of Former Glaciation in the Tararua Ranges, North Island, New Zealand.
[Read before the Wellington Philosophical Society, 6th September, 1911.]
The discovery of evidence of former glaciation in the Tararua Ranges was made by the writer in February, 1909, and two years later (March, 1911) further discoveries were made and the previous ones confirmed. The glaciated areas and the memorials of former frost-action so far discovered are situated on the highest ranges of the Tararuas—viz., on that part of the Dundas Range lying nearest the geographical centre of the mountain - system, and on the Mitre-Holdsworth Range. During the maximum phase of glaciation the heads of five river-yalleys were filled with glacier-ice: (1) Park River,* the main tributary of the Waiohine-iti River, named after the well-known New Zealand glacialist; (2) the main source of the Waiohine-iti River; (3) Dorset Creek, a left-bank tributary of the Waiohine-iti River, named after a pioneer explorer of the Tararuas; (4) Bennington Creek, a tributary of the Waingawa River, rising in the south-west foot of the Mitre Peak, named after a companion of Edward Dorset; and (5) the Mangaterera River, another tributary of the Waingawa.
The phenomena resulting from the presence of glaciers now non-existent consist of—(1) U-shaped valleys; (2) glacial cirques; (3) rock basins; (4) glacial hanging valleys; (5) fluviatile hanging valleys.
In order to give a clear and correct impression of the extent and character of former glaciation in the Tararuas, the phenomena tabulated will first be dealt with seriatim, and then the topography of the Park Valley—the locality where these phenomena attained their maximum development—will be fully described.
(1.) U-shaped Valleys.
U-shaped valleys furnish the principal evidence of the former presence of glacier-ice. So far as is at present known, they occur in five situations. The head of Park Valley is U-shaped for a distance of two miles; in the Waiohine-iti Valley the same feature extends for about one mile; in the valleys of Dorset and Bennington Creeks, for about half a mile each; and at the head of the Mangaterera Valley, about a quarter of a mile. The accumulation of scree-material, talus, and alluvium has to a certain extent obscured the U-shaped form and reduced the original steepness of the walls of these valleys, but even now their special character is unmistakable. Below their U-shaped heads the valleys contract to narrow gorges typical of fluviatile erosion.
[Footnote] * The river draining this valley has hitherto neither been named nor shown on any available map. On every available map the main source of the Otaki River is represented as draining the site of the upper portion of Park Valley.
(2.) Glacial Cirques
By far the finest example of a glacial cirque is to be found at the head of the U-shaped portion of Park Valley. This cirque is nearly half a mile across, and is bounded by mural precipices of imposing appearance. At the heads of the other U-shaped valleys the cirques are not so typically developed, the precipices being inconspicuous or absent.
(3.) Rock Basins.
There is every reason to believe that a rock basin exists in the floor of the cirque at the head of Park Valley. Since the disappearance of the ice it has been filled in with alluvium, and therefore its existence can only be demonstrated by evidence supplied by the general topography of the valley-floor. This evidence will be set forth below (p. 314).
(4.) Glacial Hanging Valleys.
Three glacial hanging valleys open into the cirque at the head of Park Valley. They lie at heights of from 360 ft. to 510 ft. above the surface of the alluvial flat forming the present floor of the cirque. The largest has a length of about 15 chains, and the other two, which lie close together and are only divided by a low rocky ridge, are about 6 chains and 8 chains in length respectively. The lips of all three glacial hanging valleys have been cut by the streams that have drained the latter since the disappearance of the ice.
The floors of the glacial hanging valleys of Park Valley, and particularly that of the largest—and the evidence is therefore the more conclusive—show some signs of downward curving at the points where these valleys terminate and open into the main cirque. For this reason it is clear that the ice in the U-shaped hanging valleys must have descended to the head of the main glacier as icefalls; the upper surface of the ice in the main cirque—i.e., the head of the trunk glacier—must therefore have stood somewhat below the level of the floors of the U-shaped hanging valleys, and probably attained a thickness of 500 ft. If the surface of the ice forming the head of the trunk glacier had stood above the level of, or even on a level with, the floors of the U-shaped hanging valleys, the terminal downward curving of their floors would have been absent, and the tributary glaciers would have joined the main one at grade. This they may have done during the maximum phase of glaciation, the icefalls and the wearing of the lips of the glacial hanging valleys by them being referable to a later date.
