CHAPTER I. ON THE CAUSES WHICH HAVE LED TO THE FORMATION OF THE CANTERBURY PLAINS.
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REPORT
ON THE
FORMATION OF THE CANTERBURY PLAINS.
CHAPTER I.
ON THE CAUSES WHICH HAVE LED TO THE FORMATION OF THE CANTERBURY PLAINS.
IN a former Report, of October, 1862, I stated my opinion that the Canterbury Plains, in their higher eastern part, belonged to the boulder formation, or so-called glacial period, and that they had been deposited below the sea, the material having principally been derived from icebergs, carrying on their surface enormous loads of debris or moraine accumulations, which, stranding, would be deposited on the bottom of the sea in either roughly stratified or irregular masses, consisting of sand, loam, and boulders, partly angular and partly rounded, without any order; that over them deltaic accumulations of large glacier rivers had been thrown down in fan-shaped masses, forming a thick capping of well stratified shingle, sand and silt, sloping slightly towards the sea, and also that near the sea, the present rivers, instead of excavating their deposits, were raising their beds above the lower plains, thus shifting continually their channels on the apex of their fan-shaped subserial deltas. To the first of these conclusions I came by finding, some miles below the gorge of the Rangitata, a beautiful vertical section in the banks of that river, consisting of different beds of boulders, for the greater part rounded but sometimes angular, interstratified with sand and loam, exactly resembling the boulder clay of Europe. These beds, of small extent only, are generally quite horizontal, but are sometimes irregularly disturbed as if tilted up by the stranding of an iceberg.
Although I had met with true moraine deposits higher up the river, showing that formerly the glaciers were of much greater dimensions than at present, I shrank from the conclusion that these accumulations could have been derived from such an enormous extension of the present glaciers, reaching in pleistocene time so far down as to have had their termination on the Canterbury Plains. Such a conclusion would have shown that in that epoch glaciers existed in this island of a length of fifty miles.
The Tasman Glacier, the main source of the River Tasman, of a length of eighteen miles and a breadth of nearly two miles at its terminal
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face, was, within the last year, considered the largest glacier in any temperate region, surpassing in magnitude by far those of the Himalayas and European Alps hitherto explored. Since then Captain Godwin Austin, of the Indian Survey, has explored some glaciers in Thibet, of a length of thirty-six miles, and conclusively shown that they extended formerly as far as one hundred miles down the valley, thus proving that even now those physical and meteorological conditions exist for the accumlation of neve's 1 of such vast extent, as to offer sufficient material for glaciers of such enormous dimensions.
As I shall show in the sequel, I have since had an opportunity to trace moraine accumulations belonging to the last epoch of extension, when the glaciers of the Southern Alps had already shrunk to smaller dimensions, as low down as 1300 feet above the level of the sea and below the gorge of the Rakaia, curving across the Canterbury Plains from near Rockwood at the Horurata to the eastern base of Mount Hutt, being the terminal moraine of the great Rakaia Valley Glacier, and consisting of large angular blocks derived from all parts of the central chain near the sources of that river. These moraine accumulations have been greatly obliterated by changes in the course of the Rakaia, so that at many localities they have disappeared below fluviatile deposits; but, nevertheless, they are so clear that they are easily traceable.
Since my examination in 1861, I have not re-visited the locality in the Rangitata, but have no doubt that it will offer by further and more detailed investigation still more convincing proofs that since the tertiary deposits have accumulated on the eastern side of our ranges great oscillations have taken place.
A further reason for adopting such a theory was, that I met with apparently horizontal terraces and lines high in the mountain ranges, some of them 5200 feet above the level of the sea, together with erratic blocks to the same altitude, whilst other newer deposits, derived from lateral moraines and descending the valleys, had partly obliterated them. The latter I interpreted correctly, whilst the former--as on the other side of the valley the mountains were often not so high as to suggest the explanation that large inland lakes could have existed--I mistook for old sea beaches. This, with some other reasons, made me adopt the conclusion that the Southern Island had been submerged in the pleistocene epoch 5000 to 6000 feet; that the climate had changed by some physical causes and assumed an antarctic character; that the remaining high land had been covered with perpetual snow, from which glaciers descended to the sea, which, being carried away, their load of debris (formerly moraine accumulations) became deposited on the sea bottom; that the country rising again, the glaciers receded, till at last their terminal face rested
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on the land, and that the upheaval still continuing, the great glaciers depositing their enormous loads of morainal matter in such a manner, having obtained a comparatively long stability, that, by still further retreating, their frontal and terminal moraines formed the banks of alpine lakes.
As similar phenomena have been shown to be the cause of the extensive drift formation which we meet everywhere in the northern hemisphere--in Northern Germany and Russia, in Great Britain and Norway, as well as in North America--where vast tracts of low lands to the 41st degree of latitude are not only strewn over with large blocks of rock, but where in many localities the drift formation deposited by icebergs on the bottom of the sea is of great thickness, it was natural that similar conclusions were easily suggested to me, the more so when we remember that even now, not only in Terra Del Fuego, but at the west coast of Patagonia, in latitude 46 deg. 50 min., according to Darwin, enormous glaciers reach the sea, their terminal end as it advances being washed away and carried into the sea in the form of icebergs, often covered with enormous blocks of rock. This latitude corresponds nearly with that of Invercargill.
