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Report on the Geology & Gold Fields of Otago

Section VI. — Surface Geology

page 86

Section VI.
Surface Geology.

Having described the geological structure of the Province, and probable oscillations of level that it has at various times undergone, we are now in a position to discuss its surface geology, or in other words to attempt an explanation of the various causes that have brought about the surface contours of the country; and although it cannot be expected that in the present incomplete state of our knowledge all the numerous and complicated problems presented to us in Otago can be satisfactorily resolved, yet if our observations on the geology of the Province, and the deductions drawn from them are in the main accurate, then the errors that we may fall into in attempting the solution of some of these physiographical puzzles will be errors of detail only, and not errors of principle; and further and closer observation will no doubt enable us to correct them.

It would be out of place in this report to discuss those features that are common to all countries, such as the origin of valleys, plains, &c., for this will be found in any good text book of geology, and it is here only intended to mention those peculiarities in the rivers, valleys, &c., in Otago that require special explanations, or those points which relate specially to the gold-bearing drifts* of the Province. There is, however, one point, which as it is still considered "sub judice" by some geologists, and as it will have to be constantly referred to in the following pages, must be briefly discussed here. I mean the origin of lakes lying in rock-basins. By the term "rock-basin," is meant a hollow in the solid rock, the margin of which is on all sides higher than the bottom. These rock basins, when on a large scale, can only have been formed in one of two ways, viz., either by unequal elevation or subsidence of the area, or by the erosion of ice. A glacier hollows out a rock basin in this way. The terminal moraine at the end of a glacier covers up the rocks all across the valley, and prevents further

* The term "drift," or "gold drift," has come into such general use in the Australasian colonies for gold bearing alluvial deposits, that I think it better to use the word here; and it must not, therefore, be thought that by "drift," I mean only glacial or ice born deposits., which is the original and more correct definition of the term.

page 87denudation taking place at that point. But above it the moving ice still keeps wearing away the rocks below it, and if this action is sufficiently long continued the slope of the valley down which the glacier is moving may in time be reversed; the cohesion between the upper and lower portions of the ice enabling the upper portions, which still retain their original slope, to drag the lower portions up hill along with them. And when the glacier melts away a rock basin is left behind, which forms a lake.

If now we turn to the lakes and old lake basins in Otago, we find that all the present lakes lie in rock basins, for they are very deep, while the rivers draining them run over rocky beds. At Cromwell the lignite has been worked to 80 feet below the river level, while the river itself runs over rock in the Dunstan Gorge. The Upper Taieri Lake proves the same thing for the Maniototo Plains, and although I do not know of any sinkings that would prove the Manuherikia Valley to be an old rock basin, we cannot doubt but that it is similar to all the others. Now in order that these lakes and old lake basins might be produced by unequal movements of the earth’s crust, the land north of Lakes Te Anau, Wakatipu, Wanaka, and Ohau, and the Manuherikia Valley must have been depressed, or that south of them must have been elevated, and as these lakes are very deep, the movement must have been considerable. On the contrary, in order to account for the old lakes of the Ida-burn Valley and the Maniototo Plains, the land north of them must have been elevated, or that south of them depressed; and as the Ida-burn Valley lies close alongside of, and parallel to the Manuherikia Valley, it is evident that the flexures in this part must have been very abrupt. Now not only are no signs of these movements to be found in the geological structure of the district, but we must also remember that owing to the great thickness of the earth’s crust, it is impossible that folds of this nature could take place in any one district without a large area being similarly affected. But we know, from the horizontal disposition of the older tertiary rocks in Otago, that no such disturbance has taken place since their deposition; and as the lake basins must have been formed considerably after that date, it follows that in Otago they cannot be due to unequal movements of the surface. On the other hand glaciers in some cases still occupy the heads of the valleys in which the lakes lie, and in most of them there is ample proof that glaciers once filled the valleys now occupied by the lakes; consequently I shall in future assume that all the rock basins in Otago, whether filled with water, or with clay and gravel, have been hollowed out by ice.

