Art. XXXI.—On the Timaru Loess as a Climate Register.
[Read before the Philosophical Institute of Canterbury, 2nd October, 1890.]
In a paper submitted to this Society a few months ago I had the honour to offer what I hoped would prove a useful provisional reading of the earlier of the recent geological formations of South Canterbury, viewed as records of climatic changes. The point then reached was the close of the interglacial period. I now propose to continue the reading, from materials provided by the second great cold age, the chief among them for its instructiveness being the loess of Timaru.
In a paper read last session, and published in the Trans-
actions,* the conclusion was stated that the loess is an Æolian deposit, a heap of wind-borne dust, the dust being rock-meal wind-swept from areas of lowlands overflowed by rivers charged with glacier silt. This conclusion, I learn, has been adversely criticized by those well able to judge of the value of the facts and arguments stated in support of it, and, as the value of any reading of the characters contained in the loess depends upon the origin of the formation being correctly ascertained and admitted, I have looked over my paper to see where lies the error or weakness of my presentment of facts which in nature permit of no doubt that the loess is Æolian. I find that of the several characters described there is one, and it may be only one, which as far as I can see is quite inconsistent with any other theory, and unfortunately very little stress was laid upon this character. It is, however, twice briefly mentioned. It is stated in that paper† that the bands marking pauses in the process of deposition “curve in the spurs so as to be roughly parallel with the present land-surface;” and, further,‡ that these bands “are all flat or curved with definite relation to the present drainage-lines.”§ I cannot but think that, had special attention been drawn to this feature, the result must have been to disarm adverse criticism of the “dust-heap theory.” Not having done this before, I must take this opportunity of remedying the defect.
Every detail of the loess of Timaru emphatically denies that it is of marine origin. But if it existed under different topographical conditions most of the details might be held to be consistent with the “inundation” theories which have been proposed to account for loess deposits in Europe. By no exercise of the imagination, however, after as full an appeal to nature for inspiration as possible, can I conceive how, by any form of aqueous agency, each distinguishable layer could have been deposited upon the previous one in such a manner as to preserve the drainage-lines. Here the lines of surface-drainage, small and great, have been preserved throughout the process of deposition. Where there are rounded ridges or flattened ridges between gullies and hollows now, there have been rounded ridges and flattened ridges from the first. The hollows have not been carved out of a level continuous sheet of material. Every form of deposit of fine material by aqueous agency tends to level and smooth over the area of deposit.
[Footnote] * Trans. N.Z. Inst., vol. xxii., art. xlviii.
[Footnote] † Loc. cit., p. 407.
[Footnote] ‡ Loc. cit., p. 413.
[Footnote] § Mr. T. Goodall, in a paper on the loess (Trans., vol. xix., art. lx.), also says, “These beds curve with the hill, and do not occur in flat beds as in marine deposits.” He also gives a diagram showing the curvature.
This was not the tendency with the agent which built up the Timaru loess; on the contrary, the effect was to increase the unevenness. The bands of stratification curve with definite relation to the drainage-lines. It is easy to understand how such curves were produced, on the dust-heap theory. They are the resultants of the conflicting forces of equable deposition, and inequable denudation by contemporary rains, the latter having greater power on the slopes and surfaces near the drainage-lines. It appears to me that this curved stratification, the formation being superficial, undisturbed, and resting upon a level base, is a crucial test, and settles the question.
The loess being of Æolian origin, it necessarily follows that it belongs to a glacial age, for no other agent than ice could, in this latitude, produce material of this kind, and under such related conditions that the material could be spread out for winds to lift and bear it away to new fields of deposit. As we saw from an examination of the products of the first cold age, the Canterbury Plains were principally built up long before the loess period, but the seismic disturbances during the interval produced alterations in levels which we have few, if any, means of fairly estimating; and where those dust-fields were chiefly situated is a question it would be difficult to answer. The building-up of shingle-fans implies the overflowing of their banks by the fan-building rivers, and perhaps we should look to such action for the spreading-out of the dust which was swept over the higher lands by the breezes. In that case we should have the whole of the Canterbury Plains, and any contemporary extension of them eastward, by changing positions, as the immediate source of the dust. Against this idea, however, is the fact that the shingle of the fans appears to be free from glacier silt, and also the fact that the fans are traceable to, and appear on the whole to be contemporary with, the moraine-dams of the mountain lakes, while the loess must be older. It appears to belong to the whole of the second glacial period, and principally to the earlier stages and the culmination of its severity, rather than to the latest, the moraine and fan-building stage.
