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

(B.) The Loess Formation

(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.