Glacial (U-shaped) hanging valleys occur at the heads of some of the other glaciated valleys also. There is a tiny one at the head of the valley of Bennington Creek. The cleft cut in its lip is in its incipient stages, so that small waterfalls still descend into the main valley. The precipices of the Mitre Peak surmount the north-east side of this hanging valley, and its head lies in the side of the main watershed of the Mitre-Holdsworth Range.
Another small glacial hanging valley is situated at the head of the glaciated portion of the Mangaterera Valley. Its lip also has been cut by the small stream which now drains it.
(5.) Fluviatile Hanging Valleys.
Hanging valleys having the typical V-shaped cross-section of stream erosion, and which owe their present state as such to the former presence of a glacier, are found in Park Valley only. The best examples are situated on the left wall of the valley, about three-quarters of a mile below the main cirque. The height of the falls which descend from their lower ends into the main valley is now greatly reduced by the infilling of the latter with scree-material. In the glaciated part of Park Valley the fluviatile hanging valleys are the sole remaining relics of, its pre-glacial form—a form due entirely to fluviatile erosion. Prior to the glacial period the portion of Park Valley referred to was very much narrower, and also rather less deeply excavated than it is at the present time. From the ridges forming the watersheds on either side of the valley steep lateral spurs ran down to the valley-bottom, and the intervening gullies were in topographic adjustment with the trunk valley.
With the advent of the ice the pre-glacial topography of the upper portion of Park Valley was modified in two ways—the valley was both deepened and widened. The deepening was relatively greater in some parts of the valley than in others; in the main, cirque the valley was over-deepened and the gradient of its floor reversed. Throughout the glaciated part of the valley the deepening was sufficient to remove all traces of the V-shaped contour of the pre-glacial trench, and to give the valley the typical flat bottom of glacier erosion. The widening of Park Valley by ice-action was of even greater extent and importance. In the achievement of this result the lateral spurs were deeply truncated, the intervening gullies betrunked and converted into hanging valleys, and the sides of the main valley cut back to such an extent as to give them a steep wall-like character. The present fluviatile hanging valleys were never ice-filled, but at the time of maximum refrigeration the tributary gullies nearer the head of the main valley were filled with ice, and were moulded thereby into their present U-shaped form. These U-shaped hanging valleys owe their present state as such more to the rapid erosion of the main cirque by the process known as “plucking” than to the lateral grinding which produced the fluviatile hanging valleys.
The Topography of Park Valley.
(See map, p. 311, and Plates XXII–XXIV.)
The topography of the upper portion of Park Valley is undoubtedly of glacial origin. The valley contains the most extensive and the best-preserved memorials of the erosion of glacier-ice, and therefore it has the distinction of being the former site of the largest of the extinct glaciers of the Tararua Ranges. The general trend of the glaciated part of the valley is west by south, but it is not straight; it runs in two curves—the upper bending southward, the lower northward. From the lower limits of glacial erosion the valley turns south-south-west and south-east to its junction with the Waiohine-iti River. This part of Park Valley is narrow and gorged. Lofty ridges form the boundaries of Park Valley, and the highest points of these—Mounts Thompson, Lancaster, and Dora, and Arete Peak—encircle its head, and in the past formed the gathering-ground of the perennial snowfields which fed the old glacier.
The main cirque at the head of the valley has a diameter of nearly half a mile. The precipices forming the bounding walls of the cirque attained a maximum height of 800 ft. above its floor. Below the cirque the valley is U-shaped for about two miles, the sheer lateral walls having a height of upwards of 400 ft. The U shape of the valley is less pronounced towards its lower end, and two miles below the cirque the latter narrows, gradually becomes V-shaped, and finally gorged. The continuity of the bounding precipices of the main cirque—which are best preserved on the south-west face of Arete Peak—is broken by the three U-shaped glacial hanging valleys. The largest of these lies on the south side of Arete Peak, and rises in a rather poorly developed cirque. It has a length of about 15 chains. The other two lie between Arete Peak and Mount Dora, and rise in ill-defined cirques. They are twin valleys, being separated only by a low rounded ridge. Their length is about 6 chains and 8 chains respectively. A small narrow gorge has been cut in the lip of each of these glacial hanging valleys by the streams which now drain them.
