CHAPTER III. NOTES ON THE DIFFERENT RIVERS BY WHICH THE CANTERBURY PLAINS HAVE BEEN FORMED.
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CHAPTER III.
NOTES ON THE DIFFERENT RIVERS BY WHICH THE CANTERBURY PLAINS HAVE BEEN FORMED.
IN the first chapter, on the physical geography of that region, I have described the general features only, leaving, to avoid repetition, a more detailed account till when treating of each river separately, and I think that at the same time a few geological notes may not be superfluous. With the exception of the Waimakariri, I have seen every river, at least from the point where it issues from the ranges, and although they have some resemblance in their general course, the natural features of each are totally different, which, as I shall show, we may attribute not only to the action of glaciers according to their former magnitude, but also to the nature of the rocks through which they pass.
For instance, whilst the Rangitata enters the plains by a deep rocky gorge, a feature which also characterises the Rakaia, it is distinguished from the latter river by the steep ranges which rise on both sides of the gorge before it enters the plains, and which are almost impossible even for an experienced mountaineer to cross.
The Rakaia shows, on the contrary, a deep rocky channel, but like the canons of the Colarado, it occurs on flat ground, the river having cut through the lacustrine deposits, and afterwards through the rocks, showing very clear sections of both.
Again, the valley of the Ashburton, at its opening on the plain, has been widened by the same agencies as the Rakaia, but did not afterwards possess the same eroding power as the former, and thus forms a wide valley, the shingle of the pleistocene beds being only slightly excavated. As the rivers south of the Rangitata belong only to those of the second class, the natural features of which are not different from others of the same class, I shall at once proceed to give some notes from my journals on the
Rangitata.
In ascending the plains, and arriving near the gorge of the Rangitata, their character shows at once that they are of fluviatile origin. One can distinguish the uppermost terrace at the foot of the range where it curves round, leaving the gorge, and can follow it down as it shows how the river ran in an easterly direction towards the Gawler's Downs.
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A beautiful illustration of the power the pleistocene rivers possessed of raising their channels is offered by observing, at the entrance of the Southern Hinds into the plains, a large triangular flat now lying beyond the course of the river behind a mountain spur. This flat lies not only lower than the former courses of both rivers, but is still a swamp, having formed a part of the flood plain of the pleistocene Rangitata.
Another sign of the eroding power of water, even in smaller ranges, is here seen, and also in a still greater degree in the higher Mount Hutt ranges, namely, the existence of enormous half-cones, often several hundred feet high and a few thousand in circumference, all derived from the waste of the ranges through meteorological action since the pleistocene fans were formed.
If a mountain can thus be excavated by a small streamlet often flowing only after rain, so as to form such deep cories, destroying as it were the greater body of the mountain in such a short period, and leaving only a shell on the sides with sharp ridges, what may be the power of enormous ice masses or of the torrents issuing from them?
As the hall-cones lie on the former river bed quite undisturbed, it is clear that they and the cories from which the material has been derived were only formed since the pleistocene torrents left their channels.
About half a mile from the present river bed in the gorge the first terrace is reached, having an altitude of 70 feet.
It is partly covered with huge blocks, some of them several feet in diameter, but having their edges somewhat worn off and rounded, as from the action of running water. Immediately below it we meet, as is usual in all fluviatile terraces, the old river channel, as the motion of the current tends to throw the channels on one or the other side, undermining the banks and making it broader. After descending two more terraces of 10 and 15 feet we reach the last, which falls at an angle of about 50 degrees or even more for 110 feet. Thence perpendicular banks of rocks descend for 60 feet more, before the waters of the river, here confined in a narrow channel, are met with, the whole gorge being about 260 feet high, and presenting a most wild and majestic sight
The upper surface of the last terrace of 110 feet before the rocky channel is reached is covered by enormous blocks of rocks, some of them six to eight feet in diameter, but their edges are also rounded, just as we observe them near the terminal face of the present glaciers, showing the enormous power the waters have of moving blocks many tons in weight.
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At first sight it would appear that these blocks form, for the greater part the banks, were it not that close by, where the river makes a sharp curve, it has excavated in the banks a nearly perpendicular cliff, about 150 feet high, exposing clearly the structure of these alluvial deposits.
On the surface, and for a short distance down, the larger boulders predominate, becoming, however, gradually smaller and smaller, till they are only the size of common river shingle, and interstratified with thin layers of sand.
The lines of the terraces rising towards the upper Rangitata Plain are clearly visible, where, among the steep rocky banks, a favourable locality was offered for their formation and preservation. In examining the nature of the rocks through which the river has scooped its gorge, we find the strata consisting of old palaeozoic sedimentary rocks, mostly dioritic sandstones with shales and slates, the latter of which exhibit some of a bright chocolate colour--the change of colour and material being very decidedly marked. All these strata strike across the channel of the river, which must therefore be considered to have been cut by the outlet issuing from a glacier, and afterwards, when it retreated, from a lake, the outlet of which continued to erode this gorge deeper and deeper.
