Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations
(C) The Formation of the Canterbury Plains
(C) The Formation of the Canterbury Plains
This chapter would be incomplete were I not to offer a few notes on the formation of the Canterbury plains, about which so much has been written, and so many theories have been brought forward, that a matter of apparently so much simplicity has in course of time become quite obscured. Since my report on the formation of the Canterbury plains was published in 1864, all the levels, surveys, engineering works, together with well-sinking, have amply confirmed my views that the Canterbury plains are of fluviatile origin, that, with the exception of some morainic accumulations in the upper portion and the drift sands round Banks' Peninsula, and the partial lacustrine deposits filling the former extension of Lake Ellesmere, the whole of the plains were formed by the deposits of huge rivers issuing from the frontal end of gigantic glaciers. Mr. J. T. Thomson, the Surveyor-General, in a paper on "The Glacial Action and Terrace Formation of South New Zealand," published in Vol. VI of "The Transactions of the New Zealand Institute," gives a diagram in illustration of the fan theory on page 329, and alludes to the sluice page break page 397deposits, such as they are formed at the present day on flats like Gabriel's Gully and Weatherstone, conforming in every respect on a small scale with the deposits formed at the termination of the gigantic glaciers during the Great Ice period. Professor Hutton in his various writings has been contending that the Canterbury plains must be of submarine origin, and that afterwards they have been raised above the sea level, when the rivers excavated their present channels, as terraces could only be formed during a rising of the land, and he brings forward a formidable array of scientific authorities in support. However, similar terraces, as I have pointed out previously, occur in all Alpine regions, in the European Alps, the Himalayas, the Rocky Mountains, and many other localities, where in several instances there is overwhelming evidence towards proving a subsidence of the lands since post-pliocene times. I should not have alluded to this question did not Professor Hutton in his last work, "Geology of Otago,"* still persist in claiming for the Canterbury plains a submarine origin, although he adopts for the former province the theory that the land during the Great Glacier period stood at a higher level than it possesses at present. As according to him, river terraces prove elevation, he fails to explain how the beautiful and perfect series of terraces in the Clutha valley could have been formed during subsidence, and it is thus difficult to reconcile his statement, that the plains in Otago "have certainly been formed in the way suggested by Dr. Haast." I wish here once more to repeat that the Canterbury plains on either side of the Malvern Hills are not on the same level; on the contrary, the difference is very considerable, and as the railway and other levels have shown, the fans of each river are quite distinct. The sections of the Canterbury plains attached to this Report, Nos. 1 and 2, on plate 8, drawn from more correct information than I formerly possessed, will, I trust, settle this question definitely. The diagrams in Mr. Thomson's excellent paper, previously mentioned, giving the curves of several valleys and the theoretical bearings of the question at issue, further confirm my views, that the Canterbury plains have been exclusively formed by the accumulations of the rivers still flowing through them.
* "Geology of Otago;" Hutton and Ulrich. (page 91).
- 1 "The more a river advances from its sources, the less will be the declivity of its bed." Or stated differently—"All rivers dispose their beds 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, depositing ooze only, as well as for rivers and torrents, rolling 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 Jess will be the slope of its bed." Or, stated in other words—"The slope of the bed of a river will diminish in the same proportion in which the body of water is increased."
These two natural laws will be quite sufficient to explain all the principal phenomena which our rivers exhibit, the more so as none of them south of Banks' Peninsula have 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, its bed having been raised gradually by embankments to such an altitude that it runs in many localities above the level of the plains on both sides.
We must conclude that the Canterbury plains are formed by the outlets of enormous glaciers, large torrents bringing down with them the morainic 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 post-pliocene 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 at observing, if we take 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 page 399more so when we consider their volume was so greatly reduced. It is thus now easily understood that, for instance, the post-pliocene fan of the river Ashburton was so enormous, and that as the then glaciers, all assisting 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 the mountains began to become gradually eaten into by the action of the descending ice-masses, sharper ridges and peaks were formed, so as to lessen the extent of the surface where perpetual snow could accumulate to feed those glaciers, which consequently began to retreat. Therefore the sources of the rivers which now cross the plains were lying, in the post-pliocene epoch, much nearer to the sea, their fans were much steeper, and they continued to build them higher and higher, changing their apex continually, and at the same time forming with their lowest portion the shore line of the ocean.
