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Report on the Geology & Gold Fields of Otago

Pleistocene Deposits — Newer Glacier Deposits, &c

page 67

Pleistocene Deposits
Newer Glacier Deposits, &c.

West Coast Sounds.—Up the Cleddau River Dr. Hector describes several moraines, and no doubt they exist in all the valleys running into the mountains, as they also do on the eastern side of the range. Dr. Hector also says that in the Waipero Cove, Crooked Arm, he made a careful survey "in order to obtain its exact form both above and below water, in order to test the theory of the glacier formation of these valleys. The average depth of water is 90 to 100 fathoms, to which depth the rocky sides of the valley descend at an angle of 35°, where there is a level mud bottom from side to side, which slopes gradually towards the main body of the Sound. The bank which bars across the valley, and forms the head of the bay, is a true moraine, its upper part consisting of angular blocks of rock of every variety found in the mountain, but in front this moraine is marked by a bank which has been formed of the finer materials brought down by the streams, and shot into deep water." * I may here remark that all the Sounds are much shallower at their mouths than inside— (see ante p. 5), but we cannot infer from this that that they all have submarine moraines across their mouths, for the shallowness may possibly be caused by ancient bars formed when the land was at a higher level. The islands in the Sounds are not "moutonnées," and although some of the smaller ones are rounded, they show no sign of a lee and strike side. The precipices on either side of the Sounds are also in general quite rough, and I only noticed two localities (both previously observed by Dr. Hector), where there was any appearance of polishing. One was in Milford Sound, on the south side of the entrance to the Narrows, where the rocks have decidedly the appearance of having been polished by ice, but I could not examine them closely. The other locality is near Deas Cove, in Thompson’s Sound, but here the evidence is much more doubtful, although I am of opinion that it shows ice action. I saw no grooves nor striæ anywhere, but Dr. Hector says that in Thompson’s Sound "the rock is a granitic gneiss, the hard surface of which has faithfully preserved the grooves and polished surfaces caused by ancient glaciers."—(1. c. p 458); and of the Cleddau Valley, he says that "the rock at the sides where it forms steeply inclined slopes, is grooved and scattered like those of the Sounds."—(l. c., p. 461). Also speaking of Milford Sound itself he says "the lateral valleys join the main, one at various elevations, but are all sharply cut off by the precipitous wall of the Sound, the erosion of which was no doubt continued by a great central glacier long after the subordinate

* Geological Explorations of the West Coast, p. 459.

page 68and tributary glaciers had ceased to exist. The precipices exhibit the marks of ice action with great distinctness, and descend quite abruptly to a depth of 800 to 1200 feet below the water level."—(l. c., p 460.) But the strongest evidence in favor of glaciers having once occupied the Sounds is found in the large granite boulders that occur along the shore at the entrance to Preservation Inlet, which must have been brought by ice for some distance down the Sound. Near the entrance all these boulders have been more or less rounded by the action of the sea, but higher up, in Kisbee Bay, which is situated in slates, there are two large angular granite erratics, the larger of which is 15 or 20 feet high.

Waiau Valley.—The low land on the eastern sides of the Te Anau and Manipori Lakes, and across the Mararoa River, as far as the base of the Takitimus, is strewed with fragments of gneiss, greenstone, and other eruptive rocks, which must have come from the western side of the lakes, and have thus crossed a deep valley. All the smaller pebbles are rounded, but the larger blocks are angular. I saw no moraines either at Manipori or Te Anau Lake, except perhaps some low irregular hills covered with bush at the bend of the Waiau River into the Manipori Lake. These angular blocks extend down the Waiau to the hill between Red Bank Creek and Black Mount, but I saw none further down. I therefore suppose that this hill is the terminal moraine of the ancient Waiau glacier.

wakatipu Valley.—Besides the numerous terminal moraines in the vallies of the [gap — reason: illegible]Rees, Dart, Route-burn, &c., and in most of the valleys between Lakes Ohau and Te Anau, lateral moraines are found on both sides of Lake Wakatipu. That on the eastern side contains many angular and rounded boulders of sandstone and greenstone tuff that must have come down either the Dart or the Greenstone, and must therefore have crossed the Lake. This lateral moraine forms the false bottom of the miners, and is covered by auriferous gravels containing boulders of schist only. At Kingston a large and well-marked moraine is seen crossing the valley, the blocks in it being schist, sandstone, and greenstone tuff. The valley between Kingston and the Mataura is also covered with angular stones, but I saw no evidence of the glacier having ever extended down below Athol. The most noticeable feature in connection with the Kingston moraine is that the southern or lower side of it is not abrupt, as is ordinarily the case, but slopes gently away to the Mataura, and, losing gradually its morainic character, it passes into the alluvium of the Mataura as if both were of the same age.