Valley of the Waitaki.—That the Upper Waitaki Plains, through which the Ahuriri River now flows, once formed part of a lake connected with Lakes Ohau and Tekapo is evident. Whether the Ohau glacier at one time extended south across the present page 88valley of the Ahuriri, and into the low pass between St. Bathan’s and the Hawkdun range; or whether the southern portion of the basin was hollowed out by a glacier descending down the valleys of the Upper Ahuriri and the Omarama and, together with the Ohau glacier, emptying itself into the Waitaki by the present outlet, I am unable to say. But I think there can be no doubt that the lower part of the valley of the Ahuriri has been cut since the disappearance of the glacier. The rocks in its valley are all sharp and jagged, and I saw no moraine in any part of its course, nor in the Waitaki, below where the Ahuriri joins it.*

That the lake once stood at a much greater height than at present, is proved by the Longslip hills, which are formed of lacrustive deposits, and rise through the shingle plain to a height of 200 or 300 feet. These hills form a line that runs nearly in the longitudinal axis of the plain, but is broken through in several places by streams, such as the Quail-burn, which cut straight through them to join the Ahuriri. These plains could not have been formed by the present rivers. Another important peculiarity is that they have two outlets, one by the Ohau River, the other by the Ahuriri; and as under ordinary circumstances no lake can have more than one outlet, some special explanation is required in this case. The following appears to me to be the most probable. After the first retreat of the glacier, a lake was left which overflowed down the Ohau River, but at a considerably higher level than at present. This lake was gradually drained, and the Ahuriri River running on the north side of the Longslip Hills, and the Omarama on the south side, both joining the Ohau River, they first wore down the lacrustine deposits on either side of the Longslip Hills and then deposited the shingle plains. Afterwards the glaciers of Ohau and Tekapo again advanced, and the latter blocked up the exit by the Ohau River; this caused a lake to be again formed over the southern parts of the plain, which overflowed down the present course of the Ahuriri. This lake was drained nearly to the level of the Ohau River before the glaciers retreated again, and while this was in operation, gaps were formed in the Longslip Hills, through which the new drainage system took place. When the glaciers at last retired the terminal moraines, and the gravels that the glaciers had pushed up before them during their second advance, threw the streams flowing from them Back into their old channel, and thus separated them from the Ahuriri. It will be noticed that this explanation requires a double advance of the glaciers, the second being less than the first, which as we have already seen was most probably the case.

Further down the Waitaki, the outlier of older tertiary rocks

* Dr. Haast, in a paper in the Quar. Jour. Geo. Soc., 1865, p. 135, mentions an Ahuriri glacier 25 miles long, and a Waitaki glacier 78 miles long, and states that traces of the Waitaki glacier are still visible far down the river, but he gives no proofs of these assertions.

page 89found at Big-gully Creek, appears to occupy another small rock basin in the valley of the river, which must have been hollowed out by an eocene glacier. The importance of this inference will be seen when we come to speak of the Blue Spur.

It is also evident that the Waitaki river existed in the lower eocene period, but that at that time, after passing the Maruwhenua, it flowed through the present valley of the Waireka.

Valley of the Shag River.—This valley, although not very large, is very old, and is particularly interesting, as we can fix the date of its formation to have been in the eocene period. The proof of this is that the upper cretaceous rocks of the Horse ranges (Waipara formation) form one side of its valley, while lower miocene rocks (Oamaru formation) are found lying horizontally in the centre. The valley, therefore, must have been scooped out between the dates of the Waipara and Oamaru formations. At this time it extended into the Kyeburn, as proved by the marine tertiary rocks found near the coal mine, and it probably took its rise in Mount Ida. The eocene Shag River was therefore considerably greater than the present one.

Subsequently, during the upper eocene and miocene periods, the valley was submerged and filled up with marine and volcanic rocks, which were afterwards partly eroded out again during the older pliocene upheaval; while at the same time the upper part of the valley, as far as the hard balsaltic rocks of the Houndburn, was worn away by the Upper Taieri glacier, and formed into part of the basin that was ultimately to become the Maniototo plains.

Old Lake Basins of the Interior.—That the Maniototo plains, the Ida-burn valley, and the Manuherikia Plains, were once lakes that have been filled up, is proved by the nature of the rocks that fill them, and the occurrence of fossil fresh water mussels at Clyde, and Mainototo Plains; and I have already given my reasons for assuming that they were hollowed out by ice.