The most striking feature of the main U-shaped valley is the high development of screes. These bury the precipitous walls to a height of from 250 ft. to 320 ft. above the valley-floor. Above the screes the lateral walls rise to a height of from 50 ft. to 100 ft. In the main cirque the precipices rise 290 ft. above the apexes of the screes. Throughout the greater part of its length the floor of the main valley is loaded with scree-material; the bases of the screes on the one wall meet the bases of those on the other, and the modern drainage-channel of the valley follows the line of contact. The screes are now not in the course of formation, being clothed with tussock-grass and subalpine scrub.
In the U-shaped section of its valley the Park River is actively engaged in altering the gradient of the valley-floor. In the main cirque it is an aggrading stream, and has there formed an alluvial flat several acres in extent. Below this flat the river flows in a narrow channel of gradually increasing depth. Near the lower limit of glaciation this channel is about 20 ft. deep, and the rock floor of the valley, upon which the screes rest, has been incised by the river to a depth varying from 10 ft. to 15 ft.
The infilling at the head of the valley, and the excavation below, clearly demonstrate that the valley was overdeepened * by the old glacier. After the disappearance of the ice the rock basin was probably the site of a small lake until it was filled in by the accumulation of alluvium.
Such criteria of former glaciation as moraines, roche moutonnées, and striated surfaces have not been found in Park Valley or in any of the other glaciated areas of the Tararuas. It is highly probable that some of the phenomena enumerated do exist, but in Park Valley, and in the other glaciated localities also, the present excessive accumulation of scree-material and alluvium precludes all possibility of their detection. The apparent absence of a terminal moraine may be accounted for by the small size of the glacier. It may be, however, that some of the angular debris resting on the valley-floor near the lower limit of glaciation is morainic material laid down during the slow but regular shrinking of the glacier during its final retreat. Another suggestion is that the great piles of boulders that encumber the narrow gorges situated immediately below the lower limits of glaciation in Park Valley are the re-sorted relics of a terminal moraine. According to this supposition, the terminal moraine of the old glacier was demolished and carried to lower levels since the disappearance of the ice by the periodic floods of the modern river. In this way the angular blocks forming part of the moraine were rounded and transformed into the boulders as they now exist. The boulders in the gorges referred to are very much larger and more numerous, than any that lie within the glaciated upper portion of the valley.
The following altitudes in Park Valley were determined by the use of an aneroid set by the trig. on Mount Dundas: The saddle in the watershed of the Dundas Range at the head of the largest glacial hanging valley, 4,440 ft. above sea-level; the lip of the largest glacial hanging valley, 3,900 ft.; the lips of the twin glacial hanging valleys 3,750 ft.; the centre of the alluvial flat in the floor of the main cirque, 3,380 ft.; the summits of the precipitous rock walls of the main U-shaped valley—left wall 3,800 ft., right wall 3,670 ft.; the lower limit of glaciation (i.e., of the U-shaped part of the valley), 3,000 ft. above sea-level.
[Footnote] * The glacial hanging valleys furnish additional evidence in favour of this conclusion.