And may not those large boulders, which we meet often several miles below the gorge on the plains, have been brought down by the river after a debacle, which occurs sometimes in Alpine countries, by changes in the size or position of glaciers by which water is stowed up, and afterwards suddenly released?
About half-a-mile below the entrance of the river into the plains the palaeozoic strata disappear below the alluvial accumulations, without tertiary strata being visible.
That those tertiary strata exist in the neighbourhood is shown by some circular funnel-shaped holes in the plains (which occur frequently in limestone countries), although the sides are covered with shingle.
In a former part of this Report I stated that the greater part of the plains would consist underneath of tertiary strata. Their general dip being, towards the east, will diminish as they recede from the volcanic ranges, which they partly overlie, so that they may in many spots have been only slightly covered by the pleistocene fluviatile fans.
As soon as we cross the Hinds and reach trachytic rocks, the tertiaries, mostly here argillaceous or tufaceous limestones, make their
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appearance. They first occur on the line where the two fans of the Rangitata and Ashburton would have met, but that of the latter, having been greatly destroyed by numerous rivulets which descend from the ranges between the sources of the Southern and Northern Hinds, the junction is not clear. The isolated volcanic system, called Gawler's Downs, was another obstacle to their complete union.
In following down the course of the river, new terraces are continually formed, which, as they open more and more out, disappear entirely on the plains.
With the exception of the glacial beds four miles below the gorge, to which I alluded in the first part of this Report, all the accumulations have the same fluviatile character. There is every evidence that the river, on the plains, is flowing during its whole course on the side of the large pleistocene fan. On its southern declivity near the sea it has been destroyed by the river, whilst higher up the plains the altitude of the terraces on both sides show that there is a great difference between the northern and southern banks, the latter, of course, being more depressed. The river, finding lower ground on its southern side, has, as soon as it reached the point where it would begin to form a new fan, abandoned the task of cutting into its northern banks, having a tendency towards the south, where the rivers of smaller size did not, in pleistocene times, form accumulations of any size. As in the old moraine deposits higher up the Alps, beautiful sections of which are laid open in several localities by the action of the present rivers, the shingle of the pleistocene fans shows an equal change from the natural bluish tint to one of a rusty dark yellowish colour which coats the whole deposit. I have found in several rivers that the change from the one colour into the other is often rapid, the dark yellow shingle underneath being divided from the bluish at the summit by a very distinct line, sometimes with a small bed of loam between them, over which, at numerous spots, small watercourses are dripping down.
Another reason for the widening of the valleys and the retreating of the banks by secondary causes, is to be sought in the loosening of the shingle by the formation of ice between it when water percolates through it till it is arrested, as before described, by beds of a more impervious matter. In a country where for several months in the year so many cold nights occur that ice is formed, which is melted again by the higher temperature of the day, a change takes place which is of great importance.
Passing, after a cold night, along the nearly perpendicular banks of many rivers, I was struck by this effect. Everywhere, as soon as the
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ice began to melt, a shower of shingle fell down, forming at some spots quite a talus, which, in due course of time, would, by changes in the current of the river, be swept away.
From the tables annexed at the end of the second chapter, it will be seen that the fall of the Rangitata is still on the average 36 feet, which fact, on being compared with the fall of the Rakaia--the latter having had also to cut a new channel through the barrier of rocks at its entrance into the plains--is another proof of the existence of the natural laws quoted before, by which the action of running water is regulated.
I have not yet visited the mouth of the river, but, from information I have obtained, there is no doubt that on the northern side the old pleistocene fan continues to the sea, and like the mouth of the Rakaia, which I shall afterwards describe more fully, the southerly swell forces the water here also towards the north and forms a lagoon.
River Ashburton.
This river has, in a minor degree, the same characteristics as the other principal rivers of the plains, but in its present size it was not able to excavate in any degree the pleistocene fluviatile formation now filling its broad valley.
Only when it set towards its northern bank are terraces of any size formed, which, in entering the plains, gradually disappear.
In some slips of these terraces a distinct line of demarcation is visible between the shingle and gravel of different colors, blue shingle in its natural color lying above, with ferruginous beds of the same material below.
The broad valley of the river, which presents quite a different appearance to any of the other valleys, is bounded on both sides by tertiary rocks, tufaceous limestones and sandstones with a calcareous cement, as well as flaggy limestones of fine quality.