The general configuration of the surrounding mountains shows that the Maniototo Lake was chiefly formed by the Upper Taieri glacier, and Ida-burn valley by the Pool-burn glacier, both these glaciers flowing north. Also the rounded form of the low narrow ridge of schist which separates the two where the Rough Ridge joins the spurs of Mount Ida, makes it appear probable that the Maniototo glacier joined that of the Ida-burn at that point, and that both together flowed round Blackstone Hill into the Manuherikia glacier. There is also some confirmatory evidence on this point. In the neighbourhood of Naseby large quantities of boulders of a white or pinkish highly metamorphosed quartz conglomerate occur, and blocks of the same rock are also common in the Maruawhenua on the other side of the Kakanui mountains, and are occasionally found as far down as the Awamoko. I have not been able to find this rock in situ, but from the dis-page 90position of the boulders I think that it will probably be found somewhere between Mount Ida and Kyeburn Peak. Now boulders of this conglomerate are found all down the valley of the Manuherikia as far as Clyde, and as the streams do not run in that direction now, it follows that they must have been taken into their positions by a different drainage system to the present one.* If this view is correct, the terminal moraine of all these glaciers would have been at Clyde and Alexandra, and the river running from it would have flowed down the valley of the Clutha. I have already said that I saw no trace of a moraine in this position, but it must be remarked that the mountains from which these glaciers would have been fed are round-Backed, and tolerably equal in altitude; consequently they would be almost entirely covered with snow. Also the Clutha has had plenty of time to remove any moraine that might once have existed there.

We have now to try to account for the change in the system of drainage that took place after the disappearance of the glaciers, and which caused the Maniototo Lake to overflow into Strath-Taieri, and the Ida-burn Lake to break through the Raggedy Range near Black’s.

When the land began to sink, and the glaciers to retire, those of the Maniototo and Ida-burn would retreat much faster than that of the Manuherikia, as they were fed by the snows on lower ranges of mountains which are of nearly equal altitude; and this difference would be the greater if, as is probable, the Manuherikia glacier was a continuation of the Ohau glacier. If now we assume, as we reasonably may, that the Maniototo and Ida-burn glaciers had been greatly diminished, or altogether disappeared, while the Manuherikia glacier was still continued as far as Blackstone Hill, lakes would be formed in the Ida-burn and Maniototo valleys, separated from one another by the Rough Ridge; and if the ice at Hills Creek was sufficiently high, these lakes would be dammed Back until they overflowed at the lowest place along their margins, which in the case of the Maniototo, would be into Strath Taieri, and in the case of the Ida-burn, into Manuherikia, above Blacks. Gorges would then be cut down in these plains which, by the time that the Manuherikia glacier had retreated to the Hawkdun Mountains, might be sufficiently low to continue the drainage.

To test the truth of this conjecture, it will be necessary to compare the levels of the tops of the gorges where the Taieri and Ida-burn quit their plains, with the lowest part of the ridge dividing them, and ascertain what thickness of ice would be required at Blackstone Hill to cause the lakes to overflow there. It will be noticed that this hypothesis makes no mention of the second advance of the glaciers. This is because the second elevation was not suf-

* The boulders containing tertiary fossils found at Alexandra may also have come from the upper Shag Valley.

page break
Geology of Otago Plate IIIOutlet Of Wanaka Lake.

Geology of Otago [gap — reason: illegible] Plate III
Outlet Of Wanaka Lake.

page 91ficiently
great to form glaciers of any size in the Lammerlaw Range, which is quite in accordance with our supposition that these glaciers had retired while the Ohau glacier was still at Blackstone Hill, because, as we have already seen, the second elevation was only supposed to have made the Ohau glacier advance so far as to block up the Ohau River. It also explains why these old lake basins are entirely filled up, while those situated more in the mountains, having been re-excavated by the second advance of the glaciers, are still unfilled.

The great length of time that has elapsed since the ice passed over this district, is shown by the extraordinary way in which the rocks are weathered. At a distance these hills look quite smooth and rounded, but on closer inspection the surfaces are seen to be formed of numberless projections of solid rock, some ten or twelve feet high, with grass growing between them. It is to this that many of the ranges owe their very suggestive names, such as Knobby Range, Raggedy Range, Rough Ridge, Rock and Pillar, &c. This very peculiar style of weathering is different from anything that I have seen elsewhere, and I can offer no sufficient explanation of it; but I would remark that it is confined to the dry inland district, and gradually disappears towards the coast.

