(1.)Evidence for a Recnet Elevation
The existence of peat-beds, as well as buried logs—that is, an old land surface—is proved by artesian-well borings in the Christchurch area. Peat has been found at the following depths: at Ward's brewery, 400 ft.; at Sydenham, 500 ft.; and at Islington, 700 ft., below the surface of the ground. As the first two places are less than 20 ft. and the last only 112 ft. above sea-level, the evidence is convincing that the land stood at least 600 ft. higher than at present when the outskirts of the plains were formed. This proves a substantial elevation in recent geological times; and as artesian borings are put down further a greater elevation may be proved, as only in the immediate neighbourhood of the Port Hills has solid rock been reached by such borings.
Additional positive evidence of an increased height of the land is to be found in the bays which surround Banks Peninsula. They are all, or nearly all, drowned valleys, and were formed when the land was higher. In most cases they are valleys which have been formed wholly by water action. In cases of Akaroa and Lyttelton Harbours, the original craters of volcanoes have, perhaps, been enlarged by explosions, but certainly have been further amplified by water erosion and extended into the valley form. The exposed floors of these valleys grade into the submerged portion. The usual depth of the bay near its outlet to the ocean is from 6 to 8 fathoms—that is, from 40 ft. to 50 ft.—and this gives the minimum elevation necessary to allow the valleys to be formed. But all the bays have been filled to a marked extent by mud washed from the hillsides, so that no accurate estimate can be made of the depth of the rock floor beneath. Borings in search of artesian water-supply put down in the valley behind summer failed to reach either water or solid rock at a depth of 200 ft.
The date of this elevation is difficult to determine in the absence of any fossil evidence or any other accurate time indication; but, taken in conjunction with the evidence from artesian wells, it is, I think, of fairly recent date. Another proof that the land has recently been higher is afforded by the shape and position of the valleys of the streams near Timaru. In most cases they are submerged where they enter the sea.
The evidence from the valleys as well as that from the wells proves conclusively that the land was recently much higher, certainly as much as 600 ft. This elevation would produce a great extension of the land eastward, as the sea-bottom is sensibly flat till the hundred-fathom line is reached at a distance of about forty miles from the present coast-line. Then the depth increases very rapidly to over 1,000 fathoms within the next
few miles. This submarine bank or shelf no doubt marks the utmost eastward extension of the land since Pliocene times. The fan of the Rakaia and Ashburton at one time stretched further east than the present coast-line, as pointed out by Sir Julius von Haast. This would probably have been so extended during a period when the land was at a higher level. On depression setting in, the outer segment of the fan was swept away owing to its being exposed to the full force of the heavy seas and the strong northerly drift on the coast; and this would, no doubt, contain that portion where the streams were actively aggrading their beds. In the case of the Waimakariri, however, this portion has only been submerged, not actively eroded, owing to the protection afforded by the volcanic mass of Banks Peninsula and its submarine easterly and north-easterly extension. Soundings marked on charts show this extension, and also show that the depth increases very gradually form the mouth of the Waimakariri for some distance out into Pegasus Bay. The coast-line here is not marked by any cliff such as occurs on that part of the Ninety-mile Beach near the mouth of the Ashburton River and on the coast near Oamaru. In this place erosion of the coast-line by the action of the waves is extremely rapid, and threatens serious loss in the near future unless adequate protection is given.
An elevation of even 600 ft. would have considerable effect on the climate of the country. In the first place, it would largely increase the extent of country above the snow-line, and hence cause a great extension of the glaciers. The present terminal face of the Tasman Glacier is 2,460 ft. above sea-level; in increased height of the land of, say, 600 ft. would bring it down nearly to the upper end of Lake Pukaki, which is 1,550 ft. above sea -level—that is, supposing the glacier would reach the same distance above sea-level in time of high land as of low land. This supposition may not be strictly accurate, as it is quite possible that the glacier would come down further owing to the increased accumulations of snow; but even if not, the effect of the elevation would still be very marked.
The effect of high land is easily seen on comparing the size of the glaciers at the head of the Waimakariri and Rakaia with those near Mount cook. Even allowing for the increased average height of the peaks in the last-named locality, the glaciers are of enormously greater importance and come down to a much lower level. The height of the terminal face of the Tasman Glacier is 2,460 ft., while that of the Lyell Glacier at the head of the Rakaia is 3,568 ft., and that of the Waimakariri 4,162 ft. above sea-level.
It is possible, therefore, that, owing to increased snowfall
due to elevation and to larger collecting-grounds, the proved elevation of 600 ft. would cause a marked glacier extension; it might even cause a glacier epoch. Captain Hutton has previously explained the advance of the glaciers as due to increased height of the land, and pointed out, form biological evidence, that there could have been no marked refrigeration of the climate.
Another effect of elevation of the land would be to cause desert or steppe conditions to prevail on the plains at the foot of the mountains. Owing to their increased height, the mountains would intercept more of the moisture brought by prevailing westerly winds from the Tasman Sea, which, owing to its depth, probably existed under the same conditions then as now. The mountains would then intercept more moisture, and cause it to fall as snow on the higher levels. Their eastern slopes near Mount Hutt and Mount Torlesse receive their chief rainfall from the west; but, when the higher central ranges cut off this moisture, the eastern slopes would receive much less of this westerly rain. Further, owing to the coast-line being so far to the east of its present position, there would be on the plains a smaller rainfall from the prevailing winds which at present supply the coastal lands. Even at the present time the plains of Canterbury experience a modified steppe climate. The average rainfall for Hokitika is about 119 in. per year, while at Lincoln, near the eastern border of the Canterbury Plains, it is only 25in., and in one year it fell as low as 14 in. The average annual rainfall for Christchurch is only 23 in. These steppe conditions would be intensified during a period of elevation, and the climate would resemble that of Patagonia or Thibet as it is at present.
The present steppe conditions are marked by the great number of xerophilous plant as which are found in Canterbury and Otago, and there are indications from their life history that the desert conditions were at one time more severe.
In his admirable paper on the “Plant Geography of the Waimakariri,” Dr. Cockayne draws special attention to the present climatic conditions of the country, and emphasizes the existence of steppe conditions. When speaking of the eastern climatic plant-region he says, “The Œcological condition of this region is essentially xerophilous. This is not to be wondered at when the small rainfall and constant drying winds in conjunction with the usually stony soil is considered.” In this same paper, in giving expression to an opinion of Diels, the great Œcological and systematic botanist, he further says, “Diels was much struck with the extreme xerophilous character of many plants, which he considered out of all proportion to any severity of climate they have now to endure. At the present
time the driest regions of New Zealand are less arid and possess a more equable climate than Middle Europe, so he considered Carmichaelia, Hymenanthera, Corokia, and some others to be descendants of a forest flora which had been forced to retreat northwards during a rising of the land, which led to the formation of a dry easterly steppe region, where survivors of the forest had become modified and assumed the structure and physiognomy of desert plants.” If this opinion of Diels is correct, I think the conditions are easily explained by an increased height of the mountains modifying the climate. However, Dr. Cockayne shows in his paper that the present conditions are severe enough to account for the plant modification.*
[Footnote] * Dr. Cockayne has told me privately that he has latterly modified his opinion somewhat, and now thinks that present conditions are hardly severe enough to account for the xerophilous plant forms.—R. S.