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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

(B) Western Slopes of Southern Alps

(B) Western Slopes of Southern Alps.

In several portions of this Report I have already given some account of the most striking characteristic features displayed by the deposits of the Ice-age along the western slopes of the central chain. Whilst the remains of the most northerly moraines in Westland belonging to the old Taramakau and Arahura glaciers, consisting of a large circumvallation around the lower end of Lake Brunner (227 feet), and a smaller one in the valley of the Arahura, do not reach the sea within twelve miles, the next ones belonging to the great Hokitika glacier are separated from the sea-shore by a small strip of low land of recent littoral origin only. The next large moraine belonging to the Mikonui-Waitaha glacier forms already a very conspicuous object page 393in the landscape, rising as Bold Head, a remarkable headland, from the coastline. It was doubtless the frontal moraine of a huge glacier descending so far towards or into the sea, and its structure can well be studied by passing along it. Rising in the centre to about 250 feet it consists principally of till, with numberless blocks of all sizes and shapes, mostly angular, imbedded in it. These blocks are derived from the Mount Torlesse and Waihao formations, typical metamorphic or igneous rocks being of rare occurrence. Some of the imbedded rocks are of enormous size, often larger than the celebrated Pierre-à-bot in the Jura. One of the most striking examples is the large block lying in front of the centre of this Bold Head moraine, to which the latter owes, without doubt, in some degree its preservation. This block, consisting of contorted clay-slate with layers of quartz, is from 30 to 40 feet in diameter, and covered on its summit with a luxuriant vegetation. In the upper portion of the confused mass of morainic accumulations are two large deposits of fine glacier silt, in which also angular blocks of various sizes are imbedded. However, what gives an additional interest to Bold Head, is the existence of an alluvial deposit 30 to 40 feet thick, having a considerable slope to the south, and separating the morainic beds into two distinct portions. It contains a quantity of subangular shingle and gravel, its greatest portion being derived from the metamorphic and igneous rocks at the western base of the Southern Alps, a clear proof that at least one great oscillation during the Glacier period took place, when these oldest rocks became exposed in their turn to fluviatile action. Bold Head has also a considerable slope towards the East. It then joins a ridge forming its eastern continuation, and rises again, gradually and steadily, till it abuts against the western foot of the Alps.

Having passed along a long sandy beach of several miles in length, we come to the morainic accumulations of the Wanganui glacier, reaching for a distance of more than ten miles from the Pukuaro cliff north, to the northern banks of the Puerua River south. These deposits consist of the united moraines of the Wanganui and Puerua channels, both draining a considerable portion of the Southern Alps. It is a wild piece of coastline, of which the photo-lithographic view of the Wanganui Bluff, with Mount Oné-Oné in the distance, gives a faithful representation. It is evident that, owing to the resistance offered, all the headlands consist of those portions of the moraines in which the greatest number of large blocks are enclosed; in fact, they correspond generally with the central and lateral moraines. At the page 394same time their partial destruction has offered the means for their further protection by depositing at their base large masses of rocks, extending into the sea for a considerable distance, and against which the waves break violently but ineffectually; whilst the softer beds between them not having this protecting talus, are, if not constantly, at least during the frequent gales along this coast, quite unprotected from the fury of the waves, and are much less able to withstand their destructive power. The observer passing along this Bluff must come to the conclusion that a great portion of it has already been washed away, as for a considerable distance erratic blocks lie in the sea. Some of them are of enormous size, of which a remarkable instance is seen in the view of the Wanganui Bluff. A section of this portion of the coast No. 3 on Plate 8, shows the somewhat complicated character of these deposits. Beginning in the north we have first a true lateral moraine, consisting of the usual detrital matter, blocks of all sizes imbedded in sand and silt, the whole having a rough anticlinal arrangement. All these rocks are derived from, or at least from near, the summit of the central chain, and belong to the Waihao and Mount Torlesse formations. The southern slopes of this moraine disappear below an alluvial bed, forming a vertical wall of about 30 feet altitude. The latter consists of a well stratified bed of subangular boulders and gravel deposited here by a large river—after the retreat of the glacier— towards the sea. Although generally consisting of the same kind of rocks as those enclosed in the moraines, the presence of true metamorphic and igneous rocks proves the retreat of the glacier to such a high position, that the lower slopes of the Alps were exposed to glacier and fluviatile action. After one mile and a half a new moraine, having the same rough arrangement as the former, rises again below the fluviatile deposits. It represents doubtless the central moraine, belonging to the same glacier as the former. A new river bed of similar length follows, and then a third moraine, the southern lateral moraine of the same glacier, makes its appearance. Thus, with the facts before us, we can conclude that the northern branch, forming the trunk glacier, was here about five miles broad, consisting of two principal branches higher up, and being covered with comparatively scanty morainic matter only.