As described in my paper of last year, the loess contains marks of several pauses in its deposition, in bands containing (a) drought veins,* the product of a dry climate; (b) rust-granules, the product of a wet climate; (c) multitudes of birds” crop-stones, which I shall presently suggest have an interesting significance as an index of climate; and (d) at one level certain alterations of texture produced by extreme severity of climate. Deposited upon areas elevated above the reach of rivers, this
[Footnote] * Previously described under the name “evaporation veins.
growing dust-heap played the part of an observant bystander, taking notes of certain climatic phenomena as they successively arose. The record of the lowest separable layer, marked off by a band in which both drought-veins and rust-granules occur, may, I would suggest, be read as follows:—
1. A phase of cold, producing great icefields and glaciers in the highlands, which send down floods of sludgy waters, inundating the lowlands, and creating fields of dust, from which the winds picked up and deposited here a bed of loess up to 10ft. thick where the contemporary denudation was slight. (This is the thickest of the layers.)
2. A phase of improving climate, during which the glaciers diminished and the supply of dust ceased, probably in part through the trapping of the glacier silt in lakes or pools, occupying basins scooped out by the previously extended glaciers. The climate here continued wet, however, for even where the slope of the surface afforded good drainage the rust-granules characteristic of wet soils were formed.
3. The climate further improved, becoming dry enough in summer to crack the ground to the depth of a few feet, and drought-veins were formed.
4. The moist climate returned, the formation of drought-veins ceased, and that of rust-granules was resumed.
5. With increasing cold the glaciers again advanced, and the supply of dust was resumed, this recommencing the series.
If right in the main, this reading may be wrong by containing a redundant clause. There may have been but one phase, not two, of wet climate, giving rise to the production of rust-granules. If but one, there is some reason to suppose that it was related to the return of the cold phase rather than to the retreat of that phase. A ground for this supposition will be stated later on, in the suddenness of considerable improvements in climate.
The series of variations of climate registered in the first layer of loess appears to have been fully repeated but once. There are only two of the buried subsoils, so far as my observations show, that contain the drought-veins produced by dry climate cracking the ground, the second of these being near the top of the deposit. I have not been able to determine the exact number of marks of pauses in the deposition, but there are in one good section five or six distinct and a few indistinct ones. As the faces of the cuttings are coated with rain-wash it is not easy to count these soil-bands with certainty. As each of them indicates a long period when the supply of dust ceased, and as we trace the dust to an origin dependent upon a certain condition of climate, each of these bands registers an absence of that condition—in other words, an important alteration in the climate.
Towards the close of the loess record two new characters are introduced, the interpretation of which for a long time puzzled me completely. From 2ft. to 4ft. beneath the summits of the ridges the loess shows in weathered road- and railway-cuttings a projecting band 1ft. to 3ft. thick, the material in which differs from that above and beneath it only in being more compact, and thus resisting the weather better. The presence of drought-cracks nearly everywhere has enabled the rain to carve this band into elongated bosses. On the slopes of the spurs this character gradually gives place to another, which is quite unique in the formation. Where well developed, as on the slopes of the larger gullies, this consists of hard flaky layers, some rusty, some not so, some even whiter than the loess generally; the whole generally but a few inches, but in some places a few feet, in thickness, and frequently, or rather generally, separated into small roughly cubical fragments. The whiter portions look just like the “pugged” clay formed beneath landslips and seen where such slips have been sectioned in roadwork. The two related characters show more or less clearly in every spur in cliff and cutting at the coast, and the pugged layer in most of the sidling road-cuttings on the slopes of the gullies all over the plateau. The only explanation I can find to account for these characters is that they register the phase of greatest severity of the ice age to which the loess belongs—the second glacial period; that they show that the summits of the ridges at that time were compacted by a heavy load of ice, while the surfaces of the slopes were “pugged” by the ice creeping over them to form ice-streams in the gullies. Adopting this explanation, the extensive denudation of the dolerite and underlying gravels seen in the larger gullies and their branches becomes comprehensible; whilst it must be simply a cause of utter bewilderment at the time required, if we must believe this denudation was effected by the trifling surface-drainage now at work.