Valley of the Upper Clutha.—The old lake formed by the united glaciers of Wanaka and Hawea appears to have passed Cromwell, and extended up the Bannock-burn and Fork-burn Creeks, the outflow of the river from the glacier being probably into the Frazer River, and so into the Clutha. Not only does the shape and position of the old lake basin indicate this, but there are also reasons for concluding that the Dunstan gorge, through which the river now flows, was not then in existence. In the first place, the abruptness and narrowness of the entrance to the gorge at Cromwell shows that it has never been the outlet for a large body of ice; and in the second place the lignite deposits under the township at Clyde must have been formed in still, shallow water, which could not have been the case if the Clutha then rolled through the gorge in the way it does now.

It has already been said that lakes Hawea and Wanaka are terminated by low moraines, which pass gradually into the broad gravel plains lying beyond them.—(See Plate III.) The peculiar shape of these moraines may perhaps be owing to the glaciers in their second advance having pushed the gravel beds before them. The plains themselves also are of interest as bearing on Dr. von Haast’s theory of the formation of the Canterbury plains by rivers running from glaciers. The glaciers of Hawea and Wanaka, separated by a range of mountains, present a parallel case to the glaciers of Waimakariri and the Rakaia separated by the Malvern Hills, and the plains in Otago have certainly been formed in the way suggested by Dr. Haast. But while the Canterbury plains on page 92either side of the Malvern Hills are within a few feet of the same level, those of Hawea and Wanaka, although much closer together, differ in height by more than 200 feet, thus showing the great improbability of the level plains of Canterbury having been formed in the same way.

Lake Wakatipu District.—I have already said that the moraine that circles round the end of Lake Wakatipu behind Kingston, which is one of the best marked moraines in Otago, proves that the lake was once occupied by a glacier; and I have already mentioned that a lateral moraine is found in places along the east side of the lake, which moraine is in part composed of rocks, such as sandstone and greenstone tuff, which only occur on the western side. Now these sandstone boulders can be traced from Queenstown as far as Frankton, where they are covered up by the debris brought down by the Shotover, and I have not been able to find any more of them further down the Kawarau until we come to within a few miles of Cromwell, where sandstone boulders again occur, that have come down from Lake Hawea. From this we must infer that the glaciers that descended the Arrow and Shotover valleys did not continue down the Kawarau, for they would have taken the sandstone boulders with them, but that they joined the Wakatipu glacier at Frankton, and then both together went down to Kingston, and on to Athol and the Dome Pass. When afterwards this glacier had retreated to Kingston, a lake was formed between there and the Dome Pass, which overflowed into the Nokomai, and cut the Mataura Gorge opposite Athol. Previous to this the Nokomai had been the source of the Mataura.

While the main glacier of the Wakatipu was retreating, the smaller ones of the Arrow and Shotover were doing the same, and they were unable to reach Frankton long before the main glacier had retreated so far. Consequently a lake would be formed between Frankton and Arrowtown, which, dammed Back by the ice of the Wakatipu glacier, overflowed into the Kawarau, cutting a gorge through what is now known as the Arrow Bluff, which was previously the line of watershed between the Arrow and the Kawarau. By the time that the Wakatipu glacier had retreated as far as Queenstown, this gorge must have been cut so deep that it was lower than the lowest part of the moraine at Kingston, and consequently the whole of the drainage of the lake took this channel. Lake Hayes was afterwards separated from Lake Wakatipu by the detritus brought down by the Shotover.

It must not be supposed that this method by which glaciers alter the former drainage system of a country is not paralleled by cases occurring at the present day, for the Lake of Moril, in Switzerland, is now held Back by the ice of the Aletsch glacier, so that in floods it overflows into the Rhone by another channel, and if the present condition of things is continued long enough, this page 93channel will be cut sufficiently low to drain the whole of the lake that will be left when the Aletsch glacier melts away.