In several localities near the river funnel-shaped cavities occur, partly obliterated by shingle, which may be considered as conclusive evidence that these tertiary (miocene?) rocks, striking across, have not entirely been denuded.
A very interesting fact, and full of suggestive matter, is exhibited in the Ashburton. This river, having first excavated its bed in its upper course, shows a tendency in its middle course to raise it; whilst again,
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before entering the sea, it has cut a deep channel into the plains, which, near its mouth, according to Messrs. C. Davie and E. Jollie, rise about 50 feet above the mouth of the river.
The plains, therefore, from the railway section to the sea, deducting the altitudes of the cliffs from the total sum, fall only 24 feet per mile, whilst the Rangitata shows that they fall 33 1/3, consequently about 11 feet per mile more.
As the sides of a fluviatile fan are also steeper than it is on its central apex, this may be considered as a supplementary evidence that the axis of the united pleistocene fan was situated near the mouth of the Ashburton. Moreover, it is consistent with the laws alluded to, that unlike the other rivers, instead of raising its bed before entering the sea, the original pleistocene deposits having here such a greatly diminished fall, it has the power of excavating it according to the volume of its water and the distance from its source.
Rakaia.
The next river to be considered is the Rakaia, at present the principal river of our plains, draining, as a look at the map will show, a very large tract of country.
I have not yet visited the sources of the main branches, but judging from the volume of water in the Whitcombe, at the junction of the Cameron, the glaciers at its source cannot be much inferior in size to those which form the Godley River, the affluent of Lake Tekapo, coming from the same central system.
The river, before it enters the plains, crosses a gorge, cut first through old tertiary lacustrine strata, with seams of brown coal, and afterwards through quartzose trachytes, striking here across, as the southern continuation of Snowy Peak.
In a former chapter I have already alluded to the fact that the remnant of a former moraine curves across the plains a few miles below the gorge from which, in consequence, during pleistocene times, the glacier torrent issued, and by which that part of the plains has been formed.
Let us now examine the nature of the deposits above these curving moraines, and judge whether they offer any evidence to such an hypothesis, and to do so it will be necessary to consider what would take place if the glacier, after having been stationary here for some time, were to retreat.
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First, the tongue-like projection of the glacier would disappear, and the waters, liberated by its destruction, would assist in melting the glacier washed by it much quicker than otherwise would have been the case. Consequently, the lake thus formed would become larger the more the glacier, from climatological causes or oscillations in the level of the country, retreated--assisted materially by the first-named agency.
The glacier being at last separated from the lake by morainal accumulations, the river would fall into the latter, just as the present rivers do into our alpine lakes, forming with the shingle brought down, a delta at its upper end, whilst the glacial mud would be deposited at those localities where the waters were least disturbed.
The delta advancing, would, in course of time, fill up the lake, forming a shingle plain, over which the river would form its course, till it would be so gorged, that it would excavate a new channel through hard rock.
Now, all these phenomena clearly exist above and near the gorge of the Rakaia. On the rocks of the gorge, scooped out or rounded by the pleistocene glaciers, we meet with horizontally stratified sands or silt; or on the sides where the rocks have an incline, these loose beds also dip often at a very considerable angle. Higher up, the lacustrine strata filled the former large lake in the usual manner.
At other localities old river beds are visible above those lacustrine strata; but, being so raised by shingle as to impede its further flow, the river had to erode a new and deeper channel through the rocky barrier. As water has only power when in motion, a river tends to cut more easily a new channel through the hardest rocks than to remove shingle which has been dammed up in its previous channel by former changes in its fall, because, whilst in one case it would add shingle to that already deposited, it is easier for it to cut by erosion through any barrier, as experience has shown in gold-fields and elsewhere, as, for instance, the Falls of Niagara, which illustrate clearly the point in view.
The greater part of the rock basin left by the retreat of the Great Rakaia Glacier was thus filled by the shingle brought down by its affluent, with the exception of the smaller Lake Coleridge basin, which, partly protected by the mountain ridge on its southern side, and partly by changes of altitude in the bed of the River Harper, was left a remnant of the former large lake. The outlet of this lake has also a reverse flow, namely, in an opposite direction to the slope of the terraces.
The occurrence of old river channels above the lacustrine deposits,
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which necessarily enveloped or obliterated the outer moraine, may be considered another cause that it is not so well defined as around the other lakes of our Alps, which have not yet been entirely invaded by the encroachment of the shingle and gravel deltas. The commencement of the terraces in the gorge is very clear, and offers every proof that they are formed by the action of running water, their slope diminishing in ratio as their starting-point is nearer to the present level of the river.