Does this not suggest the conjecture, that slight changes in oceanic or atmospheric currents, as in the configuration of the land, will cause very great difference in the temperature, and affect it to such an extent as to change entirely its climate, and with it its fauna and flora? As my researches were directed more towards the older formations, these conclusions were adopted as the most natural, at least at first sight. But as the formation of valleys and plains, the shape of the mountains, in fact, the configuration and orographical features of the whole Southern Island are intimately connected with the glacial, or as perhaps better named, with the pleistocene epoch, and more detailed examination was necessary than the former hasty one for the thorough understanding of the phenomena in connection with it, with which the observer is struck, as soon as on ascending the plains he approaches the outrunning spurs of the Southern Alps. There is perhaps nowhere in the well-known regions of the earth so easy of access, and in such comparatively low positions above the level of the sea, such clear and fresh signs of the existence of enormous glaciers during the pleistocene epoch, than in this part of the Southern Island of New Zealand.
On the plains, or close to them, large moraines are met with striking across valleys several miles broad, and so fresh that the blocks of which they are composed seem as if only lately detached from their original position. Combined with them, the straight courses of the broad valleys, the rounded outlines of the lower mountains, terraces, and many other phenomena of an equally striking character, thrust the
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conviction upon the observer that the same causes which we see at present at work in the arctic and antarctic zones, and in a lesser degree in the Himalayas, the New Zealand and European Alps were in the pleistocene epoch repeatedly in operation to give to the country, which we have selected for our second home, its present configuration. Questions of the highest geological interest are involved in the explanation of those phenomena; and of late men of the highest intellect, geologists, natural philosophers, and astronomers, have attempted to explain the occurrence of a glacial period and to account for its remains, met in all parts of the world within the temperate zones.
When writing my first Report, alluding to those questions, many difficulties suggested themselves to me, which I hoped further investigation would clear up, and as I wish that also the unscientific reader may follow my deductions, I shall point out the two principal ones which then presented themselves to me. These difficulties since then have shown that the theory was not tenable, even had I not found new facts which, combined with those revealed by the re-examination of those localities which first induced me to the theory of submergence, showed clearly that there is only one other explanation possible.
If, as the level terrace lines pointed out, the country had been submerged for several thousand feet, Banks' Peninsula which would, even admitting the possible occurrence of a very great unequal ratio of subsidence between east and west, have been entirely, or, at least, for the greater part below the level of the sea, and in consequence would have offered a very favourable locality for the stranding of icebergs and the deposition of their detritus loads; but, after the most minute examination, I was not able to find the least sign of any boulder or pebble of another than local origin; and although clear and conclusive evidence is offered that in the quarternary, or even recent era, a small subsidence of Banks' Peninsula, and a subsequent emergence has taken place, the details of which I shall offer in the course of this Report; even at its foot no boulders or rocks derived from the western chains are found. Such an occurrence would have been unavoidable at the western slopes of the volcanic system in question, which naturally would have been most exposed to the stranding of icebergs.
A further examination of the dolerite plateau on the southern extremity of the plains, and on which Timaru is situated, offers, if the submergence theory had been correct, still greater resistance for its adoption. This dolerite plateau is covered for many feet with silt or loam, in which, at several localities in the cliffs near the sea, I found the remains of shells still inhabiting our littoral zones, which naturally indicates that in the post tertiary or recent times a rise has taken
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place. Higher up in the cliffs, formed by some creeks which have cut through the sheets of dolerite, I was not able to find any shells.
In ascending this plateau we come almost directly upon gullies about one hundred feet deep and of a most picturesque character; the sheets of dolerite form here escarpments, which allow a descent to the valley in very few places only. Above them lay deposited about thirty feet of loam or loess, but no boulders of any kind amongst them. From an examination of that part of the country, it is evident that these thick layers of loam have now the character of having either accumulated in a shallow sea, to which no rivers of any size had access; or, that they are at least partly derived from the decompositions of the volcanic rock itself through atmospheric influence.
There is no doubt that the loam deposits of Banks' Peninsula, found as high as one thousand feet above the level of the sea, are generally slope deposits, owing their origin to the decomposition of the volcanic rocks, either in situ or washed down from higher parts. I met with several very instructive sections, showing the very slight gradation from the hard and undecomposed volcanic rock into the yellow loam without being able to find the line where the first one ended and the latter began.
Now, it is evident from the physical features of the country, had this island during the pleistocene epoch been submerged to such an extent as the level lines in the Alps seemed to indicate, and the formation of the icebergs being favored by the causes above stated, that besides Banks' Peninsula no better locality for the deposition of detritus from icebergs could be found than on the dolerites of Timaru, which cap so conspicuously and for such a large extent, tertiary beds in the southern part of this province.
These observations, made in December, 1862, convinced me that the former theory was untenable; and thus, in my further journeys, I examined eagerly all localities which seemed to throw more light on that important question. Finally, during this year, when my investigations in the character of certain carboniferous strata brought me again into the same regions, where the level lines occurred, I devoted some time for their re-examination, using this time a spirit-level to be certain of the value of the results obtained. At the same time I examined minutely the nature of the deposits, which had led me to the former conclusions.
