Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations
Chapter I. General Considerations
Chapter I. General Considerations.
Looking at a map of the earth on Mercator's projection, we observe isolated from all continents and far from the innumerable islands in the Pacific Ocean, two larger islands and some smaller ones running in a south-west and north-east direction, and as my friend Professor Dr. F von Hochstetter so justly observes, "situated almost in the centre of a great continental ring, which, with a rich and varied world of shores, encircles the great Pacific Ocean."*
These islands have, by Tasman, their first discoverer, been named New Zealand, and although consisting of two main islands and another small one, separated from the South Island by Foveaux Straits, and called Stewart's Island, they must nevertheless be considered as forming a whole. A large longitudinal mountain chain running from south-west to north-east, forms the axis; beginning at the south-western end of the South Island, it runs to the East Cape of the North Island, separated by a broad gap formed by Cook's Straits New Zealand thus forms a remarkable line of elevation in the Pacific Ocean, unconnected with any other continent or island, self-existing and independent in position, in fauna and flora, as if the creative Wisdom had thus indicated to mankind the high position destined for it in the future, under the hand of the Anglo-Saxon race.page 173
The south-west and north-east coasts of the Southern Island present us with picturesque fiords—enormous mountain masses rising here abruptly from the sea, and deep indentations running for many miles inland. The parallel between this coast and the south-western coast of America or those of Norway and Scotland is very striking, and just as South America has an insulated continuation in the Terra del Fuego, so we find Stewart's Island separated from the Southern Island by Foveaux Straits. May we not therefore attribute to one common cause the fact, that nearly every continent has been devastated by some destructive force coming from the West, until it has been arrested by huge mountain chains, forming a saving barrier to the low lands lying at their eastern base?
This longitudinal chain, appropriately called the Southern Alps by Captain Cook, reaches its highest elevation and greatest development in the Provinces of Canterbury and Westland, its principal watershed forming the boundary line between them. Extensive fields of perpetual snow repose on its slopes, from which large glaciers descend far into the valleys. I may also mention, that in the adjoining Province of Otago to the south—Mount Earnslaw and other peaks, and in the Northern Province of Nelson—the Spencer Mountains rise considerably above the perpetual snow-line, where also true glaciers, although of smaller extent than in the Southern Alps proper, take their origin. Numerous diverging ranges of large extent branch off from this stupendous chain, running towards the east coast, and when reaching the western foot of the Canterbury plains, are still of considerable altitude (six to seven thousand feet). As before mentioned, the divergent ranges or secondary ridges on the western side are far shorter, the Alps falling here abruptly towards the coast. Although the form and altitude of the divergent chains, branching off from the Southern Alps have been greatly modified by the effects produced during the great Ice Period of New Zealand, the position of their valleys, their direction and peculiar main features may be traced to abyssological disturbances having taken place during a much older period in the earth's history, and of which I shall treat more fully in the geological portion of this publication. However, a few characteristic points bearing upon the physical features of the country may here be shortly noticed, as, without doing so, it would be difficult to understand the structure of the chain under consideration.
As pointed out by me in previous reports, the Southern Alps consist almost entirely of stratified rocks of palæozoic age, thrown in huge page 174steep folds, which have been so much denuded, that the synclinals or lower portions of the folds now form the summits of the mountain ranges, whilst the valleys are generally formed along the anticlinals or saddles. In some instances, the valleys run, at least for some distance, with the strike of the beds, but in others, diagonally or across it, and we have to search, therefore, for some other cause by which this peculiarity has been brought about.
Whoever examines our mountain ranges must be struck by the enormous waste which is going on without interruption amongst them. There are mountains 7000 to 8000 feet high, which are covered with angular fragments of rock from summit to foot, and from which only here and there strong buttresses of rock prominently stand out, more or less split up on their surface. In examining these angular fragments (usually called shingle in New Zealand, whence the expression " shingle slips " for these huge taluses of debris covering the mountain sides), we find that they have a polyhedrical shape, their planes being sharply defined, and cutting, as seen in the conglomerates, clean through even the hardest pebbles of which the latter are composed. If, on the other hand, we examine the rocks in situ, either on the mountain sides or in the river gorges, which in many cases have been cut through only in comparatively recent times, we observe that all these rocks are jointed in a very striking manner. These joints run in such various directions, and appear so conspicuously, that in many instances they conceal the original stratification altogether. The latter occurrence is chiefly observed where large series of very thickly bedded sandstones form the ranges. It is well known, and has been practically proved, that joints are the result of great pressure exercised upon the rocks, the joint having been formed at right angles to the direction of the pressure.
In the geological portion of this report, I shall return to this interesting subject, but wished to refer to it here so far as the physical features of the country have been influenced by this peculiarity in the structure of the rocks. There are mostly five distinct sets of joints observable, of which some are more or less conspicuous. In many localities, one of the principal joints generally appears at right angles to the stratification, the others passing in various but well defined directions. It is evident that the internal forces by which these joints have been formed must have acted from various directions, sometimes nearly opposite to each other. Mr. E. Dobson, C.E., when laying out the Otira road, made some important observations on page 175the subject, in that district, and I may here be allowed to quote from his valuable report,* the principal passages having reference to the subject:—" The next feature to be noticed is the jointed structure of the rocks. Although they cross each other in all directions, apparently without order, there are two systems of joints which are to be met with throughout the whole valley of the Waimakariri, and which have an important influence on the configuration of the Passes. These are—(1st) A system of vertical cross joints at right angles to the stratification, and running in unbroken lines for great distances with such regularity that they might easily be mistaken for planes of stratification, were it not for the frequent occurrence of beds of trap rock, the outcrop of which marks unmistakably the true bedding. (2) A system of joints more or less inclined to the horizon, not running in parallel planes, but arranged in a series of curves radiating from a common centre. The effect of this system of jointing is to produce a rectangular arrangement in the plans and sections of the ravines; the rivers and water-courses running either on the strike of the beds or in the direction of the cross joints, or in a zigzag course, following alternately these two directions, as in the annexed sketch (fig. 2), which shows the character of the valleys connecting the Waimakariri and Teramakau, which have generally a northerly direction, thus making an angle of about 22 deg. with the strike of the strata. In consequence of the vertical position of the strata, and the inclined position of what maybe called, for convenience of description, the horizontal joints, the sides of the ravines present either sheer precipices or dangerous slips, according to the extent to which the rocks have been loosened by exposure to the rain and frost; and the result is, that to form a road inside cutting through any of these ravines is simply impossible."
* Report to the Secretary for Public Works upon the Practicability of Constructing a Bridle Road through the Gorge of the Otira, and upon the character of the Passes through the dividing range of the Canterbury Province. By E. Dobson, C.E., Engineer of the Lyttelton and Christchurch Railway. With forty illustrations and an appendix.
For those of my readers who are not acquainted with Mr. Dobson's able Report, I have annexed a copy of his ingenious map, on which all the principal features in question are clearly indicated. It is true, as before observed, that the Hurunui-Teramakau line shows this radiating structure most strikingly, but all the other valleys, although not so clearly indicated, have similar features: thus, to give a few instances—there is Harman's Pass at the head of the Waimakariri, leading into the Whataroa; and the deep depression between Mount Cook and Mount Stokes, the direct continuation of the Tasman river and Hooker glacier valleys. A similar well defined opening is the alpine saddle between the Moorhouse range and Mount Holmes, at the head of the Dobson river; whilst Docherty's Pass, at the head of the Clarke river, might be pointed out as continuing the direction of the makaroa line over Haast's Pass to the starting point in question.page break
The general structure of the Southern Alps reduced from a map by E. Dobson C.E.