Report on the Geology & Gold Fields of Otago
Section V. — Historical Geology
Section V.
Historical Geology.
Having described all the facts known to me relating to the geology of the Province, it remains now to collate them, and try to deduce from them the causes to which they owe their origin, or in other words the geological history of the Province. I have kept this section distinct from the last because it stands on a differeut footing. The last section simply described my observations, which are open to all to examine and verify or correct as the case may be; while the present section is an attempt to theorise on those observations, and to build up from them a history of the geological changes that have taken place in this portion of the earth’s surface. But as these observations are necessarily very incomplete, and indeed sometimes probably erroneous, it is evident that too much stress must not be laid on the accuracy of the conclusions drawn from them. Even if all the facts were as accurately known as possible, the interpretation of them would not be so easy as might be supposed, for sometimes the facts admit of more than one explanation. Nevertheless whatever may be the difficulties in the way, we must try our best to overcome them, for not only will a knowledge of the geological history of New Zealand be of the utmost importance to general theoretical geology, but it will also throw much light on the deposition of our auriferous alluvia, and it has therefore its immediate practical, as well as its future theoretical value. Consequently it is of no use saying that we ought to wait until our knowledge is more complete before attempting to theorise, for if we did so we might wait for ever, and still not be satisfied as to the fullness of our evidence; while if we carefully endeavour to make out from the observations before us the most probable history of the geological changes in Otago, we shall at any rate be in the best possible position that our present knowledge admits of for solving accurately the practical problems presented to us.
Eozoic and Palæozoic eras.—Our knowledge of the eozoic and palæozoic rocks of Otago is too limited to enable us to speculate much about them. It is, however, evident from the facts previously mentioned (page 40), that the metamorphism of the Manipori formation has no connection with the eruption of granite at Pre-page 75servation and Chalky Inlets, and that most probably the metamorphism of the Wanaka and Kakanui formations took place during or after the deposition of the rocks of the Kaikoura formation; and, as the Kaikoura formation is not more altered where it rests on the Manipori formation than it is at other places, it follows that the more intense metamorphism of the latter formation must have occurred at an earlier period than that of the Wanaka and Kakanui formations. This shows a considerable difference in age between the Manipori and Wanaka formations, which is further proved by the abundance of dykes in the former which do not appear to penetrate any higher.
The slight unconformity that exists between the Kakanui and Kaikoura formations, shows that, at an intermediate age (perhaps during the Devonian period), an extent of low flat land existed in Otago which underwent a slight denudation. This land, the earliest of which we have any evidence in New Zealand, sank during the deposition of the Kaikoura formation (perhaps the carboniferous period), and was afterwards elevated again into dry land in the Permian period.
This last elevation is proved by the unconformity that exists between the Maitai and Kaikoura formations, which, however, is not so great in Otago as it is further north. But even in the north there is perhaps no evidence that New Zealand, during the Permian period, was a mountainous region, for there are no signs of any great local denudations having taken place, such as would be produced by mountain streams, and it is more probable that New Zealand then formed a very subordinate part of a large continent, which, judging by the similarity of the shells and plants found in the following formations with those found in Australia, India, and Europe, probably stretched away far to the northward.
Mesozoic era.—At the commencement of the triassic period, this continent began in New Zealand to subside below the sea; but from the occurrence of plant remains throughout the Maitai formation, it is probable that this subsidence was not very great, and that the southern portion of the Permian continent was not yet quite submerged, when another slight elevation, accompanied by an outburst of volcanic action, took place, which caused the continental vegetation again to advance to the southern and western portion of New Zealand. This was followed by subsidence, during which the mixed estuarine and marine Putataka formation was deposited. The granite also, erupted into the palæozoic rocks during the deposition of the upper part of the Maitai formation, was exposed at the surface by denudation, and fragments of it were brought down by rivers running from west to east and were mixed with the gravel on the sea shore. At the close of this formation (that is to say towards perhaps the middle of the jurassic period), the whole of the country was again elevated, and the chain of the New Zealand Alps formed.
page 76The proofs of this elevation are (1), That all the rocks of precretaceous age partake in the main foldings of the strata (see page 24); (2), The absence of all rocks belonging to the upper jurassic and lower cretaceous periods, and (3), The total unconformity between the Putataka and the Waipara formations, which is especially well seen in the Provinces of Canterbury and Marlborough, where rocks belonging to the Waipara formation are found filling up valleys of erosion in the Maitai and Putataka formations, proving conclusively that the country had previously undergone a considerable amount of subærial denudation. The foldings of all the rocks older than the Waipara formation, show that this upper secondary elevation was no local affair, but part of a large movement, which probably resulted in the upheaval of a continent. How far this antarctic continent extended we have as yet no means of knowing, but that it probably extended to South America is shown by all the formations later than this upheaval containing fossils related to, or identical with those of Patagonia and Chili. Since this upheaval the New Zealand Alps have never been submerged.