Five miles below the gorge in the banks of the river on its northern side a tertiary outlier, consisting of calcareous sandstones, greensands, (pepperstones), and marls, is exposed, full of characteristic fossils of our middle (miocene?) tertiary series, and known to the colonists as the Curiosity Shop. These beds, which are exposed for only a short space, rise to an altitude of about 60 feet in the lowest terrace about 180 feet high, and well deserve this name, as they are literally stuck full of fossils, of which teeth of Lanina and different species of sea urchins (Echinoderms), as well as of Terebratula, Waldheimia, Pecten, Scalaria, Cucullaea, together with cetacean bones, are the most numerous, showing at the same time that the tertiary strata, found at Mount Somers, in the Malvern Hills, will, in many localities in the plains, underlie the pleistocene fluviatile accumulations.
Descending the course of the Rakaia, the deposits of the older fan, having more than twice the fall of the present watercourse, soon disappear below the now forming deposits, according to the natural laws previously explained; but it may not be deemed superfluous, if I point out the desirability of fixing, at the Rakaia, the exact spot where the change between the excavation of the former, and the formation of the present fan begins, although secular as well as paroxysmal movements, and an unequal ratio of elevation or subsidence, between east and west, or north and south, may produce repeated alterations.
This Island is now rising, as is shown amongst other indications to be treated of afterwards, by the steady advance of our glaciers. The point of change will move towards the gorge, therefore the new fan of the Rakaia and of the other rivers will become larger.
I have not the requisite knowledge, nor do I wish to treat of purely engineering questions, although geology, in its practical bearings, will give great assistance in solving many difficult engineering problems, but as the advancement of the present fan, like that of the Waimakariri, would occasion great losses to our community, I think that it would be very important to ascertain, by careful levels, assisted by geological examination, where this change is taking place, as well as the amount of rise, during a given time, produced by the river on this new fan.
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The Rakaia, below the ferry, shows very conclusively that it flows here also on the sides of the large united pleistocene fan. Not only do its southern banks soon begin to rise above the level of the river, but its northern banks are invaded to such a degree that in all probability it will, at some short time, again find or force its way into Lake Ellesmere, or reach the sea nearer to it.
Mr. Edwin Fereday's 1 homestead, situated ten years ago about half-a-mile from the river, is now only a few hundred yards distant from it; the river, which continues here raising rapidly its bed, already at present invades the plains in some localities on their northern side in heavy freshes. In following the northern bank towards the sea, three miles from the mouth of the river, it begins to rise above it, and about a mile from the mouth of the river is 16 feet high, and consists of finely stratified silt, which has a dip towards the south of about 7 degrees. This silt, arranged in layers more or less ferruginous, fine, or sandy, is evidently a lacustrine deposit, such as will now be formed in Lake Ellesmere, although in a minor degree, no river of such a magnitude as the Rakaia entering it.
There is no doubt that these beds are part of the raised bottom of the Lake Ellesmere extension, described in the first chapter, consisting of more or less finely triturated matter, according to the pure or impure state of the water of which it was the recipient. In following these banks to the mouth of the Rakaia, we find them separated from the sea by a lagoon running parallel to the coast, the formation of which will be afterwards described.
It rises here to an altitude of about fourteen feet as a nearly perpendicular bank, its strata dipping slightly towards west, and consisting, for six to eight feet from the summit, of the same stratified silt or loess. Below it there is a layer of coarse white sand for about six inches; the rest is formed by shingle densely packed, but dipping also towards the west, or against the general fall of the plains.
Thus, if additional evidence were necessary to prove the rise of the country, this former lake bed raised above the sea would be sufficient to give it.
May we not deduce from this observation the fact that during the last subsidence of the country the extension of Lake Ellesmere reached to such a distance, and that the Rakaia also fell into it, of which even the southern banks of that river show signs.
The country rising again, these strata, offering more resistance than shingle accumulations, were not so easily destroyed by the Rakaia, while
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the newly-formed shingle spit, beginning at the southern bank of the Rakaia, protected them from the inroad of the waves of the sea. And if the rising of the country continues, or even remains stationary for some time, this lagoon running parallel to the coast will be filled up by the shingle and silt brought down by the river, through which the latter will afterwards force its way, and, assisted by the shingle and sand travelling to the north, its own sediment will build outside a new wall, behind which the river will flow, till after heavy freshes, it has forced again a new channel directly into the sea.
An examination of the sea shore at the southern bank of the Rakaia offers beautiful sections, showing the structure of the fluviatile fans which the action of the sea has laid open.
There is generally a capping of silt on the top, more or less coloured or veined by oxide of iron, below which shingle or gravel in its peculiar fluviatile form and position appears.
These cliffs are now protected by banks of beach shingle (easily distinguished by its flatness), so that the sea cannot reach and destroy them, as was the case during the period of submergence or stability.
According to the data before us, the rise may have been from twelve to eighteen feet since the change of the last oscillations took place.