All my observations show at the same time that the three great post-pliocence torrents, namely, the Ashburton, Rakaia, and Rangitata were united without doubt in one large water-course before they reached the sea, having a common fan at their mouth, following the law that rivers which unite, endeavour to do so by the shortest line. This fact is well illustrated near the present mouth of the river Hinds. This river flowing between the fans of the Rakaia and Ashburton above the point where they united into one, has been forming a swamp of considerable extent, being four to five miles broad and eight miles long. It has partly been drained by natural water-courses, partly by artificial drains under process of formation, all passing through the higher shingle beds in front. The united existence of such a fan is also instructively shown at the coast, where the sea has worn away the protruding arc between the southern bank of the Rakaia and the northern bank of the Rangitata. These cliffs, consisting of true river shingle and river sand, are at the south side of the Rakaia about six feet high, and rise gradually to an altitude of 70 feet near the mouth of the Wakanui Creek, four miles north of the Ashburton river, where consequently the united fan has suffered the greatest destruction. Having reached this altitude the coast cliffs page 400towards the south diminish in height, so that near the mouth of the Ashburton they only rise to 18 feet.* A few miles north of the Ashburton we reach again the line, where the sea-coast meets the level of the fan under consideration.
I have already pointed out that there has been an oscillation of about twenty feet, the upward movement occurring towards the end of the Great Glacier period. This rising of the country was of course of great importance for the low ground forming in front of the shingle fans. Thus with the rising of the country this large lagoon became partly filled either by the shingle of the rivers falling into it, (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, drift sands 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 favourable 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 slight oscillations taking place, through which the rivers changed their channels, and either removed the clays deposited in the lagoon, or buried below their newly-forming shingle-beds the peat swamps, vegetable soil, and even forests, of which well-sinking in the neighbourhood of Christchurch has offered many illustrations. In this way those swamps were either covered or partly drained, whilst others, not so favourably situated, continue to this day to remain in the same condition, as, for instance, portions of the Rangiora swamp. Many of the raised beaches are easily traceable, and with them the former western banks of the great Lake Ellesmere extension. This accounts also for many peculiarities we meet in the centre of this ancient, partly drained and filled lagoon, which, without such an explanation, would be unaccountable.
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 now traversing the Canterbury plains have page 402lessened their fall, although still having the character of true torrents, descending in their course at a more uniform but lesser rate than the post-pliocene 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 reach a point where they intersect the line, below which the old fan falls more considerably, and thus the present rivers, instead of excavating, begin to fill up, raising their beds above the older deposits and forming new fans. The present rivers repeat simply on a smaller scale the action of the large post-pliocene torrents forming 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. Consequently it has been found by actual survey, that the contours taken at an equal distance from the beginning, or perhaps better stated, from the emergence from the older beds, 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. It was, therefore, of the greatest value to the district to ascertain not only the exact spot where the recent accumulations begin to 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. The fact, however, that only the Waimakariri, and in a minor degree the Rangitata on its southern and the Rakaia on its northern banks, for a small extent 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 post-pliocene deposits described previously. Here some rich 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.
In the following four tables the data collected during a number of years, as to the fall of the plains, and the rivers which formerly built them up, are given. They are taken from my Report on the formation of the Canterbury plains, to which a number of details obtained since and also some corrections have been added.
|Name Of Rivers.||——||Difference between two Stations.||Length of Plains.||Fall of Plains Per Mile.|
|Rangitata||From beginning of gorge to mouth mean||1443||32||45|
|From beginning of gorge, 1443 feet, to railway crossing 292 feet||1151||23¼||49½|
|*From railway crossing 292 feet to sea||292||8¾||33⅓|
|Ashburton||From Two Brothers to sea mean||1500||35½||42¼|
|From Two Brothers 1500 feet, to railway crossing 305 feet||1195||25||48|
|*From railway crossing 305 feet to sea (50 feet above sea level)||255||10½||29|
|*Plains between Rakaia and Ashburton (railway crossing), altitude above sea 412 feet, cliffs 60 feet||352||12½||28|
|Rakaia||*From gorge to sea mean||1480||37½||39½|
|*From gorge 1480 feet, to railway crossing 378 feet||1102||21½||51¼|
|*From railway crossing 378 feet to sea||378||16||23½|
|Waimakariri||From upper gorge to sea mean||1580||44||36|
|From junction of Kowhai 1410 feet, to lagoon at gorge hill 1182 feet||228||5||41½|
|From lagoon 1182 feet, to the so-called 18th mile peg 355 feet||827||18¾||44|
|*From 18th mile peg 355 feet, to last raised beach near North-road 33 feet||322||13¼||24¼|
|*From last raised beach 33 feet to sea||33||4||8|
|Selwyn||From entrance into plains to Lake Ellesmere||790||29||27¼|
|From entrance into plains 790 feet, to railway crossing 214 feet||576||16½||35|
|*From railway crossing to Lake Ellesmere||214||12½||18|
* The numbers to which an asterisk has been prefixed are results obtained with the spirit-level, by the Public Works Department.