Subsequently (upper cretaceous period) this continent subsided, and at the close of the mesozoic era, the north part of this island was 6000 feet, and Otago was 3000 feet lower than at present. This is proved by the height to which the Waipara formation extends in Marlborough (Benmore) and Otago (Mount Hamilton) respectively. That the Alps were not entirely submerged, is shown by the Waipara formation, which was deposited during this depression, abutting against the older rocks, and occurring in such positions as to show that it could never have overspread the whole of the Alps. This, however, can be better proved in Canterbury and Marlborough than in Otago. It was at this time that the sea swarmed with huge saurians (plesiosaurus, &c.), and although none of their remains are as yet known to occur in Otago, they may still be found between Shag Point and the Otepopo River in the hills behind Hampden.
New Zealand was now reduced to a narrow chain of islands, and from that time to the present it has always remained isolated from any large continental area.
* I mention these facts, although referring to districts outside of Otago, as we are by means of them enabled to roughly estimate the difference between this and the former elevation. They are all derived from personal observation of those districts.
It was this lower eocene upheaval that gave the final form to the internal geological structure of the Alps, for all the rocks of a later date are, with local exceptions, more or less horizontal, and circle round the bases of the hills formed by the mesozoic and palæozoic rocks. Thus showing that since the deposition of the tertiary strata no great internal changes in the position of the rock masses have taken place, although vast external changes, caused by denudation, have torn the tertiary rocks into fragments.
* Reports of Geological Explorations, 1872·3, p·44.
During the upper miocene period the whole of the South Island was considerably depressed, and, if I am right in supposing that Mount Prospect near Lake Te Anau is composed of rocks of that age, Otago must then have stood at least 3246 feet lower than at present.
No marine rocks of older Pliocene age are known in any part of New Zealand, and consequently I infer that at that time the whole country stood at a higher level than at present; but at Wanganui there is a bed of blue clay containing marine shells, of which about 24 per cent, are now extinct, which, therefore, we must refer to the newer Pliocene period. Also in many places round the coast of New Zealand, such as Oamaru, Mataura, Wanganui, Taranaki, Cape Kidnappers, &c., we find pleistocene deposits containing marine shells, in some of which all are still living, and in others there are from 5 to 10 per cent. of extinct forms. Consequently there can have been no general elevation of the land extending over a long period since Pliocene times.
The Southland plains are distinctly terraced, and as Dr. Hector remarks are "marked by ridges which doubtless indicate the successive channels which were formed by the Mataura and Oreti rivers at the time they poured their waters into an extension of the present bay, prior to the last elevation of the land."* The network of gravel beds which surround the Hokanui and Moonlight ranges also shew evidence of marine action, for it is impossible that the alluvia of the Mataura, the Oreti, and the Jacobs River should all join one another with gradual slopes behind the seaward range of hills, without the intervention of some uniform widely acting cause, such as the sea; and we have further evidence of this action in the flat topped beds of eocene limestone in the Waimea plains. This would show that the land here has been lately about 600 feet lower than at present, which corresponds very well with the height of the silt formation at Oamaru. The chain of small islands between Paterson’s Inlet and Ruapuke are also quite flat-topped, the result evidently of marine denudation. They are between 140 and 200 feet high.
In the West Coast Sounds nothing perhaps is more striking to the geological eye than the contrast between the immense, almost perpendicular, cliffs found up the Sounds and the absence of any high sea cliffs between them. This certainly implies that the land must be moving either up or down, and constantly therefore presenting a new line of action to the surf. For if the land had been stationary, the furious swell of the Pacific Ocean, impelled against the same line of coast by the almost constant succession of westerly gales, must have long ago eroded them Back so far that an iron bound coast line of high cliffs would have been formed all along that part of the coast where the mountains approach close to the sea. But this is by no means the case. At all the headlands between the Sounds the spurs of the mountains, covered with bush, slope at angles of between 25° and 50° to the sea, and only a line of low cliffs is seen at their base. (See fig. 21.) That this movement is one of elevation there are several proofs.
