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

Chapter XIV

page 366

Chapter XIV.

(A) Raised Beaches.

Before proceeding to treat of the Great Glacier Period of New Zealand, I consider it convenient to speak first of two other formations, partly deposited before this important era in the geological history of this Island began. The first of these formations consists of a deposit of marine shells, raised about 100 feet above the sea level, near the mouth of the Motanau river, situated in the northern portion of the East Coast. This bed, which I was not able to trace to the south, contains, according to Professor Hutton's researches, only recent shells, with the sole exception of Mactra rudis. It is thus of post pliocene (or quarternary) age, but I am unable to say if the deposit in question is of pre-glacier age, no morainic or alluvial deposits, formed during the Great Glacier period being found in its proximity. One fact, however, is certain, namely, that the land in post pliocene times in the northern portion of the province along the east coast stood at a lower level than at the central and southern portions. Thus, whilst the raised beach near Motanau rises about 100 feet above the sea level, gradually getting lower and disappearing near the mouth of the Waipara altogether, in all the more southern portions of the coast, where the country has not been subjected to fluviatile action, only beds of loess or silt are met with near the sea level. They either cover the hill sides for several hundred feet or form downs of large extent.

page 367

If such beaches had been formed on the lower slopes of Banks' Peninsula we should certainly have ample evidence as to their existence; however, beyond a small oscillation averaging at most 15 to 20 feet in vertical height, no rising of the ground has there taken place. Everywhere on the lower slopes, where the volcanic rocks are not exposed, thick beds of loess invariably cover them. And even where shells occur as high as six to eight feet above high water mark, it is very possible that they might have been placed there by the agency of an exceptionally high tide. The same may be said of the Timaru plateau, consisting in its upper portion of beds of loess, which can be followed about ten miles inland. Either volcanic rocks or loess beds form bold cliffs along the seashore, where splendid sections are open to our examination, but nowhere is there any sign of raised beaches or littoral deposits, any marine shells found at the foot of the cliffs having been brought there by the tides. Still more to the south, there is ample evidence that the cliffs of loess now situated several miles inland were at one time washed by the sea. Deposits of boulders and sand travelling northwards with the sea currents, and enclosing sometimes to the west of them lagoons and marshes, gradually shut these cliffs off from the ocean. The most careful examination has never revealed the least sign of any raised beaches amongst them.

The following shells have been collected in the bed in question occurring near Motanau:—Fusus corticatus, Fusus dilatatus, Fusus plebæus, Buccinum costatum, Struthiolaria nodulosa, Galyptræa maculata, Crypta contorta, Imperator imperialis, Rotella zealandica, Polydonta tiarata, Gibbula nitida, Siphonaria denticulata, Ampullacera avellana, Dentalium pacificum, Mulinia notata, Tellina deltoidalis, Mactra rudis, Mesodesma cuneata, Mytilus magellanicus, Modiola albicosta, Pecten latieostatus, Pecten zealandia, Terebratella rubicunda, and Rhynchonella nigricans.

(B.) The Loess Formation.