Beginning in the Ribbonwood Range, I found, in ascending, that the eight lower terraces which seemed to be horizontal, when using only the clinometer as done formerly, were simply enormous moraines rising one above the other to an altitude of 2000 feet above the terminal
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moraine which surrounds Lake Heron, showing that they belong to a former extension of the glaciation far down the valley of the Ashburton and having been here protected by a projecting spur of the Ribbonwood Range.
The enormous glacier, of which the lateral moraines are the deposits, came down the Cameron as far as Lake Heron, where it joined another large glacier descending from the head of the Whitcombe, the main stream of the Rakaia. This united glacier continued to the junction of the Valley of the Ashburton, where a third large glacier was welded to it, emerging from its narrow gorge and mingling its lateral moraines with those of the main glacier. This accounts for all these moraines having nearly a level appearance, as they fall very slightly towards the Ashburton Valley, whilst where the Ashburton moraines join and mix with them they are in their general outline nearly level, and rise afterwards slightly towards the gorge of the latter. The spirit level showed this at once, besides which the character of the blocks of rock which show here and there above the general outline of these deposits pointed to their moraine origin.
In following them in different directions, both up the Cameron and the Ashburton Rivers, they very soon begin to have a more considerable fall; and to the eye, unassisted by any instrument, their fall and nature, even when standing in the valley, is there quite clear. I made this highest line 4180 feet above the sea level.
These uppermost moraine deposits lie against the range, leaving a small valley-like depression between, just as the lateral moraines of our present glaciers lie against the side of the mountains, leaving space for small rills of water.
Ascending the range still higher, there occur two more ledges, confined to a smaller horizontal extent, now covered with debris fallen from above, but, as it seemed, cut into the rock. They had also a very slight inclination down the valley, as a proof that they were neither sea nor lake beaches, but either old river channels, or perhaps cut, during a still greater glaciation of New Zealand, into the rock. It is evident that during that greatest glaciation enormous ice masses filled all the valleys; not confined only to the present main valleys, but that every available depression was invaded by them.
So, for instance, the great Rangitata Glacier sent its superfluous masses into the Ashburton Plains, whilst a branch of the great Rakaia Glacier came by the Cameron Valley across the Lake Heron country to join the same. Again, this trunk glacier sent down branches by three
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different valleys, namely the Ashburton, Trinity, and Puddingstone Valleys, covering, in fact, the whole country with a network of ice streams, over and through which rose the harder rocks which had successfully resisted their eroding power.
They had necessarily a very slight fall, being so extended that they were unable to find sufficient outlets for their united masses down the existing valleys, which had not yet been eroded to such an extent as they are at present.
It is not necessary to give a picture of the desolate aspect of the country in those pleistocene times; but when reading the descriptions of Dr. Kane, of Greenland, and of other arctic or antarctic explorers, it brought vividly before my mind that this Island during that era would have presented a very similar appearance. Or, may we perhaps not more appropriately compare it to the inner Thibetan Glacier regions, as they at present exist?
There remained some other localities which, from the level terraces or beaches occurring there, gave to them such striking features that the attention of every visitor of that part of the country was directed towards them--I mean the so called downs between Forest and Butler's Creeks, on the Rangitata, where the bed of the river widens considerably, showing at a first glance that there once existed a lake basin.
Also here, when examining at first the geological structure of that country, I was struck by the appearance of level lines, which I could only explain by adopting the theory that they were raised sea beaches. This year I devoted several days to their re-examination, of which the results were very conclusive, and of which I shall give only a resume. After ascending a great succession of newer pleistocene lateral moraines, which rise to an altitude of 2860 feet above the level of the sea, or 1185 feet above the river, near Forest Creek we pass, in order to reach the higher lines, across a swampy tract of country, in which we meet numerous lagoons, the whole having the character of an ancient river-bed running along a lateral moraine, similar to the torrent-beds existing sometimes along our present glaciers.
Having crossed this ancient river-bed, we ascend again. The terraces become steeper, which, although looking quite horizontal and smooth from the valley below, exhibit, nevertheless, everywhere large angular blocks, which clearly indicate that they are also the remnants of glacier deposits, but somewhat modified by the action of running water, which was at work here for a short period. These terraces have still a slight fall; but at 1620 feet above the river, or 3095 feet above the sea, I met with one terrace perfectly level, even when examined by aid of the
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spirit-level, which proved by its configuration and other characteristic features that it had been the shore of a lake, which, although now covered by grass and obliterated here and there by boulders thrown down upon it from the higher terraces and by small watercourses, showed, nevertheless, distinctly all the characteristic beach features.
Ascending still to a higher level, new moraine accumulations were met with, but also somewhat disturbed by running water. Here the uppermost one but two of the whole series (more than twenty in number), 3464 feet above the level of the sea, and about 1980 feet above the valley, had all the peculiar features of a portion of an ancient river-bed, having a fall of about 50 feet to the mile, but sloping towards its existing western banks, which it had undermined, whilst at one spot it had branched and left an island in its centre: 140 feet above it, the highest terrace is reached, of rather an indistinct character, having a slight slope down the valley, from which roches moutonnees, like hillocks, rose in several spots.
These observations indicate clearly that many causes of a very complex nature were at work to produce the deposition of such various formations which were protected by favorable circumstances from being entirely swept away.
