(2.)Evidences of Depression.
The evidence for the lowering of the land below its present level is as follows:—
(1.) Marine terraces occur at Kaikoura, Port Robinson, Amuri Bluff, Motonau, Conway River, and at Banks Peninsula. They are found as high as 600 ft. above present sea-level at Amuri Bluff. The first five of these have been recorded previously by Haast, Hutton, Hector, and McKay, but the last case has not been previously noted as far as I am aware. The evidence for this is as follows: Round the coast of Banks Peninsula the headlands have in many cases flat extremities. The lava-flows which form them dip outwards at low angles, but the edges of the streams are truncated and cut level on the upper surface. The greatest height at which I have noted this marine terrace is at Lyttelton heads, where the elevation is over 450 ft.; the same phenomenon can be seen at Whitewash Head, near Sumner, and at the Long Lookout Point. It is well marked, besides, in other places. The height of this terrace diminishes, as a rule, on those parts of the coast-line which would be exposed during submergence to strong currents and heavy seas. It is low on the southern side of the peninsula. I have not come across in any place traces of marine organisms, but it is not likely they would occur plentifully, or be preserved when they did occur, in such a position. One of the principal conditions which promote rapid erosion on rocky coasts seems to be the presence of strong currents, which can sweep away the material dislodged by wave and other action. Headlands which stretch out far into the sea, particularly if the water be deep on either side, will therefore commonly show a marked wave-cut
terrace, while an even coast-line will show none. Thus we have the remarkable shore platforms at Kaikoura Peninsula, but hardly any sign of them on the steep hills to the north and south. The conditions would be extremely favourable for the cutting of distinct shore platforms on the spurs of Banks Peninsula during a period of depression.
(2.) The existence of the silt deposit or loess was held by Captain Hutton to be a proof of subsidence. If it is a marine deposit, it undoubtedly proves that the land was much lower—quite 1,000 ft.—as may be inferred from the distribution of the deposit, and its present occurrence so far above sea-level would be a proof of subsequent elevation. However, there are strong reasons for believing it is a wind deposit, and I know from conversations with Captain Hutton that he was not quite satisfied with some of his evidence. One difficulty which strikes me with regard to Captain Hutton's contention is the following: The so-called silt must have been formed of glacial rock-flour during a period of sever glaciation—i.e., during a period of marked elevation of the land. All observers are agreed, I believe, in this. Now, Captain Hutton's theory demands that it should have been distributed into its present position by marine action during a time of depression of the land. It is absolutely impossible that the two processes could have gone on simultaneously in the Canterbury area. If the silt were swept down by great rivers issuing from the glaciers and distributed by the sea at their mouths, the area of deposition would be forty or fifty miles to the eastward of the present coast-lines. Further, if the sea advanced to cover the Canterbury Plains, the glaciers would then have disappeared, or have lingered on only the very highest parts of the Southern Alps. The sea must therefore have distributed the silt during a time of depression posterior to the time of elevation when glaciation was at its maximum. It would have been expected that the silt would be thickest in the hollows and on lower ground. Such in not the case, however; it shows a marked tendency to be thickest on the spurs and to thin out on low ground. In this way it closely resembles the distribution of the loess in the Valley of the Mississippi, to explain which the aid of the sea has never been called in.
Professor A. heim, of the University of Zürich, an observer of wide experience, and an authority of the greatest weight on glacial and allied problems, differed with Captain Hutton on this point. After a visit to New Zealand he published in Zürich, in the year 1905, a paper which has many valuable observations on geological problems in this country. The following is a translation of his remarks in this work on our so-called less:
“When the great glaciers which were thrust forward to the outlets of the alpine valleys receded, and the ground moraines which were left behind were dried up by the north-west wind (Föhn), then the fine dust was blown far over the surface right up to the sea. The deposit of dust accumulated in the form of the fertile loess. The, as we see in many parts of Germany, the loess covered the land-surface, sometimes from half a metre to a metre in thickness, and sometimes form 10 to 15 metres. Where it breaks away on the upper edge of the river-bed region it forms perpendicular walls, and here long-buried moa-bones frequently appear. But even now the loess formation is going on. We have ourselves-seen how thick are the clouds of dust whirled up form the broad, shingly river-beds by the north-west wind and spread over the cultivated land. The rain, when it falls afterwards. unites the dust with the agricultural land. A part of the fertility of the eastern plains depends on the loess covering.”
After a general consideration of the evidence, and from my own observations, I have come to the conclusion that the loess has not been beneath the sea. It is very thick on the hills between Tai Tapu and Birdling's Flat, but is completely swept away from those places which have been exposed to lake or sea erosion. It could not exist in its peculiar position on the tops of spurs, &c., if they had been washed by the sea since it was laid down. Further, if it had been a marine deposit it should have covered the whole landscape irrespective of its form, and it is unlikely that it has been removed by denuding agents from so many places and left comparatively untouched on the spurs and the sides of valleys. I am therefore inclined to think it was a wind deposit during the steppe conditions of a higher land and drier climate, with severe windstorms sweeping from great river-beds greater clouds of dust than are seen now in the Rakaia and Waimakariri, although these are by no means of insignificant proportions at the present time.
The deposit of loess covers up the old shore platforms on the south-west side of Banks Peninsula, therefore the depression during which they were formed was pre-loess, and therefore before the great glacier extension. If this is really so, it serves to emphasize the recency of this extension. The general order of events would therefore be a period of low land, when the marine terraces were formed, then an elevation in glacier times, followed by a depression till now, with probably minor periods of slight elevation. there is a slight elevation going on now, as may be seen from the wave-worn caves at Summer now several feet above high-water mark, and the bands of sand-dunes between Christchurch and the sea. This, no doubt, accounts for the low,
broad terraces to be seen in the lower reaches of the Avon and Heathcote Rivers.
The elevation of the land is always considered a most important point in causing terrace-development, but this is chiefly in those cases where rivers have been near their base-level. Subsequent elevation causes them to form terraces owing to the restoration to them of their power of corrosion. This is the case of the Avon and Heathcote terraces just mentioned.
Now, the Canterbury rivers have a remarkably steep grade, and a depression of the land would hardly be felt in their upper and middle portions. I think it very probable that if the land were lowered till the shore-line corresponded with the main line of railway, the erosive power of the streams near the gorges would be only very slightly altered. Further, if terracing were due to elevation it should be progressive upstream from the coast, whereas the contrary is the case: the terraces are highest in their upper portions.
I do not think that change in the height of the land has materially affected the erosive power of these rivers. Near the sea-coast it has undoubtedly exerted some influence, and the raising of the bed of the Waimakariri near Belfast is most probably due to continued depression of the land.