I used before the expression river shingle in contradistinction to beach shingle; and, as it will be sometimes of the highest importance to distinguish between the two, I may give the principal difference. Whilst the effect of a river is to give to the boulders which lie in its bed, and have originally been angular, a sub-angular shape, by turning them, the shingle which lies within reach of the waves is washed up by the surf, which retreats so rapidly, that, remaining behind, it can only slide down, the water amongst it not being sufficient to insure for it a rolling motion. Beach shingle will in consequence be flattened. The same holds good with the shingle forming on the shores of lakes, where the prevailing winds are often so strong that a regular surf is formed, and the same process goes on as on a sea beach.
The numerous rivulets, which owe their origin to leakage in higher parts of the plains, and which are forced upwards as soon as they come upon a more impervious stratum, and which unite to form the little Rakaia, have a southward course along the shingle reach till they join the big Rakaia.
This shingle reach or spit, in its northward course, separating Lake Ellesmere from the sea, becomes more and more extensive, as is natural,
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the swell of the ocean being here towards the north. Thus, when the lake has reached its highest elevation, it always opens a channel at its southern end, where the shingle hank is not only lowest, but also where it joins the main-land. A beautiful illustration that shingle, even in a lake, travels with the swell occasioned by high winds is afforded in Lake Ohou. Here a bay, which turns suddenly towards the west, is completely cut off from the lake by a dam of shingle as regular and flat on the summit as if thrown up by the hand of man.
River Waimakariri.
In treating of this river, it is only with diffidence that I approach the subject; not because I consider that the geological evidence before me is not sufficient, but because an immediate danger for the property of many of our fellow settlers on its banks has made it a point of local discussion into which I have no wish to enter; but, notwithstanding, I consider it my duty to give all the information which geology offers us. It will show that it has a great practical bearing upon the subject, if we study the causes and their effects already before us.
Near the junction of the Kowai with the Waimakariri, we can observe that, like all the other rivers, it has formed also its fan. Here, on the surface of the pleistocene deposits, a clear river bed exists, which, running in a southerly direction, went across to the Malvern Hills, reaching them at Abner's Head, and then flowing along them, with Little Racecourse Hill as its northern side, being here traceable all the way.
On that part of the plains between the junction of the Kowai and the Gorge Hill, there is not only evidence of rocks occurring at different localities, which still crop out above the shingle, but also that before the pleistocene fan was formed older formations of a similar nature existed here, as they do below the gorge of the Rangitata, but probably not of the same age.
Little Racecourse Hill, as well as Trigpole Hill, near the junction of the Kowai with the main river, rise like islands above the shingle plains, whilst amongst other localities the same formation is met with on the eastern and northern slopes of Abner's Head.
Instead of the well-rolled shingle of the pleistocene fans, we observe that these older accumulations, to which the name drift can be properly applied, consist of blocks of large size-some as much as four feet in diameter--sometimes a little rounded at their edges, but very often perfectly angular, and of such a fissile and fragile nature (slates and shales), that only icebergs or glaciers could have brought them into such a position, because running water as moving agent would very soon have destroyed them.
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The form of Little Racecourse Hill is that of a crescent about fifteen feet high--the concave side towards Abner's Head, whilst the higher Trigpole Hill, near the junction of the Kowai with the Waimakariri, running from W. N. W. to E. S. E., with a razor back, and about 350 paces long, has such a position that a glacier descending Mount Torlesse by the Rubicon Valley would reach the plains and form its moraine in such a manner. As at the head of the Rubicon Valley roches moutonees occur, changes such as stated in the first chapter will easily account for the formation and extension of glaciers from Mount Torlesse, but such an explanation could not well be accepted for the Little Racecourse Hill, which lies so much farther from the higher ranges. It will be, therefore, better not to attempt at present to explain the causes of their formation, which proves beyond a doubt that before the pleistocene glaciation of New Zealand great oscillations of the ground had already laid the foundation upon which the present plains were formed.
If there are any points, which, by their peculiar beauty, are remarkable in our plains, the Waimakariri Gorge deserves certainly to be classed first amongst them. Here a hill rises only about a hundred feet, if so much, above the plains, consisting of very hard sandstones, gritty or dioritic, alternating frequently with shales and slates. In them I found some small fossils resembling Tentaculites of the upper Silurian epoch.
Such an age, considering the lithological character of the rocks, would well agree with their stratigraphical conditions. It will be difficult to decide if the rocks strike across from the Malvern Hills to the Gorge, so as to form a barrier, or whether the Gorge Hill stands isolated, through which the river had to cut its way, when its channel everywhere else had been choked up by shingle. The rocks at some spots are exposed for several hundred feet, and the river does not run parallel to their general strike, but cuts across them at nearly a right angle.