And in order to show how many oscillations took place in such a comparatively short epoch, as it bears directly upon the formation of the Canterbury Plains, I shall sum up the various changes which have here succeeded each other within the pleistocene epoch:--
1. An enormous glacier filled the whole valley of the Rangitata, crossing probably the saddle between the plains and Coal Creek, and of which some signs are met with below the gorge, and during the existence of which the valley was not yet so deeply excavated.
2. The glacier retreating so far as to have its terminal face above the junction of the Butler with the Rangitata, its outlet displaced or re-assorted partly the lateral moraines of more ancient origin, either undermining them, as the rivers do now, or forming over them a bed for its outlet.
3. The glacier still retreating laid its bed deeper, as shown by the lower terminal moraine lines.
4. Through changes or oscillations in the existing level of the country, a lake was formed, of which the shores are still visible and traceable for more than half a mile.
5. The lake was filled up and again scooped out, as several, but not clearly defined, watercourses show. Many of the large moraine
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boulders have their edges rounded, whilst others are quite angular, as if just detached from rocks in situ.
6. The scooping action continuing for a long period, the glacier at last again advanced, forming the lowest moraines, which are separated from the older ones by a belt of swampy ground, the bed of an ancient river.
Thus these loose accumulations, which have a breadth of nearly three miles, being protected by a projecting spur of Black Peak, combined with some other favorable circumstances, were not destroyed when newer changes took place.
Does this not show clearly that a detailed examination of the geological structure of a country can very often, and principally in the beginning, alone protect us from erroneous conclusions; and that, very often, only by repeated investigations, the true causes of great physical changes can be brought to light?
There is, in consequence, no doubt that these accumulations are of glacier and fluviatile origin, and that the level lines were not ancient sea beaches, but of lacustrine origin, giving additional reason to doubt the submergence theory.
I may mention two other localities, where horizontal layers of silt exist in lateral valleys--the first in a small branch of the Tasman River, rising to an altitude of 3500 feet above the sea, which I at once recognised as being deposited in an ancient glacier lake; the second one, which I re-examined this year, occurs also in a lateral valley of the Forest Creek, and its origin and mode of deposition are beautifully shown. A glacier advancing down the main valley of Forest Creek, the waters, collecting in a lateral valley, were stowed up by the ice; the latter valley was, in the course of time, filled up by the silt derived either from the glacier itself, or, what is more probable, from the sides of the mountains, brought down by streamlets, depositing their silt on the bottom as well as on the steep sides of the mountains around it.
The other phenomenon difficult of explanation, if not adopting the theory of the emergence or rising of the land, was the formation of terraces along the river beds in the upper part of the Canterbury Plains by the scooping action of their waters.
This difficulty, however, is easily removed by demonstrating, as I shall do in the sequel, that only an elevation of the lands, going on slowly or intermittent, could not have produced such a result; but had this been the case, or should it be the case in the future, the rivers would
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raise their beds, which they were scooping out during the sinking of the country.
We have not to lose sight of the fact that, besides the formation of successive terraces in river valleys through elevation of the land, two other causes may be alluded to, which will produce such a result, namely, the retreating of the sources of rivers, and secondly, the permanent diminution of their waters through changes near or at their sources.
I shall demonstrate in the sequel that both these latter conditions were existing since the pleistocene periods, and had alone the effect of lowering the rivers, thus giving rise to the deep gorges and fine-terraced valleys between the ranges and at the beginning of the Canterbury Plains at the eastern foot of the Southern Alps.
I may be pardoned that I have devoted so much space to the preliminary explanation before entering into the main subject of this Report; but I thought it desirable, wishing to give also the necessary information for the general public, to show on which corallae my former deductions were based, and the reasons which led me to abandon them and to adopt a different theory, which, I think, will stand the test of the most careful scrutiny.
Causes of the Pleistocene Glaciation of New Zealand.
During the tertiary period, the Southern Island of New Zealand was repeatedly submerged, and extensive strata of calcareous, tufaceous, and argillaceous sandstones, greensands, marls, limestones, and shales, with beds of lignite, were deposited. The country emerging again, the physical feature was a high mountain chain, plateau-like, but with depressions existing before the tertiary submergence, but now partly obliterated, running generally either on the junction of two formations, on the lines of faults, or on the break of bold anticlinal folds.
As soon as the country had risen so high as to reach the line of perpetual snow, the accumulation of neves began, which were the more considerable as glaciers and large rivers had not yet begun their task of ridge-making in contra-distinction to the action of waves and currents of the sea on submerged lands, which tends to wear off all eminences, filling the submarine valleys with the debris.
The configuration of the area now forming the Canterbury Plains would have been a broad arm or channel of the sea running along cliffs of tertiary rocks from Timaru to Double Corner, and surrounding Banks' Peninsula as an island. The waters derived from atmospheric sources had already begun during the emergence of the land to open an outlet for themselves from the higher regions by clearing the natural water
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courses, but had only in a minor degree attacked the tertiary strata, filling the valleys in favorable localities as high as 4000 feet above the level of the sea. The neves, considering the insular and peculiar position of New Zealand, its principal range or backbone running from S. W. to N. E., thus lying at a right angle to the two prevailing air currents, the equatorial north-west and the polar south-east, both bringing moisture with them, would soon have attained an enormous extent, and would have considerably lowered the line of perpetual snow, even had the land not been raised to a higher elevation than at present. The consequence would have been that glaciers of much larger extent would have descended down the natural outlets, grinding down the rugosities of bottom and sides.
