Art. XXXI.—Note on the Mechanical Composition of the So-called Loess at Timaru.
[Read before the New Zealand Institute, at Christchurch, 4th–8th February, 1919; received by Editor, 12th February, 1919; issued separately, 16th July, 1919.]
The origin of the so-called loess at Timaru, and of other similar deposits on Banks Peninsula and elsewhere in the South Island, is connected with the larger problem of the formation of the Canterbury Plains, and with the question of the direction of the more recent movements of the land in that area. Haast* was of opinion that the deposit is similar in origin to the loess of China described by Baron von Richthofen—that is to say, it is an aeolian deposit. In this opinion he has been supported by Hard-castle,† by Speight,‡ and by Professor A. Heim§ of the University of Zürich. Marshall‖ has also declared his adhesion to Haast's view. Hutton¶ always strenuously opposed this theory, maintaining the deposit to be a marine silt laid down during a period of submergence of the plains; and he quotes Professor Boehm, of Freiberg, in support of his arguments. Since analyses of the deposit as seen typically at Timaru may shed some light on the problem I submit this note, though I admit my contribution of original observations is small when the importance of the question and the time given to it by previous workers are considered.
The following results were obtained by the method of mechanical analysis adopted by the British Agricultural Education Association (see Journ. Agric. Science, 1906, vol. 1, p. 470). The particles are freed from one another by treatment first with dilute hydrochloric acid to dissolve inorganic cements, then with dilute ammonia solution to dissolve organic cements. They are then separated by sedimentation.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Name of Fraction.||Diameter of Particles in Millimetres.||(1) 2 ft. from Surface.||(2) 20 ft. from Surface.||(3) 30 ft. from Surface.|
|Dissolved matter and hygroscopic moisture||N.d.||N.d.||N.d.|
[Footnote] * J. VON Haast, Geology of Canterbury and Westland, Christchurch, 1879.
[Footnote] † J. Hardcastle, Trans. N.Z. Inst., vol. 22, p. 406, 1890.
[Footnote] ‡ R. Speight, Trans. N.Z. Inst., vol. 40, p. 33, 1908; Trans. N.Z. Inst., vol. 49, p. 386, 1917.
[Footnote] § A. Heim, Quoted by Speight, Trans. N.Z. Inst., vol. 40, p. 33, 1908.
[Footnote] ‖ P. Marshall, New Zealand and Adjacent Islands, p. 31, Heidelberg, 1912.
[Footnote] ¶ P. W. Hutton, Trans. N.Z. Inst., vol. 37, p. 465, 1905.
Haast, when he set forth his theory that this deposit is of aeolian origin, followed Richthofen's explanation of a supposedly similar deposit in China. The required conditions are (1) the production of waste in an impalpable form, (2) steppe conditions for its desiccation, (3) winds to transport it, (4) vegetation to fix it where it settles. In our case it is presumed that the material is rock-flour produced by the Pleistocene glaciers and brought down by rivers; and, since it is generally agreed that the extension of the glaciers was due to the land being at a greater elevation, it is assumed that the coastal plains then exhibited the characters of a steppe region, so that the silt deposited on the flood-plains of the rivers was in condition for transportation by the wind. Under these conditions winds also must be granted. But now comes a difficulty. According to the experiments of Udden,* the average largest size of quartz-particles sustained in air by strong winds is 0.1 mm. in diameter, and Emerson† says that 70 per cent. of loess-particles range from 0.05 mm. to 0.01 mm. in diameter, a statement that is apparently based on the following analyses given by Merrill.‡ It is unfortunate that I have not been able to find any records of mechanical analyses of the loess of China.
|Name of Fraction,||Diameter of Particles in Millimetres.||(1) Upland Loess, Virginia, Illinois.||(2) River Loess, Virginia, Illinois.||(3) Loess, Nebraska.||(4) Dust from Snow Rockville, Indiana.|
|Very fine sand||0.1–0.05||7.68||24.84||23.14||0.00|
It will be noticed that mechanical analysis by the method generally adopted in the United States separates the material into fractions that differ somewhat in size from those obtained by the British method.
Now, when we look at the composition of the Timaru deposit we find that from 28 to 40 per cent. of the particles have a diameter ranging from 0.04 mm. to 0.2 mm., while there is also a certain small quantity of material over 0.2 mm. in diameter. We have here, therefore, a very strong argument against the aeolian hypothesis.
Richthofen further stresses the importance of vegetation as a means of accumulating the fine particles. Haast quotes him as styling the loess beds “a graveyard of innumerable generations of grasses.” In the Geological Magazine, vol. 9, p. 297, 1882, Richthofen writes, “Where this dust falls on barren ground it is carried away by the next wind; but where it
[Footnote] * J. A. Udden, Journ. Geol., vol. 2, p. 323, 1894.
[Footnote] † F. V. Emerson, Journ. Geol., vol. 26, p. 532, 1918.
[Footnote] ‡ G. P. Merrill, Rocks, Rock-weathering, and Soils, New York, 1906.
falls on vegetation its migration is stopped.” Quite as we should therefore expect, we find in one case nearly 5 per cent., in another nearly 12 per cent., of organic matter in the analyses cited above from Merrill. But the Timaru deposit contains merely a trace. Hardcastle attempts to explain this remarkable fact by assuming that “the growth of the deposit was so slow as to nearly allow the rootlets of each generation of plants to suck up the last remnants of the decay of previous ones.” But any one who understands what goes on in a soil clothed with vegetation knows that this cannot take place. An article by Sir A. D. Hall in the Journ. Agric. Science, vol. 1, p. 241, 1905, may be consulted in this connection.
Speight (loc. cit.) has written, “The general absence of marine fossils, the presence of remains of Dinornis and other land-birds, as well as its peculiar distribution, is against a marine origin being assigned to it.” As regards marine fossils, setting aside the fact of those recorded by Hutton (loc. cit.), which cannot be explained on the aeolian hypothesis, we must surely allow Hutton's contention that negative evidence on such a question is of little weight. There are other cases of undoubted marine deposits which are marked by a poverty of marine fossils. The greywacke is one such; the Buller series of sandstones and conglomerates is another. The Amuri limestone and the Weka Pass stone are also poor in the larger forms that we might reasonably expect.
The presence of remains of Dinornis and other land-birds must be interpreted rather as evidence in favour of marine origin than the reverse, the bones having been washed down and covered in the silt by local stream-action. It is obvious that birds require vegetation for their sustenance, and moas could scarcely have lived on Banks Peninsula at a time when loess was supposed to be forming, when it is considered that there is proof that no vegetation was present on the part covered by the deposit—at least there is no trace of it left. It may be noticed, in passing, that the advocates of the aeolian hypothesis call in local stream-action to account for the cases of distinct stratification noticed by Hutton* at Lyttelton and elsewhere, but are unwilling to allow the same agency to account for the covering of the moa-bones.
As regards the peculiar distribution of the deposit, that also is, I think rather evidence in favour of its marine origin. Its occurrence within the Lyttelton caldera is as reasonably explained by deposition from water as by supposing, as is necessary to the aeolian hypothesis, that while it was deposited on the lower portions of the outer slopes it was also carried over the intervening ridges into the Lyttelton caldera without leaving a trace of its path.
[Footnote] * F. W. Hutton, Trans., N.Z. Inst., vol. 15, p. 411, 1883.