At the lower end of the gorge some clear sections are exposed, showing how the plains have here been formed; and I will therefore give a section in ascending order--
Feet.
Siliceous slates with tentaculites.........20
Loess, loams stratified in small lavers, coloured by oxide of iron...............60
Sands, sometimes loamy, with occasional layers of small shingle ...............25
Fluviatile deposits, stratified horizontally, or at very slight inclinations with layers of boulders amongst them...............12
Loess or loam...............10
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Thus the greater part of these accumulations were deposited in comparatively quiet water, produced by the shelter of higher rocks immediately above them.
Above the gorge no terraces of any size have been formed, whilst below it they are extensively developed, and I should have to repeat what I stated in respect to them, when treating of the Rakaia and other rivers.
These terraces fall gradually till they disappear in the plains, and at the eighteenth mile peg we reach the spot where the river leaves the channel excavated in the pleistocene deposits, and begins to form its new fan by raising its channel continually, and thus repairing, by shifting again and again, all former inequalities.
The labors of Messrs. E. Dobson, Beetham, Doyne, and several others, have shown conclusively that this is the case, and if we wanted more evidence that rivers of such a nature build up their channels, and leave them as soon as they have risen above the general level of their fan to commence the same operation at some other spot, we have only to consult the works of Italian engineers, where we shall find many instances of disasters which have taken place in populous districts, until by great and costly engineering works they have been averted.
The Waimakariri is the only river in our plains which may be said to have all the true characteristics of a river, because all the others, as they fall 23 to 35 feet in the mile, can only be called torrents, whilst the former, some distance above Kaiapoi, begins to have a much more diminished fall, its capacity of transporting shingle to lower regions ceases, and only sand and mud is carried down towards the sea.
It is natural that with such oscillations of the ground as alluded to, the alteration in the formation of the new fan, and its advancement towards the sea, must also change: and in fact I have been assured that at the beginning of the settlement, vessels or boats were able to ascend the Waimakariri much higher than they do at present, which, assuming that the country is rising, would give additional force to such an hypothesis, were it not based upon still more reliable data.
We must not lose sight of the fact that all the changes in the Waimakariri have to be calculated from the gorge of that river, and that it is quite natural that it will always tend to equalise its lower fan, taking as starting point the gorge in the plains. If there be any unequal ratio of rise or subsidence, the level of the gorge as well as that of the mouth of the river will come in different relations of altitude to each other than at present, it is evident that the gorge will be of primary importance for the changes in sequence.
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But even without changes of level, would it not he safe to assume that in due course of time a river like the Waimakariri will, according to the natural laws before explained, so equalise its fan between the sea and its upper course as to advance gradually to the sea?
From the eighteenth mile-peg, to which the river has been levelled by Mr. Beetham, we find that the mean fall from that point (860 feet) to the sea is in round numbers about 20 feet per mile. Such a fall of the river is, I think, more than sufficient to extend its shingle fan gradually to the sea, and if peculiar changes in the present configuration of the country do not take place, such an occurrence must follow as a natural consequence. Of course I always speak of geological time, leaving to the engineers to calculate the effect of actual data; and even now, if Mr. Doyne's levels are extended to the exact spot where those of Mr. Beetham ended, we may be able to judge from the difference what the aberrations of the river are likely to be in future. I think it will show that the river has raised its bed for several feet, by which, even without any change of level on the earth's surface, the fall of the water from any given point to the sea will have increased, and as a consequence per se force the power of the water to carry its shingle further.
But, before proceeding, let us examine the evidence we have before us, that, not only in post tertiary times, but also since the country has been settled, sensible changes of level have taken place, because, as I shall show in the sequel, such evidence is of the highest consequence.
That a secular rise along the East Coast of the Island occurs, is placed beyond doubt by many independent observations made by others as well as by myself, whilst it is equally proved that the West Coast is sinking, the axis of oscillation or equilibrium being situated somewhere in the central chain.
Raised sea beaches of older date occur near Double Corner, whilst, as previously described, all along the sea coast south of Banks' Peninsula the rise is also clear by the shelf of shingle, ten to sixteen feet high, now protecting the loose fluviatile deposits.
But the raised beaches north of the Peninsula, which I shall describe in the sequel, are another evidence of such an upward movement having taken place. The caves of Banks' Peninsula, the deposits of recent shells high above the present flood mark, the round holes made by boring moluscs, as pholas, &c., in the rocks, are clear enough to shew that in post tertiary times a rise of at least twenty feet has taken place, whilst there are other signs of a second more local, or, perhaps, paroxysmal rise, which, as it seems, has only affected the northern side of Banks' Peninsula.