The action of the glaciers, beginning to lay open the rocks of the higher ranges, would soon offer sufficient material for moraine accumulation, first on the glaciers themselves and afterwards at their terminal faces.
The scooping action of the ice having once begun to eat into the plateau-like range, not only in the main course of the glaciers, but also in the lateral valleys, becoming more extended from day to day, would furnish more and more material for the formation of huge moraines.
Let us now consider what may have been the action of the waters during the emergence of the island upon the region over which at present the Canterbury Plains extend.
In the first instance the waves of the sea would have acted upon the tertiary strata, undermining and destroying them, till the debris of the falling cliffs would have formed a protecting wall at their foot, although frequent oscillations and changes in the ratio of elevation or subsidence may have occasioned many local diversities.
The tertiary beds, risen above the level of the sea, would soon have been eroded by the action of the streamlets or torrents, descending from the higher regions, and growing larger with the continuance of the upheaval and becoming more numerous, bringing down with them gravel and sand, more effectually increasing their eroding power.
At the same time the glaciers, descending from the enormous snow-fields, covering the large plateau-like ranges, began to fill all the existing valleys to the plains, and even advanced beyond, spreading in the plains in a fan-like shape.
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Of this occurrence, however, we have very little proofs, if it be not that the older glacial deposits in the bed of the Rangitata, several miles below the gorge, and some others rising above the plains between the Malvern Hills and the Waimakariri belong to that period.
High on the ranges, near the plains, the proofs of such a still greater glaciation of the island are visible in the ice worn sides; even the ranges themselves, in their summits and configuration, bear distinctly marks that their very form is attributable to such an almost universal glaciation. Moreover, it is evident, by judging from the study of our present glaciers, that these enormous neve and ice masses covering with an uniform sheet the higher regions of the whole island, would not offer much material for the formation of the plains, till the glaciers had begun their task of ridge-making, which took place principally in the second epoch of the glaciation of this island.
The glaciers had then so far retreated that they were only confined to the principal valleys, and of these such clear signs are found as soon as we enter the valleys between the ranges leading from the Canterbury Plains into the Southern Alps proper, that their power and its effects are unmistakeable. And as now these glaciers, as shown above, brought a much greater amount of debris from the disintegration, destruction and weathering of the mountains with them, they did not form only enormous moraines at their sides and their terminal end, but the torrents issuing from them carried down a great amount of material in the form of boulders, shingle, sand, and glacier mud--the latter being derived from the triturating effect of the ice on the sides and bottom of the channel of the glaciers. These torrents now began to raise rapidly their beds, and filled, by the continuous shifting of their regular and flood channels, all the inequalities of the surface of the lower regions, exposed by the emergence of the sea. From that moment the formation of the Canterbury Plains began.
Moreover, a momentum of high importance must not be lost sight of, namely that all the rivers which traverse the Canterbury Plains have their sources in formations which consist principally of shingle and sandmaking rocks, thus affording all the necessary elements for effecting this purpose.
Ascending the Canterbury Plains we can easily trace their continuation up the valleys, the angle of the dip of the loose deposits rising the more we approach the terminal moraines of those pleistocene glaciers. Amongst those which form our plains, I have traced all the principal ones, and I shall give in the sequel the length and the position of their terminal face.
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It will be seen from that list that at least two of those glaciers (not yet having examined the upper valley of the Waimakariri) were so large that they sent side branches into the valley of the Ashburton, so as to form in that valley one of the largest ice streams of the whole system, which thus gives the key to some peculiarities of the Canterbury Plains, and which, without the knowledge of this important fact, would be very difficult to explain.
Length and Position of the Pleistocene Glaciers which formed the Canterbury Plains.
Great Rakaia Glacier, ending four miles below the gorge of the Rakaia, spreading in a fan-shaped form across the Canterbury Plains from the Hororata to Mount Hutt. Part of the terminal moraine still existing: First period--Length, fifty-two miles.
Rakaia Glacier, branch down the Valley of the Cameron, uniting with the glaciers from the Cameron and Ashburton Valleys, as well as with the branch of the Great Rangitata Glacier, descending the Valley of the Ashburton as far as the junction of the Stour. Terminal face destroyed: First period.--Length, forty-eight miles.
Rakaia Glacier, latest extension, uniting with the Cameron Glacier, the terminal moraine forming Lake Heron. Perfectly preserved: Second period.--Length, forty-one miles.
Rakaia Glacier, latest extension, branch by Valley of River Stour: Second period.--Length, forty-two miles. Terminal moraine still perfectly preserved, and forming a little lake behind Clent Hill Station. Thus the outlet of this glacier reached the sea by the Valley of the Stour, and by that of the Ashburton, the watercourse from Lake Heron being still perfectly delineated, whilst the present outlet of that lake, uniting with the River Cameron, has now a reverse flow towards the Rakaia.
Cameron Glacier, uniting with the Rakaia Glacier, forming Lake Heron. Perfect terminal moraine: Second period.--Length, nineteen miles.
Ashburton Glacier, descending as far as the Valley of the Stour, uniting with the South Rakaia Glacier: First period.--Length, twenty-five miles.