* Geological Exploration of the West Coast of Otago, p. 441.
The entrance to Doubtful Sound is also distinctly terraced on both sides, (fig. 21), and I estimated the highest to be about 800 feet above the sea level.
On the other hand Dr. Hector has brought forward the following evidence in favour of the view that these Sounds are at present undergoing subsidence. Speaking of the streams that run into Edwardson Sound, a branch of Chalky Inlet, he says "the flats along the lower parts of these streams are true valley deposits, such as may be seen in any mountain valley, formed by the gradual change of the water course from side to side; and as I did not discover any remains above high water mark of the brackish water deposit with estuarine shells, which is now slowly filling up the basin, or any trace of terraces round the mouths of the rivers, I conclude that the land at the head of this sound, unlike most parts of the New Zealand coast, is not rising; and the consideration of the page 81nature of the falls almost demonstrates that it is, on the other hand, an area that is being submerged. The examination of the walls of the chasm through which the first mentioned river—[the north river at the head of Edwardson Sound]—falls—[the falls are 25 feet high, and the river falls into a pool in the chasm 36 feet deep]—convinced me that it has been cut by the action of falling water, which may have followed a pre-existing fissure. The rock is a granular quartzite, compact and close grained in texture, but still such a rock that can be slowly worn away by the mechanical action of running water. The depth of the rocky chasm below the fall must, therefore, of itself prove that the fall was once higher, and as it now falls to the sea level, as a necessary consequence it follows that the land must then have been more elevated."*
Again, speaking of the low neck of land, some 400 yards in length, between Cunaris Sound in Chalky Inlet and Last Cove in Preservation Inlet, he says: "This neck of land is quite low, but bounded on both sides by lofty and precipitous mountains. The torrents which descend from these flow into Preservation Inlet, and accordingly add to the eastern side of the isthmus by the quantity of subangular fragments of rock that they bring down. ∗ ∗ ∗ The absence of the characteristic terraces which mark the existence of a ‘col,’ or ancient strait, was a further proof that the coast line here is not rising, as in a situation like this they could hardly fail to be present." (1. c. p. 452.)
* Geological Exploration of the West Coast of Otago, p. 451.
The deep pool in the chasm, mentioned by Dr. Hector, has, no doubt, like the sounds themselves, been formed when the land stood at a higher elevation than at present, but I do not think that the hole necessarily implies recent subsidence, for its shape might have been preserved below the water for an immense period of time. With regard to the last evidence brought forward by Dr, Hector, viz., the absence of terraces on the low neck of land between Chalky and Preservation Inlets, even if we grant that terraces are characteristic of a col, which, however, I am not prepared to do, still, they could hardly be expected in this case, for the tidal current at the head of these sounds is very slight, and even if a terrace had been cut, it would have been at once obliterated by the immense quantity of debris that falls from the lofty and precipitous mountains on either side of the col.
The Rev. R. Taylor, in his "New Zealand and its Inhabitants" (London, 1855), states that earthquakes in 1826 and 1827 so altered the form of a small cove, called "The Jail" by the sealers, about 80 miles north of Dusky Sound, that it could hardly be recognised. The cove became dry land, and trees wore seen under water near the coast, having probably been carried down by landslips into what was previously deep water.
It has been already mentioned that all the river valleys in the interior exhibit a remarkable series of alluvial terraces. These terraces are generally supposed by geologists to be caused by the rivers rapidly cutting down their beds during elevation of the land, but as Dr. Haast has stated* that there is another agency by which alluvial terraces are formed, "namely the retreat of the river sources to higher and more distant regions,"† it is necessary for me to show that he is mistaken on this point.
* Report on the formation of the Canterbury plains, p. 14. The other reason given in this Report to account for terraces, viz., "The permanent diminution of their (the rivers) waters through changes at or near their sources," need not be discussed, for it would act in exactly the opposite direction to that supposed by Dr. Haast.
† Trans. N. Z. Institute, vi., p423.