Tuapeka District.—Seated on the top of the saddle that divides Monro’s from Gabriel’s Gully, is a deep cup-shaped hollow in the schist rocks which has been filled up much higher than the present lips of the cup by gravels cemented into a hard blue conglomerate. This is the Blue Spur, famous for the immense quantities of gold that have been derived from it. On close inspection this hollow is seen to have been an old mountain tarn with smooth polished sides, which have, however, now decomposed into blue clay to a variable depth of from two to six inches. Although I could detect no striæ on the sides of the hollow, I have no doubt but that it was excavated by a glacier. This old rock basin is filled up with beds of conglomerate that dip to the east—(Fig. 6)—and as a rule the stones in the conglomerates get smaller toward the east, which, together with the direction of the dip, prove that the old tarn was filled up from the west. The conglomerates consist in great part of pebbles and subangular blocks of green quartzite and a dark purple jasperoid slate with quartz veins, which rocks do not exist nearer than the Tapanui mountains, west of the Clutha River. South east of Lawrence also, several other patches of conglomerates are found at the Blue Lead, Waitahuna, at Manuka Creek, &c., all of which probably mark the position of an old valley which extended from the Tapanui mountains through the Blue Spur to Kaitangata. No appearance of such a valley can, however, be seen at present, and its supposed course is now crossed at right angles by the Clutha, and by the Tuapeka and Waitahuna rivers. Consequently this old river must date Back to a time previous to the formation of the present valleys; and as we have already seen that the Clutha received the drainage of the pliocene glaciers of Maniototo, Ida-burn, and the Manuherikia, we must place the existence of the Blue Spur glacier during a still earlier upheaval; that is to say, we must refer it to the eocene period. The auriferous conglomerates, therefore, would have been poured by the river into the old mountain tarn after the retreat of the glacier, caused by the subsidence that took place previous to the deposition of the Oamaru formation. There is nothing extravagant in these suppositions, for in the valley of the Waitaki we have collateral proof that eocene glaciers existed in New Zealand, and that gold existed in the rocks during the eocene period we have proofs in the auriferous sandstone of the Shag River—(ante p 17)—and in the auriferous conglomerates over the coal at Coal Point (ante p. 48.)

Lower Taieri and Tokomairiro District.—It has been already mentioned that a valley extends from the Silver Stream, behind Dunedin, through the Lower Taieri plain, Waihola Gorge, and Tokomairiro Plain to the mouth of the Clutha. The old moraine on the south east side of this valley, between the Taieri and the page 94Otokaia has also been described. Now it is evident in the first place that this valley existed in the eocene period, and that its estuary was filled up by the Tokomairiro and Kaitangata coal formation in the lower miocene period; and secondly that the valley as it now exists was again hollowed out when the land was elevated, for the tertiary rocks are left on either side of it at Waihola Gorge and Tokomairiro. It appears therefore probable that during the greatest elevation of the land in the older pliocene period, glaciers came down Strath Taieri (it must be remembered that the gorge between Strath Taieri and the Maniototo Plains was not at that time in existence), which being met by another large glacier coming down the Waipori, turned eastward, and had its terminal moraine between the Taieri and Otokaia. The river running from the end of this glacier appears to have cut down the gorge between Taieri mouth and the bridge, and when the glacier retired, this channel took the whole of the drainage of the Taieri, now reinforced by the addition of the Maniototo Plains to its catchment basin. We must also remember that since this period the whole of the valley up to a height of about 500 feet must have been submerged below the sea.

Summary.—In this section we have seen that since the last great extension of our glacier system, the old lakes of Maniototo, Ida-burn, Manuherikia, and Upper Clutha have been filled up by debris brought clown by streams; that the surfaces of the rocks that were formerly covered with ice, have been weathered to a depth of 10 or 12 feet; and that the gorges of the Kawarau, Dunstan, Mataura, and Upper Taieri have been formed. So that in many places the drainage system of the Province has been completely altered since that time, and if to this we add the evidence adduced in the last section, viz., the alteration in the external forms of the older moraines, the older river channels being filled up with gravels, and the depth to which the rivers have cut their new beds we can have no hesitation in placing the date of our last great glacier period far further Back than the glacial epoch of the Northern Hemisphere. That all our lakes are not filled up is probably owing to the second advance of the glaciers which partially scooped them out again.