Knowing that in the discussions which took place some years ago upon the glaciation of New Zealand the view was strongly combatted that there had ever occurred such a degree of glaciation as to involve the lowlands of Canterbury, I have been the more cautious in adopting it. It is, however, well supported by plainer evidence in the immediate neighbourhood.
The map and descriptions of the icefields and glaciers of the great glacier period, in Haast's “Geology of Canterbury and Westland,” can by no means show the full extent of the glaciation of Canterbury. The map shows, the text describes, only the larger glaciers originating in the Southern Alps. Besides these there must have been many minor snow-fields with their glaciers. One of these minor fields evidently existed on the north side of Mount Misery (otherwise Cave
Hill), immediately west of the Timaru plateau. This snow-field gave rise to a small glacier which, gathering towards the north, swept by east and south to the Pareora, along the western side of Mount Horrible (the summit of the Timaru plateau, 1,100ft.), and gouged away the mountain on that side, with its thick cap of dolorite upon soft marine-beds, into a precipitous face 800ft. or 900ft. high. This stream, joining another flowing from the upper Pareora country by the south of Mount Misery—rather a broad sheet of land-ice, perhaps, than a mere glacier-stream—gouged away the southern side of Mount Horrible and lower portions of the plateau in a similar manner. Moreover, the summit of Mount Horrible, though of no great area, yielded a glacier which, flowing down a slight slope northwards, scooped out on that side a wide yet ravine-like gully, this stream then joining that from Mount Misery. Mount Horrible is thus blocked out by precipitous faces on three sides, in a manner giving a western aspect justifying its name. No other agent competent to do the work could have operated here; no other agent operates in such a way; and, besides the general character of the denudation, there are a few other marks, of glacier-work on the mountain, in perched blocks, and a small lateral moraine piled against the remnant of an enormous slip from the precipitous southern face. Having studied the enormous gouging of Mount Horrible, evidently the work of ice, I have no hesitation in attributing the two peculiar characters in the upper loess to the glaciation of this deposit. These marks, then, show that the severest phase of the second ice age occurred near its close. Another layer of loess was afterwards added, but it is of less thickness than earlier ones, and should, I think, be referred to the earlier stages of the retreat of the ice, as in later stages the glacier silt would scarcely escape being trapped by rock-basin lakes, which later on were filled with shingle.
Among the accidental constituents of the Timaru loess, the multitudes of bird-stones in most of the bands marking long-persistent land-surfaces are surely the most remarkable. Their distribution also is remarkable. According to my observations they are decidedly much more numerous at the coast than a couple of miles inland, and also more numerous at the north end of the coast cliffs than a few miles further south. So far the only good section of the whole deposit that has been made inland is the “stripping” at the Harbour Quarry, and here the bird-stones are few, certainly much less numerous than at the coast. The number and the partial distribution of the pebbles may be accounted for by the supposition (besides that of an arctic climate, otherwise found necessary) that the Timaru plateau never extended much
further eastward than it does at present. We may then see in this well-drained north-eastern angle of the dolerite and loess plateau the mustering and alighting-ground of swarms of birds migrating to and from less frigid regions with each recurring year. I cannot see how the partial distribution is to be otherwise accounted for. But there is another difficulty. Sea-birds do not use gizzard-stones, their use is confined to granivorous birds, and these seldom or never discharge them. Are these multitudes of stones, then, a mortuary talus? If so, why their markedly greater number near the coast? I have never met with any statement on the subject, but I think it more than probable that birds about to take a long migratory flight will unburden themselves as much as possible before starting, including the discharge of their gizzard-stones. It is worthy of note that bird-stones are numerous in some places in the clefts in the dolerite, deposited there before the loess began to fall; and also that the loess in the upper portion, including the compressed and pugged layer and a few feet beneath it, is barren of them. From the last fact it is to be inferred, if the general explanation is correct, that the climate became too severe to allow the birds to visit this region.