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That this rise within a few years has been two feet is beyond doubt, as Mr. E. Dobson has shewn me on the Government Jetty at Lyttelton such clear and conclusive signs of it that it would be convincing to the greatest unbeliever. But, as such a fact is of the highest importance in a practical point of view, I consider that it would be desirable to have all the evidence on that subject not only collected, but also that additional proofs should be sought, which will, without doubt, be easily found, although in a hasty examination round Lyttelton Harbour among the cliffs some weeks ago I could not find any conclusive confirmation.
Since then, Mr. Stoddart has informed me that on the southern side, at Diamond Harbour, no such rise has taken place; and this gentleman assured me if any change were visible there, it would be certainly to prove the submergence of that part of Lyttelton Harbour. So that we have to assume that a movement along a fault, running parallel with the harbour, is taking place.
The present rivers carry an enormous mass of silt with them to the sea, which, as the conditions are wanting to form deltaic accumulations at their mouths, is carried away by the waves to be spread either over the bottom of the sea, or to be deposited by the waves in localities favorable for its reception, as, for instance, Lyttelton and Akaroa Harbours, and the other bays.
Future minute observations of the change in the thickness of the mud bottom in Lyttelton Harbour will show this clearly; and thus we may safely assume that the mud, which now for a great thickness covers the bottom of that Harbour, has not mainly, as supposed, been derived from the decomposition of volcanic rocks of the Peninsula and brought down by rivulets or washed down by rain, but that the sea itself brings the greater part of this mud with its tides and swells.
When examining last year the mud on Gebbie's Flat, in Governor's Bay, I observed in it many small scales of silvery mica, forming generally a part of crystalline or metamorphic rocks, quite different from the rubellan mica of our volcanic rocks in Banks' Peninsula, and for the existence of which I could not otherwise account.
At the same time I have been assured that the mud-flats at the head of Port Victoria have augmented considerably in length, which some persons attributed to the deposition of fresh mud, and others to a rise of the land. A careful examination of the harbour would set all these questions at rest, and a chemical analysis of the mud would at once show whether it is derived from the volcanic rocks around or from the arenaceous and argillaceous sedimentary and partly metamorphic strata of the eastern side of our central chain.
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In order not to be misunderstood, I may here add, that the accession of land in any estuary or locality near the sea shore favorable for the reception of sediment, is not due only to the elevation of the country, but also, as I have shown elsewhere, to the action of tides and currents, which bring with them a great amount of silt, which is brought by our large rivers, even in the finest and dryest seasons, to the sea.
That this sediment is enormous I have already alluded to when speaking of the Rakaia (Mr. Fereday's garden), and I hope to be able not only to ascertain in the course of my examination into the physical geography and geology of the province, the amount of water brought down by the different rivers to the sea in a given time, but also to find by actual test what is the amount of sediment which the waters have in suspension in their different states, in freshes, or when the rivers are very low and clear, as this will offer not only data of scientific interest, but also of no little practical value.
It is natural that such silt being deposited near the coast in places favorable for its reception, mud-flats will be formed which will soon rise above the low water line receiving accession with every tide. Vegetation, in the form of sedges and rushes, will begin to spread over the highest or most sheltered places, which, invaded by high water, and still later only by high spring tides, will act as a filter for the sea water invading them. Thus that ground will rise higher and higher, till it will reach at last above the rise of the highest tide and form a considerable accession to the coast without any rise of land by abyssological causes.
Of course the accession of aerial rocks (sands, and dust, blown on the higher part of the mud-flats, and among the sedges by the winds) will give further assistance to the land gaining upon the sea.
The Heathcote estuary, among others, offers a beautiful example of this operation in nature going on uninterruptedly, and the fact that after a very high tide, the sedges, &c., are coated entirely by silt proves best what an enormous amount of mud is thus left behind by the waves.
Looking at the course of the Waimakariri on a map it at once suggests the idea that it has been deflected towards the north by a local upheaval of Banks' Peninsula, unless a careful contour levelling of its fan proves to us that the formation of its northern part is not yet so far advanced as the southern.
In the first part I stated that whilst the shingle at the sea-shore was travelling from the south towards Banks' Peninsula, it did the same on its northern side from the north, and that thus a lake of large dimen-
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sions had been formed. Examining the country between the mouth of the Waimakariri and Kaiapoi, a great difference in the character of the beach is manifest. Whilst south of Banks' Peninsula the beach consists mainly of shingle, here to the north of it the sea-shore is formed by quartzose sand, derived from the decomposition of the extensive tertiary strata near the Waipara, which has been brought down the coast by the north-easterly wind and swell; so that even without a rise of the country each year new accessions will be made to the land. That the swell is north-easterly is shown by the ripple-marks running from N. W. to S. E., which, as they run at right angles to the direction of the swell, point to that quarter.