When speaking of Banks' Peninsula, I have already referred to the remarkable slope deposits, by which the middle and lower slopes of this isolated volcanic system are extensively covered. They consist of an unstratified yellowish loam, friable in small pieces, but very tenacious and consistent in large masses. This loam, to which in the future I wish to apply the term loess, consists mostly of argillaceous matter with small grains of felspar, minute fragments of mica and page 368hornblende, with some small per centage of carbonate of lime. It contains also remains of land shells and moa bones, the latter generally surrounded by marly concretions. The eminent German traveller and geologist, Baron Ton Richthofen, has thrown a great deal of light upon the mode of its formation, through his researches on the nature of loess deposits in China, where they cover districts of enormous extent, and reach a thickness of 500 to 1500 feet, measured in a vertical direction. He has shown in his last publications, that the loess in China could only be of subærial origin deposited by agencies, which at the present time are still at work in forming that rock. Atmospheric currents, together with the growth of grass and other vegetation during an untold number of years, are the principal agencies by which the loess has been deposited. In the first instance, rain-water running down the more or less steep slopes of the country carries with it fine particles, which are partly retained by the grass or amongst its roots, whilst the wind blowing across the land takes up a great amount of fine sediment, afterwards also partly caught and retained by the grass. However, a third and most important agent is to be found in the roots of the plants themselves gradually decaying, and thus raising the ground. There is a peculiar vertical capillary texture observable in the true loess, deriving doubtless its origin from the decaying of the numberless rootlets during many past generations of grasses, to be also noticed in numerous localities in Banks' Peninsula. Thus von Richthofen correctly styles the loess beds a grave-yard of innumerable generations of grasses. Of course I do not wish it to be understood that all beds of the nature of loess have been formed in that way. Many have been deposited in lakes and lagoons, others by livers overflowing in heavy freshets the low ground along their banks; but the general character and position of the principal loess (or loam) beds in this province prove clearly that they have been formed by the modus operandi pointed out by von Richthofen. There is, however, one difference which I wish to point out, and that is the absence in the Canterbury beds of the peculiar small marly nodules so common on the Rhine, the Danube, and China, where they are named loess babies, little loess men, and stone ginger—if we do not consider the large marly concretions surrounding the moa bones their equivalents. The remarkable regular concretions assuming so many curious forms obtained in the gorge of the Rakaia and near the junction of the Acheron, have been formed in argillaceous beds of lacustrine origin. They resemble the so-called Morpholites of Ehrenberg found at Denderah in Egypt, or the Marlekor of Sweden, both having been page 369collected from similar deposits of clay, in which they occur often with a linear arrangement. The land having been gradually raised for several hundred feet after the formation of the Pareora beds, all that portion of the country, not being subject to fluviatile action, now became exposed to physical conditions favourable to the formation of loess. The deposition of these beds where the ground has remained in its virgin state, is going on still without interruption. It may therefore truly be said, that the loess formation, commencing in pliocene times, has not yet come to its termination. Thus during the Great Glacier period of New Zealand, next to be treated—beginning towards the end of the pliocene and ending in the post-pliocene period —during quarternary and recent times, the loess beds have gone on accumulating steadily, so as to reach such a considerable thickness, as we find them amongst other localities on the lower slopes of Banks' Peninsula and on the Timaru plateau. Where they occur in the neighbourhood of the channels formed by the great glacier rivers, they are sometime overlaid and preserved by fluviatile deposits. In some other instances loess beds of smaller extent are interstratified with the latter. Finally towards the end of the Great Glacier period, when the rivers descending from the Southern Alps began to lay their channels lower by cutting into and removing the fluviatile deposits previously formed by them, the remaining portion of the plains became in its turn, and wherever favourable circumstances presented themselves, extensively covered by loess beds formed in the manner previously described. These beds, on close examination, are easily to be distinguished from the deposits of silt or ooze formed during great freshets, when the muddy waters spreading as one broad sheet over the country in the neighbourhood of the river channels, cover it with deposits resembling to the casual observer the former, in their lithological character.

Extent.

I have already alluded to the fact that the volcanic system of Banks' Peninsula having doubtless remained above the sea level since its formation, and being at the same time not subjected to fluviatile denudation possessed most favourable conditions for deposition of loess. In the north of the province the downs rising from the Canterbury plains at the foot of Mount Grey and reaching as far as the Ashley and the Moeraki Downs, are capped by these beds under review. In the Malvern Hills they are also well represented. More to the south page 370they appear again on the southern banks of the Orari, stretching to the Timaru plateau. They also cover the coulées of volcanic rocks, of which this plateau is formed. They reach to near the summit of Mount Horrible and gradually thin out, as we follow them from the sea. Some of the cliffs, formed entirely of loess are 70 to 80 feet high, and exhibit throughout the whole section exposed along the sea, and in railway cuttings, the same characteristic features. South of Timaru they again overlie younger tertiary sedimentary strata and form undulating downs reaching several miles below the Waihao, after which they have generally been destroyed by fluviatile action. An outlier is situated about 15 miles above the mouth of the Waitaki, south of Elephant Hill. In some localities they cap the shingle formation of the Canterbury plains to a thickness of ten feet. In Westland, in some sheltered localities, beds of loess have also been noted by me.

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