Ashburton Glacier, reaching to the Upper Ashburton Plains, and striking across the valley from Trinity Hill to Mount Rowley or Clent Hills. Its moraine is for the greater part destroyed: Second period.-- Length, eighteen miles.
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Great Rangitata Glacier, down the main valley to the beginning of the gorge. Terminal moraine destroyed: First period.--Length, forty-two miles.
Great Rangitata Glacier, branch uniting with the glacier descending Potts' Stream, and afterwards with the Ashburton and South Rakaia Glaciers: First period.--Length, forty-one miles.
Great Rangitata Glacier, branch uniting only with the Potts' Stream Glacier, and throwing moraines across Puddingstone Valley and uniting with the Ashburton Glaciers across Trinity Valley and across the Upper Ashburton from Trinity Hill to Ribbonwood Range: Second period.--Length, thirty-four miles. This glacier retreating formed Lakes Acland, Tripp, and Howard, and had three outlets, one by Puddingstone Valley to the Rangitata, another by the Trinity Valley to the Ashburton, and a third and principal one, uniting with the outlet of the Lake Heron Glacier, followed the present Ashburton Valley. The terminal moraines across Puddingstone and Trinity Valleys, and across the large valley between the Ashburton and Rangitata, in which the above-mentioned lakes lie, are beautifully defined, curving across in succession one within the other. Also here the lateral moraines, and the terrace-like steps cut into the rocks by the ice, are well defined, and in such an almost endless succession that those on Mount Harper have appropriately been called by the shepherds "Jacob's Ladder," while those on Mount Potts go by the more colonial expression of the "Devil's Staircase." The deposits which form the Canterbury Plains begin properly at the foot of those moraines; and although the erosion of the rivers has in a great degree destroyed them in many instances, nevertheless it is possible to follow them all along the valleys to the very terminal moraines.
I observed before, that since the glaciers had begun their ridgemaking operations, the amount of debris brought down by them had been enormous. The same action is still going on at present; so that the ratio of debris brought down by the larger glaciers is infinitely greater than of the smaller ones, even taking their relative size into account.
But, before proceeding with the physical description of the plains, it will be necessary to give some of the general laws which regulate the course and fall of rivers and the formation of their beds, as without this knowledge it will be impossible to understand the action of our own rivers in past and present times.
The Italians were also in this, as in many other sciences, the first people who by the nature of their country were obliged, as far back as the beginning of the middle age, in order to protect their magnificent towns
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and villages, to study the nature of their watercourses, that they might define the laws by which their apparent aberrations are regulated; and men like Guglielmi, Manfredi, Frisi, and many others, obtained such eminence as engineers for hydraulic undertakings, that the engineers of all nations have studied with great advantage their works and writings, which offer a great source of information on all subjects on hydrostatics even to the present day.
The following are some of the general laws which, as far as I am aware, have been first propounded by these Italian natural philosophers and engineers.
1. "The more a river advances from its sources the less will he the declivity of its bed." Or stated differently--"All rivers dispose their bottoms in the lower course on a less slope than they had in the upper, so that the declivity diminishes in proportion to the distance they have to run from their sources." This law holds good for slowly flowing rivers which deposit only loess, as well as for rivers and torrents, which roll boulders, gravel, and shingle, because the largest material is gradually left behind.
2. "The greater the ordinary body of water is in a river, the less will be the slope of its bed." Or, stated in other words--"The slope of the bottom of a river will diminish in the same proportion in which the body of water is increased."
These two natural laws will he quite sufficient to explain all the principal phenomena which our rivers exhibit, the more so as none of them south of Banks' Peninsula has formed a delta properly speaking.
The only river possessing such complete features is the Waimakariri, which in its lower course shows great resemblance with the lower part of the Po, in the plains of Lombardy, which for many centuries has caused anxiety for the safety of the surrounding country.
The bed of this latter river has been raised gradually by embankments, and to such an altitude that it runs at present on the top of a high mound, and it has, to quote an instance in Ferrara, its surface more elevated than the roofs of the houses.
I stated before, that the rivers forming the Canterbury Plains had, properly speaking, no deltas; and as such an opinion, to many of my readers, will seem to he incorrect, I may add that the term "delta" in geology is confined to the alluvial deposits of a river at its mouth, falling either into the sea or into a lake; but thinking that giving such a name to the alluvial accumulations of the rivers in this island, showing some peculiarities, would impart an erroneous impression, Dr. Hector and
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myself, in drawing up a synopsis of the geological formations of New Zealand, have adopted for the formation of those subaerial accumulations the expression "Fan," for those of regular watercourses; and of "Half-cone," for those of intermittent mountain torrents, and we shall for the future use these two expressions.