Only after some time I was enabled to find any pebbles on the beach, consisting of hard dioritic semi-crystalline sandstones, brought down by the rivers north of the Waimakariri.
Between the sea and Kaiapoi a succession of old sea beaches is passed, with swampy ground between them, till we reach the town of Kaiapoi, where we cross the last one; a road having been cut through, has exposed to view and confirms the fact that true flattened beach shingle is lying below it; thus showing clearly that when these beaches were formed, the rivers from the north, or, perhaps, the Waimakariri itself, had sufficient power to bring quantities of shingle not only to the coast, but that the currents were powerful enough to carry them along.
That these beaches were raised even within the short historical time since the Maoris inhabited the country can be safely concluded from the fact that one mile and a half from the coast, behind the third raised beach, numerous kitchen-middens are met with, the shells lying on heaps, showing by their nature that they were collected for food by the natives, who certainly would not have carried them so far across deep swampy creeks and lagoons, but would have stayed near the beach where they were collected, and where sufficient drift-wood would be available for the purpose of their preparation. The last raised beach, crossing Kaiapoi, is towards west, bounded by terrace-like lines, as if they were the shores of an ancient lake; and, in fact, when seeing the Rangiora Swamp stretching before us, standing on that spot, the former hypothesis of the large inland lake or lagoon receives a still greater confirmation.
That the mouth of the Waimakariri is first thrown towards the north, and before it enters the ocean towards the south, is the conflict of two causes: first, the local rise of the northern part of Banks' Peninsula; and, secondly, the N. E. current. Thus, if the river by the building of its fan, takes a northerly direction, the water will always afterwards be deflected towards south.
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This action will invariably take place till the river has advanced on its raised embankment, consisting of shingle to the very sea level, and may account for the fact that the Rangiora Swamp--a part of which is the ancient extension of Lake Ellesmere--has not become in a greater degree the receptacle of the shingle of the Waimakariri.
It has also been asked, why the Waimakariri, with its northern tendency, would nevertheless try to find an outlet to the sea towards the south--(Christchurch). The answer, for many reasons, is obvious, if we consider the principal forces at work, the curve of the river course itself illustrating this question very clearly. The Waimakariri first had its nearest natural course to the sea by reaching it where now Lake Ellesmere is situated, and only when the embankment thrown up became so high that it forced the waters from the slopes of this so-formed fan, it gradually went towards north, filling up the lower ground in that direction, assisted without doubt by a local rise in Banks' Peninsula.
A resume of the subject in contemplation may thus be given.
The question that affects the settlers is dependent on the causes at present in operation, and any reference to remote, i.e., geological causes, must be only used to explain them.
The causes at present in operation are as follows:--
1. Set of the swell from the north-east. This tends to deflect the mouth of the river towards the south.
2. Current and wave action, which is intermittent, and tends to reclaim land by heaping the sand during high gales and tides.
3. A gradual or intermittent (or both) elevation of the coast line, greatest towards Sumner, and tending to deflect the course of the river towards the north.
N. B. --Shingle rivers lying more towards the north have therefore their fall diminished, and might for the same reason at one time have brought down shingle, though they have now ceased to do so. This observation is thrust upon us by the last raised beach, cut through by a Kaiapoi road, which has a shingle line.
4. Gradual extension of the shingle down the river from the base of the old pleistocene fan, consequent to the natural laws stated in Chapter I.
It is evident that No. 1, 2, and 3, all tend to impede the outlet of the river, and therefore to cause it to seek new channels, whilst No. 4
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tends to fill up the upper ends of these channels in an irregular manner, that cannot be provided against, except by engineering works of great extent.
I am, therefore, of opinion, that nothing can be recommended, except the protection of the bights of the river by piles or otherwise, or by providing a new straight channel to the sea, which would probably involve us in expenses of such magnitude that we could not provide for them at present.
The only other efforts to avert the destruction of the endangered land that could be attempted is, to encourage by deepening and straightening the course that the main channel tends to take: but this also would be an expensive work, and should not be thought of until exact details have been ascertained of what the actual destruction has been and the effect and cost of the ameliorative effects.
In conclusion, I may state my belief, on geological grounds, that the danger for the lower grounds, although pending, is not so imminent as it would at first appear; that every year changes in the bed of the river may take place higher up, which may alter entirely the whole aspect, as for instance, that a heavy fresh may score out a new channel close to the old bed or straighten it, and that thus the danger may be postponed until another period.
Careful levels taken, and geological examination from time to time, will be of the highest practical value, because here the Engineer has to work with the Geologist, and the results obtained by both will, combined, have much more weight than merely facts collected and worked out by the latter or the actual surveys of the first alone.
