Report on the Geology & Gold Fields of Otago
Pleistocene Deposits — Newer Glacier Deposits, &c
Pleistocene Deposits
Newer Glacier Deposits, &c.
* Geological Explorations of the West Coast, p. 459.
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.
* Trans. N. Z. Institute III., p. 276, and VI., p. 312-313.
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.
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.
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.
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)
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.