In this connection must be mentioned that in the lowest layer of the deposit stones are to be found which could only have been used by large birds of the moa family. I have found an odd one or two higher up, and also fragments of bones; but in the lowest layer I have seen two nests, so to speak, of large stones. In my last paper on the red gravels, it was stated that bones of large birds have been found beneath the dolerite; therefore there is nothing to be wondered at in finding relics of such birds in the later formation. There is this curious fact, however, in connection with the majority of the pebbles in one of the nests referred to: that they were unquestionably picked up by the bird or birds from an exposed sea-beach. They are of the same or very similar rock material as those forming the bulk of the present sea-beach, i.e., Waitaki shingle, and they have the discoid shape produced by the action of the waves on the beach. I have been very careful to make sure that these pebbles are really imbedded in the original deposit, and not merely surf-washed and mingled with slip stuff, cases of which can be seen all along the cliffs. A vague and unsatisfactory inference might be suggested from the fact stated, that the beach from which the pebbles were obtained was not far away, and, from this, the further inference that the land in that case could not have been at a much greater elevation than at present. In the second nest referred to, the pebbles were all well-rounded.
Before leaving the loess, I would suggest that the upward
range of the loess on Banks Peninsula may be a peculiar evidence of the contemporary climate. I read that it extends only some 800ft. up the hill-sides. Why so? It cannot be that was the limit—plus the amount of any subsidence since—to which winds could lift, the loess-forming dust. Does not this limit of height mark what was practically the limit of vegetation capable of entrapping and retaining dust—in other words, the snow-line?
If we suppose the ice retreated from the field of its greatest extension to the limits marked by the great lake-dams, we have to deal with a great amelioration of climate, and the change appears to have been as sudden as it was great. There is an absence of marks of gradual retreat over the country generally, and the great moraines have almost as definite a limit of commencement as of termination. The second great thaw, under the influence of which the glaciers retreated to something like their present dimensions (or it may have been further back), also appears to have been a sudden thaw. It was certainly very rapid, compared with the length of time occupied in building up the great terminal moraines. The fans, which originate from those moraines, also appear to have been completed by an extraordinary, one might say a cataclysmic, rush of water, not by ordinary river-action. The small terraces and gutters which make old river-beds rough travelling are absent from the surfaces of the fans, where these have not been smoothed over by a surface of soil. A sweeping rush of water would account for this. Probably, in some cases such a rush of water might be attributable to breaches in lake-dams, but scarcely in all cases. One of the most remarkable instances of the smoothing of the surface of a fan is that of the small fan of the Waihi (Woodbury), of which the material is, at the gorge, very coarse and bouldery. The river at its highest now is but a small stream, that could scarcely cover the levelled upper portion of the fan. It seems to me that this fan could only have been levelled in the manner it was (before subsequent terracing on one side) by a powerful rush of water. And there is no lake on any of its branches, but it drains a bulky mountain, which at the time glaciers filled the great lakes probably carried a heavy load of snow.
Much has been written about the great glacier period, but I do not remember having seen attention drawn to the sudden termination of the work of building the moraine dams. The compactness of those great terminal moraines, their small breadth in proportion to the length of the glaciers which piled them, suggests a corresponding steadiness of climate. This, however, may be misleading; the moraines may be the record not of a continuous and equable period of cold, but of a series of maxima of nearly equal intensity. If such was the case,
evidence of the fact might be found in excavated moraines, such as has been described in the Nelson Province. Be this as it may, the cessation of the work of piling up the terminal moraines clearly points to the occurrence of marvellously sudden ameliorations of climate. The absence of marks of slow retreat—in morainic matter—of the ice from the lowlands appears to tell a similar tale of an earlier age. How such sudden changes were brought about is a question on which I can offer no opinion; but I think the condition of the alpine lakes, with their high and steep moraines, clearly proves that the ice retreated from them with great suddenness. If this were so, as it appears, the lesser changes registered by the loess may likewise have been sudden changes—that is, occupying but a brief time compared with the duration of each fixed phase.