In consequence the Canterbury Plains are formed by the outlets of enormous glaciers, large torrents bringing down with them the moraine matter, thrown in their course at the terminal face, raising their beds and shifting their channels at the same time so as to form fan-shaped fluviatile accumulations, consisting of shingle, gravel, sand, and glacier mud. In applying the preceding rules, we shall find that as the sources of the pleistocene torrents were at the terminal face of the glaciers, they lay much further to the east than the present ones, and at the same time, those glaciers being on such a gigantic scale, the torrents issuing from them must naturally have been so much the larger. In consequence we ought not to feel any astonishment to observe, taking into account both circumstances of the retreat of the glaciers combined with their diminution, that the rivers cut new courses into the older deposits in recent times, the more so when we consider their volume was so greatly reduced. It is thus now easily understood that, for instance, the pleistocene fan of the River Ashburton was so enormous, and that as the then glaciers, which all assisted in the formation of these large deposits shrunk back to their proper valleys, the outlet from the one remaining Ashburton glacier does not stand in any relation with its former fan. Consequently it has not been able to lower its channel to any extent, whilst the Rakaia, which is still the outlet of considerable glaciers, and has retreated much more than the former, has been able to cut its channel much deeper and to prepare a much more uniform gradient for its bed.
As I shall show in the second chapter of this Report, we have plenty of opportunity to study all gradations in the effect of the "halfcone," forming intermittent mountain torrents, descending into the plains from the mountains bordering them, and throwing new debris over them to the united outlets of the large glaciers forming the Rakaia, and which has chiefly an excavating power.
From the physical features of the country it is evident that, for a long time, the pleistocene glaciers were comparatively stable, from which we may conclude that an upheaval of the land still continued, but in a slower degree. As already the mountains began to become more and more eaten into by the action of the descending ice-masses, sharp ridges and peaks were more and more formed, so as to lessen the extent of the localities or basins in which perpetual snow could accumulate to feed those glaciers. But soon the glaciers began retreating, without doubt a slow sinking of
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the land, combined with the ridge-making action, being the principal causes.
No sufficient data are in my possession to state with any degree of approximate correctness to what extent those oscillations of level have taken place, although the caves of Banks' Peninsula and certain cliffs seem to point to the conclusion, that the lowest limit of the last oscillation was between 20 and 25 feet below the present level, and that since that time the country is again slowly rising. I speak here of the general oscillation of the eastern side of the Canterbury Plains, whilst a further momentum is to be taken into consideration, namely, that local disturbances and oscillations may have taken place in Banks' Peninsula (besides other localities), which, for the immediate neighbourhood, are of great importance, and of which I shall have to speak when treating of the River Waimakariri.
I think I have sufficiently shown that the sources of the rivers, which now cross the plains, were lying, in the pleistocene epoch, much nearer to the sea; that, consequently, their fans were much steeper; that they continued to build them higher and higher, changing their apex continually, and at the same time that the latter reached into the sea. Moreover we ought not to overlook the fact, full of suggestion, that as the land was afterwards again rising, the boulders, shingle, and sand, brought down by the glacier torrents, were probably able to repair the damage done by waves and currents upon the loose masses, forming the shore of the pleistocene seas, and that as the land emerged, probably with some paroxysmal movements, these fans became longer and broader. All my observations show at the same time, that the three great pleistocene torrents, namely the Ashburton, Rakaia, and Rangitata, were united without doubt in one large watercourse before they reached the sea, having a common fan at their mouth, following the law that rivers which unite endeavour always to do so by the shortest line.
We see thus the pleistocene Rakaia deflect immediately below the gorge southwards, whilst the pleistocene Rangitata flows in an easterly direction from its gorge, to unite on the great fan of the pleistocene Ashburton; without doubt, as shewn previously, the largest of the whole series, and, as it seems to me, also the highest of them all. The existence of such a fan is instructively shown at the coast, where the sea, during the sinking of the land, has worn away the protruding arc between the southern bank of the Rakaia and the northern bank of the Rangitata; whilst the fact that the sea does not reach within ten feet or more from those cliffs at the mouth of the Rakaia, which are now protected by a talus of shingle and sand, formed by the southerly swell, is the best proof that the land is again rising. These cliffs, consisting of
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true river-shingle, river-sand, and loess, are, at the south side of the mouth of the Rakaia, six feet high, and rise gradually to an altitude of sixty feet near the mouth of the Ashburton, where the fan has consequently been most destroyed. They fall again towards the mouth of the Rangitata, thus reaching again the line where the sea coast is intersecting the old pleistocene fan. To understand fully the combined action of the agencies at work, it will be necessary to remember that the largest fan was necessarily formed by the most extensive glacier, and that in consequence the fan of the pleistocene Waimakariri could not reach so far into the sea, belonging to that era, as the united Ashburton one, built up by the southern rivers. In adopting the present data before us as the basis of our reasoning, we shall find that, as before observed, the pleistocene southern fan began near the mouth of the Rangitata, advanced into the sea till it reached a breadth of several miles near the mouth of the Ashburton, and crossed again near the mouth of the Rakaia, the whole now being washed away.
Continuing the same arc of this fan, we shall reach the Selwyn several miles above its junction with Lake Ellesmere, where the fan of the Waimakariri joined it, and, in consequence, the Selwyn is flowing upon the junction of both fans.
During the greatest depression of the Island in the post tertiary era, there was doubtless a narrow arm of the sea which ran along the western foot of the Peninsula, and of which we have ample evidences in raised beaches near it.
The oceanic swell south of Banks' Peninsula, travelling towards the north, assisted by the action of the waves, very soon began to disintegrate the pleistocene accumulations. The shingle and sand derived from that destruction, travelling northwards, were augmented considerably by the material of the same nature brought down by the rivers, and, assisted by the two prevailing winds, finally formed a dam from the mouth of the Rakaia to Banks' Peninsula, becoming every year more considerable.