Clutha Valley.—Both Lake Wanaka and Lake Hawea are bounded at their southern ends by moraines; but both of these moraines, especially that of Wanaka, are much worn, and slope page 69gradually away into the alluvial plains. This latter feature is
Fig. 13.—Terminal moraine of Lake Hawea.

Fig. 13.—Terminal moraine of Lake Hawea.

not so much marksd in the Hawea moraine (fig. 13), as in that of Wanaka, (fig. 14.) Indeed the latter is so much worn, and so
Fig. 14.—Terminal moraine of Lake Wanaka: a gravel and silt; b, grey silt; c, yellow clay, (Wanganui formation.)

Fig. 14.—Terminal moraine of Lake Wanaka: a gravel and silt; b, grey silt; c, yellow clay, (Wanganui formation.)

covered with silt at the surface, that if it were not for its crescent shape, the irregularity of its surface, and the occasional occurrence of large angular boulders, its presence would hardly be detected; for the alluvial deposits slope from the Cardrona quite to the top of the moraine. At the junction of the Lindis with the Clutha, a large moraine appears to have once existed; but it is now quite destroyed, and marked only by the number of large angular blocks that lie on the surface, and are mixed up with the alluvial deposits. These angular blocks are found in the alluvium overlying the lignite formation as far down as Cromwell—(see fig. 12, g), and are very conspicuous at the entrance to the Kawarau Gorge on the Cromwell side. Below Cromwell I have seen no glacier deposits, neither have I seen any in the valley of the Waitaki, nor in any other part of the Province except those described.
Dunedin.—It has been stated * that evidence of ice action in the form of striated boulders, rock groovings, and boulder clay, existed in the neighbourhood of Dunedin. I have several times carefully examined the evidence upon which these statements rested, once in company with J. T. Thomson, Esq., and Dr. Berggren, of the University of Lund, in Sweden, who is well acquainted with ice marks, and on another occasion with L. O. Beal, Esq, and Mr. Ulrich, who has a most extensive knowledge of rocks and minerals. We found the supposed glacial striæ very numerous on rounded basalt blocks. But although remarkably like true ice scratches, they were found on close examination not to be parallel, and even occasionally slightly curved. They were also found to be chiefly on the edges of the blocks, and often went round a corner. These facts were decisive against the scratches being due to ice action, and we came to the conclusion that they were caused by decomposition, bringing into view a previously hidden internal structure

* Trans. N. Z. Institute III., p. 276, and VI., p. 312-313.

page 70of the rock. This opinion was subsequently confirmed by finding similar striæ on hard cores of basalt in a decomposing lava stream, where of course they could never have been exposed to any abrading action. It is these decomposing basalts, out of which the harder undecomposed cores project and look like rounded boulders irregularly scattered through a clay, that have, in the neighbourhood of Dunedin, been mistaken for a glacial boulder clay, or till. The grooved rocks in the valley of the Kaikorai were found to be caused by water running over calcareous sandstone, and dissolving narrow gutters out of it.

Shore Deposits—At Oamaru a horizontal thick bed of fine grey silt, interstratified in its lower portions with several beds of well rounded gravel, rests unconformably on the volcanic and sedimentary tertiary rocks (Fig. 7, a). This deposit, which descends nearly to the sea level at Oamaru jetty, wraps over the highest points (470 feet) of the volcanic rocks forming Oamaru Cape, and descends again on the other side nearly to the sea level, The lowest gravel beds on both sides of the Cape are full of marine shells, and out of them I obtained the following:—Ranella vexillum, Euthria lineata, Voluta pacifica var γ, Cominella virgata, Calyptræa maculata, Turbo smaragdus, Diloma æthiops, Diloma nigerrima, Polydonta tiarata, Patella margaritaria, Mactra æquilatera, Mactra sp. nov., Mesodesma novæ zealandiæ, Mesodesma subtrian gidata, Chione costata, Chione mesodesma, Chione gibbosa, Dosinea anus, Tapes sp. nov., Mytilus lotus, Mytilus dunkeri, and Ostrea purpurea.

Of these, 22 species all but two are still living, but Cominella virgata is not known to live as far south as Oamaru at the present day. Many of the bivalve shells appear to be thicker than they are now, but Diloma æthiops, Diloma nigerrima, Polydonta tiarata, and Patella margaritaria still partly retain their color.

In the same bed Mr. Forrester has obtained rolled specimens of Natica solida and a Cucullæa, but these are very much water-worn, and have evidently been derived from an older formation. This bed is not more than 10 or 12 feet above the sea, but marine shells occur in other gravel beds filling old caves, to a height of 60 feet on the north side of Oamaru Cape.