|Name of Rivers.||——||Difference between two Stations.||Length of Rivers.||Fall of Rivers Per Mile.|
|Rangitata||Whole course from the Clyde Glacier main source 3762 feet to sea mean||3762||75||50|
|From Clyde Glacier 3762 feet, to junction with McCoy 3269 feet||493||3||164⅓|
|From junction of McCoy 3269 feet, to junction of River Havelock 2192 feet||1077||11||98|
|From junction of Havelock with Clyde 2192 feet, to beginning of Canterbury Plains 1180 feet||1012||29||35|
|From beginning of Canterbury plains to railway crossing 257 feet||923||23¼||39¾|
|*From railway crossing 257 feet to sea||257||8¾||29|
|Ashburton||Whole course from Ashburton Glacier 4823 feet to sea mean||4823||67||72|
|From Asliburton Glacier 4823 feet to Camp Creek 3717 feet||1106||4||276½|
|From Camp Creek 3717 feet, to junction with Clearwater Creek 1832 feet||1885||16||117¾|
|From junction of Clearwater Creek to beginning of plains, near Two Brothers 1400 feet||432||11½||37½|
|From Two Brothers 1400 feet, to railway crossing 300 feet||1100||25||44|
|*From railway crossing 300 feet to sea||300||10½||28¾||
|Rakaia||Whole course from Ramsay Glacier 3354 feet to sea mean||3354||85||39½|
|From Eamsay Glacier to junction of Cameron 2034 feet||1320||13||101½|
|From junction of Cameron 2034 feet to junction of Mathias 1688 feet||346||6||57⅔|
|From junction of Mathias 1688 feet, to junction of Wilberforce 1357 feet||331||9½||35|
|From junction of Wilberforce 1357 feet, to Gorge Island 875 feet||482||19||25½|
|*From Gorge Island 875 feet, to railway crossing 372 feet||503||21½||23½|
|*From railway crossing 372 feet to sea||372||16||23¼|
|Waimnakariri||Whole course from Waimakariri Glacier 4162 feet to sea mean||4162||93||44¾|
|From Waimakariri Glacier to junction of two main source branches, [unclear: 2607] feet||1555||5||311page 405|
|Waimakariri||From junction of two main source branches to junction of Crow River 2273 feet,||334||4||83½|
|*From junction of Crow River to junction of Bealey River 2065 feet||208||5||41¾|
|From junction of Bealey to junction of Esk River 1562 feet||503||21||24|
|From junction of Esk River to junction of Kowhai River 1003 feet*||559||17||33|
|*From junction of Kowhai River to White's old Accommodation House in river bed 605 feet||398||15||26¼|
|From White's old Accommodation House 605 feet, to tidal boundary||605||22||27½|
|From near the crossing of the North-road to sea||4|
|Selwyn||From entrance into plains to Lake Ellesmere mean||767||29||26½|
|From entrance into plains 767 feet, to railway crossing 212 feet||555||16½||33½|
|*From railway crossing 212 feet, to Lake Ellesmere||212||12½||18|
Table III.—Showing the Fall of the River Waitaki.
For comparison I may offer the principal data in my possession of the main branch of that River, and after its junction with the other branches of the Waitaki to its mouth.
|Tasman||†From the Tasman Glacier 2456 feet, to Lake Pukaki, 1717 feet||739||25||30|
|From outlet of Lake Pukaki to mouth of Waitaki||1717||82||21|
|Waitaki||If Lake Pukaki should be filled up by the River Tasman, the River Waitaki would show the following results||2456||117||21|
|The beginning of the old post-pliocene fan of the River Pukaki, the main branch of the Waitaki, is situate 150 feet above that river at the lake, which shows that the former outlet of that enormous glacier would have had only a fall to the sea of||1967||82||23|
† As for that river, the last three miles before it enters the Lake is almost without any fall, it flowing through swampy deltaic ground, I have calculated two miles less for its course than it actually has, so as to give a more correct result of its gradient.
In order to prove that the larger the river, the greater its power of scouring its channel, and consequently of lowering its bed, the following data will not be superfluous. The rivers follow according to their volume.
|From, sources to sea||21||39½||44¾||50||72|
|From beginning of plains to sea||23||23½||28||37||39¾|
The following data will show the difference in the fall of the post-pliocene rivers, from, the beginning of the plains at the foot of the mountains to the sea, for the calculation of which the surface of the Canterbury Plains offers us the necessary data:—
|Rakaia, 39½.||Waimakariri, 36.||Rangitata, 45.||Ashburton, 42½.|