The former glaciers of the Tararuas owed their existence to the then greater elevation of the country and to the more rigorous climatic conditions. At the present time the snow-line in the latitude of the Tararuas is about 8,000 ft. above sea-level. “The late Sir Julius von Haast, in his ‘Geology of Canterbury and Westland,’ estimates that during the glacial period the snow-line was 1,000 ft. lower than it is in New Zealand at the present time.”* This estimate involves only a slight reduction of the annual temperature—a reduction presumably induced by cosmic or external causes or conditions—and appears to have been based on such, other factors being neglected. The evidence furnished by the configuration of the bed of Cook Strait (as shown by soundings) and by the physiography † of the lowlands at the western foot of the Tararuas indicates that the elevation of that part of the country has been reduced since the glacial period by at least 1,000 ft. Taking for granted that these estimates are correct, and that they represent the sum of the influences that lowered the snowline, the snow-line in the Tararuas during the glacial period was, in relation to the present sea-level, 2,000 ft. lower than at the present time; in other words, the snow-line of the Tararuas formerly stood at a height of 6,000 ft. above the present sea-level. But it is evident from the known altitude and position of the extinct glaciers of the Tararuas that the lower limits of the permanent snowfields that fed them were at the most 4,000 ft., and perhaps only 3,500 ft., above the present sea-level. By taking as correct even the greater altitude—i.e., 4,000 ft. above the present sea-level)—there is a discrepancy between it and the foregoing of 2,000 ft. This lack of agreement between the tentatively adopted and the actual altitude of the former snow-line may be removed by accepting one of the following amendments: that during the glacial period the snow-line was lowered (in each case with reference to the present sea-level)—(1) by more rigorous climatic conditions 3,000 ft., and by the greater elevation of the land 1,000 ft.; (2) by climatic conditions 1,000 ft., and by greater elevation 3,000 ft.; or (3) by climatic conditions 2,000 ft., and greater elevation also 2,000 ft. The last of these is probably nearest the truth, since the estimate that the snow-line was lowered by climatic influences only 1,000 ft., as pointed out by Mr. H. Hill (Trans. N.Z. Inst. vol. 27, p. 453), “is a very small one, representing, as it does, only a difference of about 3 degrees of temperature; and this certainly would not be sufficient to bring about a glacial climate in the South Island”; and, of course, still less so in the North Island, unless it can be shown that the latter stood at an enormously greater elevation in the early Pleistocene period.
The former glaciers of the Tararua Ranges give some indication of the extent and nature of the Pleistocene glaciation of the North Island: they show that in these respects it was limited, localized, and moderate. On comparison this view is found to be in harmony with the known extent
[Footnote] * H. Hill, “On the Hawke's Bay Pleistocene Beds and the Glacial Period,” Trans. N.Z. Inst., vol. 27, 1895, p. 452.
[Footnote] † G. L. Adkin, “The Post-tertiary Geological History of the Ohau River, &c.,” Trans. N.Z. Inst., vol. 43, 1911, p. 496.
of the Pleistocene glacial development in the South Island, a development which attained its maximum in the Wakatipu ice-cap in Otago, and its lesser phases in Canterbury and Nelson, where systems of gigantic glaciers of the alpine type came into existence. In the South Island the Pleistocene ice-masses decreased from south to north, and, though at that time they made an appearance in the North Island also, they were there of even less extent than might have been expected. The northernmost of the centres of glacier dispersion in the South Island appears to have been situated in the Hardy Range, * in Collingwood. In that locality the signs of former ice-action are abundant and well preserved; yet in the Tararuas—mountains only slightly inferior in altitude, and situated in practically the same latitude—the relics of the Pleistocene glaciers are meagre, and of a less definite character. It seems apparent, then, that conditions in the North Island were not so favourable for the development of glacial phenomena, and that no widespread glaciation was experienced.
These facts and inferences are quite at variance with the idea, expressed in a paper on “Some Evidences of Glaciation on the Shores of Cook Strait and Golden Bay,”† that the bed of Cook Strait during the Pleistocene elevation was occupied by a great glacier rising in the central highlands and flowing southward. In a succeeding paper by the same author it is stated that” a large portion of the Province of Wellington suffered intense glaciation in that [the Pleistocene] period.” ‡ In keeping with these views of the extent of former glaciation of the North Island, Professor Park expressed the opinion that evidence of ancient ice-action and the products of such would probably be found, among other places, “in the Wairarapa, near the Tararuas.” The only interpretation which can be placed on this statement is that the author quoted believed that during the maximum phase of glaciation the Tararua Ranges supported glaciers which deployed upon the plains to the eastward. The evidence furnished by the Tararuas themselves is entirely opposed to such a suggestion. Until some more definite and conclusive evidence is adduced to support it, the doctrine of widespread glaciation in the Province of Wellington, and more particularly of the low-lying maritime areas of the same, is scarcely likely to gain general acceptance.
[Footnote] * See Bell, Webb, and Clark, Bulletin No. 3 (New Series), N.Z. Geol. Survey, pp. 31, 32, 1907.
[Footnote] † James Park, Trans. N.Z. Inst., vol. 42, 1910, p. 585.
[Footnote] ‡ James Park, “The Great Ice Age of New Zealand,” Trans. N.Z. Inst., vol. 42. 1910, p. 599.