In the silt overlying the gravels many bones of the moa are found at all levels from a little above the fossiliferous gravel bed to quite the upper portions, and I also obtained some fragments from the same deposit in the terrace above Oamaru.

In Fig. 15 I have given a sketch of a section exposed in a page 71gully on the north side of Oamaru Cape as it appeared in November, 1873, with, the broken pelvis and femur of a moa (Dinornis) sticking out of it (fig. 15, e). The femur and pelvis were found in their proper positions. There is also in the Museum the skull of a sea elephant (Morunga elephantina) which was obtained from this silt several miles inland from Oamrru, but I do not know the exact locality.

Fig. 15. Silt bed at Oamaru—a, silt, b, gravel; c, gravel with marine shells, pleistocene; d, [basa]t (Oamaru formation); e, moa bones in silt.

Fig. 15. Silt bed at Oamaru—a, silt, b, gravel; c, gravel with marine shells, pleistocene; d, [basa]t (Oamaru formation); e, moa bones in silt.

This silt extends inland for a considerable distance, and stretches northwards as far as the shingle alluvium of the Waitaki. It appears to present a considerable analogy to the Pampean formation of South America.* Southwards it is extensively but irregularly developed. At Hampden it is seen to rest on the blue clay of the Pareora formation (fig. 9 a). At Waikouaiti and Blueskin it passes into a sandy clay, and at Dunedin, a clay, that I consider to be of the same age, covers a considerable area at Mornington at a height of 450 feet above the sea, where it rests upon volcanic rocks.

From the Otokaia southward to Tokomairiro, beds of fine gravel occur along the shore and extend inland, covering up the old moraine deposit, previously mentioned, to a height of at least 300 or 400 feet.

From Wangaloa to the Clutha beds of horizontally stratified sands and gravels are found along the top of the sea cliff (sec. VII. a), and extend inland to a height of about 150 feet. I could find no fossils in these rocks, but they have every appearance of being very recent.

The country also between Balclutha and Clinton is thickly covered with silt, the older rocks only appearing occasionally, and generally in the valleys.

The Southland plains, extending from the Mataura to the Jacob’s River, may also be considered a shore deposit. They are composed of beds of shingle, sand, and clay, with occasional seams of lignite in the lower portions. These beds rise gradually from the sea, and attain a height of about 650 feet in the Waimea plains.

* Darwin’s Geological Observations in South America, p. 99.

It is the (1) diluvial clay of Mr. Mantell, Q.J.G.S., 1850 p. 324.

[gap — reason: illegible]Ante, p. 62.

page 72

A similar set of beds are seen on the coast at Orepuke (fig. 26, a)

Older River deposits,—Many of the rivers in Otago have on one side or the other old beds filled up with gravel and silt, and separated from the present river bed by a ridge of solid rock. In the Shotover, for instance (Fig. 16), one of these old river beds is worked for gold on the right bank of the river, a gallery being driven along the old channel, the schist rock rising up on both sides. The present river bed is cut down considerably lower than the old one.

Fig. 16. Shotover River.—a, old river channel.

Fig. 16. Shotover River.—a, old river channel.

The same thing occurs in the Lammerlaw Creek, at Waipori, where Driver’s and Bolton’s claims are worked in an old channel of the river, which is completely covered up and hidden by gravel and silt, and divided from the present river by a ridge of rock. In this case also the river is now running at a lower level than the old one did. Another example of the same thing may be seen at Few’s Creek, Lake Wakatipu; and no doubt many others of a similar nature exist.

In the Dunstan gorge the river is seen to have once run in a broad channel which has been filled up with gravel and silt, and a new and much narrower channel has been cut through the alluvial beds and deep into the solid rock below (fig. 17.) In these older silt beds moa bones are of not uncommon occurence (fig. 16,[gap — reason: illegible]a)

Fig. 17. Dunstan Gorge. —a, Position of Moa bones in silt,

Fig. 17. Dunstan Gorge. —a, Position of Moa bones in silt,

Under this head we must also arrange the river terrace formation which is so largely developed in the interior of the Province. These terraces are found in the valleys of all the rivers, but are particularly well seen in the Upper Clutha (see plate I.) These older river alluvia, which cover the still older lake deposits,* are of course very variable in character, but a general rule in their composition appears to be that the uppar beds are composed of finer materials than the lower, the whole being often capped by fine sand or silt (see fig. 12, a).

* See figs 10 and 11 (a), and fig. 12, a and b.

page break
Geology of Otago, Hutton. Plate I ARiver Terraces, Upper Clutha.

Geology of Otago, Hutton. Plate I A
River Terraces, Upper Clutha.