A small creek (Camp Creek) separates the last-mentioned beds from far larger morainic accumulations, rising several hundred feet above the sea shore. They have been brought down by a glacier of nearly the same breadth as the former, but it appears that it has been. page 395Covered entirely by detrital matter near its termination (like the present Tasman glacier on the eastern side of the Southern Alps). The two lateral and the central moraines are however best developed, all three exhibiting anticlinal arrangements, and the largest blocks of rocks, often closely packed, are found in them. The two intervening spaces are made up either of fine detrital matter, or of glacier silt, with angular blocks of small size scattered through them. An ancient river bed, similar to the one already described, reposes upon the morainic accumulations between the central and the northern lateral moraines. At the foot of the southern lateral moraine the river Wanganui enters the sea, and on its southern side, somewhat in front of the rest, the Wanganui Sugarloaf, called Mount Oné-Oné by the Natives, rises conspicuously from the sea. It is doubtless, like Bold Head, a true frontal moraine, being joined by a low ridge with another higher one, which forms the Puerua Bluff. This latter has at its southern extremity also the rough anticlinal stratification peculiar to lateral moraines. It would be impossible to give in a small section the details of the arrangement these glacier beds exhibit. There are numberless changes, from an assemblage of enormous angular blocks, to the finest glacier silt, deposited in such thin layers that one hundred or more form one inch, and often changing gradually from one into the other, or alternating with each other. The moraines of the united Waitaki-Whataroa glacier (the latter in the map erroneously named the Makaroa) rising in Abut Head to an altitude of about 400 feet, have an arrangement similar to that of the former, and consist also exclusively of rocks derived from the Waihao and Mount Torlesse formations. In some localities the whole wall consists of enormous fragments of rocks only, showing that the destruction in the higher portion of the Alps went on at such a gigantic scale, that we can scarcely form a conception of it. Two miles south of Abut Head, forming the southern bank of the Whataroa, a series of smaller morainic deposits make their appearance. They form the southern boundary of this glacier system. South of Lake Okarito the extensive glacier deposits of the Waiau glacier begin. They have a breadth of about ten miles, and exhibit the usual arrangement of the detrital matter. However, no alluvial beds are deposited above or between them. The Weheka-Karangarua moraine is also of considerable proportions, but unlike all the former, metamorphic and igneous rocks occur occasionally amongst the enclosed fragments at its northern end. Advancing towards the south the metamorphic rocks become gradually more numerous, till page 396they form at the Makowiho Bluff the greatest portion of the morainic accumulations. At this Bluff the blocks of rocks of which the moraines are composed closely resemble in lithological character those of which the circumvallations round Lake Wanaka are formed. Another interesting feature of the Makowiho Bluff is the occurrence of an ancient river bed, about fifty feet thick, deposited against a lateral moraine, covered by younger morainie accumulations. A further proof that oscillations in the position of the glaciers took place also on the western side of the Alps, and that when advancing again over a deposit of such comparatively incoherent nature, as a gravel bed, the glaciers were doing so without destroying it to any appreciable extent, we find in the valley of the Waiau. The last moraine reaching the sea-coast was formed by the Paringa glacier. It is however greatly destroyed.

The lowest signs of the Haast glacier were traced to about twelve miles from the coast. I have not ascended the Arawata high enough to reach the ancient morainic accumulations in that river bed. Some intelligent miners have however informed me, that deposits with large angular blocks are broken through by that river about ten miles above its mouth.