The evidence, therefore, appears to me to show that the latest movement in Otago has been one of upheaval, both on the east and and west coasts, and that the amount of this upheaval has been, at least, 500 feet.*
* Captain Fairchild informs me that a Weka (Ocydromus heciori (?),) is found on the Solanders; consequently, this island cannot have been entirely submerged since it was separated from New Zealand. It is 1,100 feet high, and an elevation of 400 feet would connect it with New Zealand.
† Dr. Hector and Dr. Haast have suggested that the mountains may have had a more plateau-like form, and consequently, collected more snow during our glacier period than now. This, no doubt, would tend to increase the size of the glaciers, but we must remember that the New Zealand Alps have been undergoing denudation ever since the jurassic period, and that in the upper eocene period, many of the valleys in them had been hollowed out nearly as deeply as now, consequently, during the comparatively short period that has elapsed since the date of our last great glacier period, denudation cannot have affected the shape of the mountains to such an extent as to make it worth while to take this cause into consideration.
If we now turn for confirmatory evidence to the glacier deposits themselves, and to the older alluvia, we find that the external forms of the older moraines are almost entirely obliterated, as for instance that at the junction of the Lindis with the Clutha, and that at Black Mount; while that near Taieri Mouth has not only lost its external shape, but is almost entirely covered on the seaward side by what appears to be marine gravel. Others of a rather later date have also been much modified as at the southern extremities of Lakes Wakatipu, Wanaka, and Hawea. We also find that the polished surfaces and striæ have completely, disappeared in places where we know that ice must once have passed over. In some places, as near Queenstown, the rounded, smooth surface still remains, but in others not only has the polishing and grooving been obliterated, but subsequent weathering has so altered the once "moutonneés" surfaces as to make them quite unrecognisable, as in the west coast sounds; and there are places in the Maniototo and Dunstan districts, where masses of rock ten or twelve feet in thickness have been removed since the ice passed over them.
This great amount of weathering may in part be due to the nature of the rocks, but the modifications of the moraines certainly indicate their great age, and a study of the older river alluvia leads us to the same conclusion. For instance, I have pointed out that in the Shotover there are two distinct river channels separated by a sharp ridge of rock, and that the older channel is filled with gravel (see fig 16). Now it is evident that no glacier can have come down the Shotover since the older channel was filled up, or it would have ploughed all the gravel out again and rounded the sharp ridge between the two. Indeed the V-like form of the older channel, as well as the gold found in the bottom of it, prove that it was cut out by the river, and not by a glacier, which are known not to cut sharp V-shaped but round bottomed U-shaped valleys. Remembering, therefore, that rivers deepen their beds when the land is rising and their velocity increasing, and fill them up when the land is sinking and their velocity consequently diminishing, we see that the Shotover supplies evidence that since any glacier came down it, the land has been elevated, then depressed, and subsequently elevated again. And if to this we add the first depression which caused the glacier to retire, we have two depressions and two elevations subsequent to the great extension of the glaciers. The same conclusion will be arrived at by a study either of Lammerlaw Creek, at Waipori, or Few’s Creek, at Lake Wakatipu.
page break page 85If also the old lake basins in the interior have been hollowed out by glaciers, * it is evident that an immense length of time must be allowed for filling them up.
Consequently we have proofs not only of the great length of time that has elapsed since the greatest extension of the glaciers took place, but also of a second elevation, and consequent second advance of the glaciers, which, however, was less than the first, as the ice did not a second time come down the Shotover. These two elevations we may with much probability place between the Pareora and Wanganui formations, and between the Wanganui and pleistocene deposits respectively; the earlier elevation being the greater and the longer continued. The New Zealand glacier period has therefore nothing in common with the glacial period of the northern hemisphere.
If now, in order to assist the memory, we throw the various oscillations in level that Otago has undergone into the form of a diagram, as in Plate II., where the central horizontal line which represents the present elevation of the Province, is divided into spaces proportional to the length of the different geological periods, we see that these oscillations have been more numerous and more extensive since the middle of the jurassic period than before it; and that since that period there have been three main upheavals, and two minor ones, or five in all; of which the first represents the period of the antarctic continent, and the last but one the period of the greatest extension of our glaciers. It is, however, probable that we had a glacier period during each of the main upheavals, and indeed we have proofs, as I shall show in the next section, that there were large glaciers in Otago during the middle, or eocene upheaval.
* The proofs of this will be given in the next section.