As the shingle is arrested at that volcanic system, it is at the same time evident that this bar, or shingle spit, not being able to advance further, will augment principally there in height and breadth. A peninsula was soon originated from that isolated system, rising so conspicuously in the sea, the connecting isthmus being formed by that shingle spit, behind which an arm of the sea formed a bay, moderately deep and comparatively sheltered, with an entrance from the north, into which the Selwyn, Waimakariri, and probably, at one time, the Rakaia fell.
The land rising still more, and, perhaps, with a tendency to do so
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more immediately north of Banks' Peninsula, occasioned by local causes, the shingle and sand brought down by the northern rivers, the Waipara and Ashley, it travelling with a northerly swell towards the south, threw, in their turn, a bar across the opening of the bay, thus forming a large lagoon, of which Lake Ellesmere is only a small remnant.
Of this occurrence we have sufficient evidence in the immediate neighbourhood of Christchurch and Kaiapoi. And still, up to the present time, the topographical features of the ground, which rises close to the western foot of Banks' Peninsula only about twenty-six feet in its highest neck above the sea, having on both sides low swampy ground, would give additional evidence to such a theory, were not other proofs sufficient.
With the rising of the country this large lagoon became partly filled, either by the rivers which fell into it, and of which the Waimakariri was the most important, in throwing its fan nearly across it, or assisted by the silt which the rivers had in suspension, and which was thrown down all over the lagoon in the form of loam and clay.
Moreover, driftsands invaded it from its eastern shore, coming from the north, giving additional breadth and stability to the banks which separated it from the Pacific Ocean. At favorable localities, just rising to or near the surface of the water, vegetation began to spring up, forming swamps, which, with the assistance of bog-mosses (Sphagnum), and other aquatic or semi-aquatic plants forming peat, raised considerably the ground, so as to form a suitable locality for the beginning of forest vegetation. Many were the oscillations which took place, through which the rivers changing their channels removed cither the clays deposited by the lagoon, or they buried below their newly formed shingle beds the peat swamps, vegetable soil, and even forests, of which well-sinking in the neighborhood of Christchurch has offered many illustrations.
The upward movement still continuing, those swamps were partly drained, whilst others not so favorably situated continued to this day to remain in that condition--as, for instance, the Rangiora Swamp.
As I shall show in the sequel, these old raised beaches are easily traceable, and with them their former western banks towards the great Lake Ellesmere extension.
This accounts also for many peculiarities we meet in the centre of this ancient partly drained and filled lagoon, and which, without such an explanation, would be unaccountable. But another important consideration among the causes which are now at work, to re-form the lower part of the Canterbury Plains, has not to be overlooked, namely, the
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rising of the country after the last depression. Such an occurrence would have this result, that the snow-fields would he augmented, with them the glaciers, which, consequently, would descend farther down the valleys, as actual observation has shown to be the case. Torrents which issue from them would at the same time be of larger size. All these phenomena would combine, according to well-established laws, to raise the beds of the rivers, by giving them the power to bring boulders and shingle of larger size down with them from their sources than formerly.
The consequence would be that the intersecting line, where the recent fan will begin to spread over the older pleistocene fan, would be brought higher up, and to give at the same time sufficient power to the rivers to extend their present fans, consisting of shingle, lower down the rivers, over deltaic or lacrustine formations, as among our rivers the Waimakariri alone can do.
It has been before stated, that the declivity of a river diminishes in proportion to the distance it has to run from its source, and it is thus natural that the rivers which now traverse the Canterbury Plains have lessened their course, although still having the character of true torrents, descending in their course at a more uniform but lesser rate than the pleistocene fans of the former larger rivers.
They have, in consequence, according to their present size and position of sources, cut more or less deeply into these deposits, but will reach a point where they will intersect the line, below which the old fan will fall more considerably, and thus the present rivers, instead of excavating, will begin to fill up, raising their beds above the older deposits and forming new fans, as shown in the accompanying sketch map of the plains.
The present rivers repeat simply on a smaller scale the action of the large pleistocene torrents which formed the upper plains; they also debouch from a gorge, although the sides of it consist only of shingle, and spread fan-like over the lower grounds, the axis of the course of the river changing so as to build it up regularly, and it will be found that the contours taken at an equal distance from the beginning, or perhaps better stated, from the emergence from the older beds, will have an equal gradient and altitude. Thus, as before said, the higher this point lies above the junction of the river with the sea, the larger will be the radius over which the river can roam and raise its bed, and as a consequence it will be of the greatest value to the Province to ascertain not only the exact spot where the recent accumulations begin to be spread over the older ones, but also to find, by careful measurements, what is the ratio of the present rise above the level of the sea, as well as above a given spot of the plains of older origin.
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The fact, however, that only the Waimakariri, and in a minor degree the Rangitata and the Rakaia, on their southern and northern banks, for a small extent only are subject to such an occurrence, is worth recording, and shows distinctly that the two last-mentioned rivers flow on the sides of the large pleistocene deposits described previously; but it is nevertheless true that here the richest alluvial land is situated, and thus those small strips, endangered by the aberrations of these rivers, are of far greater importance than a look at the map at first might suggest.
These questions, however, belonging to another part of this Report, I shall leave their consideration at present, and proceed at once to the second chapter.
