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Chapter IV. Plains.

On both sides of the Southern Alps, and amongst the secondary ridges, plains, varying in size and character, have been formed by the disintegration and destruction of the rocks, from which this gigantic mountain system has been built up, the detritus being brought to lower regions, first by glaciers, and secondly by fluviatile action. As previously pointed out, when speaking of the orographical features of the country, the action of the huge glaciers formerly existing, has had a most beneficial effect in preparing the surface for a dwelling place of the human race, and for following the civilizing pursuits of agriculture and manufacture. Such useful results are especially apparent in this portion of New Zealand, where, without the occurrence of the Great Glacier period, the sea would still beat against the out-running spurs on both sides of the Southern Alps, instead of washing against the low shores of broad and fertile plains, by which the great mountain chain is now fringed. Owing to the peculiar configuration of our Alps, and the effects of sea currents and tides, the largest portion of these plains is situated on the eastern side of this island. The most prominent amongst these are the Canterbury Plains, which, by their position, nature, and general characteristics, form a prominent feature in this island, and are already the centre of a rich, industrious, and large population. They begin at the dolerite plateau of Timaru, and stretch without interruption to Double Corner, in a general direction from south-west to north-east, with a length of about 112 miles. Their breadth from a few miles at both extremities, page 232north and south, augments as we advance towards their centre, having their greatest lateral extension near Banks' Peninsula, where, in a direction from east to west, they stretch a distance of nearly 50 miles to the base of the mountains. The Pacific Ocean is their boundary on the eastern side, where a long shore stretches in a line nearly from south-west to north-east—from Timaru to Double Corner being only interrupted by the volcanic system of Banks' Peninsula, which rises so conspicuously in the middle of that low shore, and to the existence of which a great portion of the loose strata composing these plains mainly owes its preservation from the destructive agencies of the waves and currents. The western boundary is formed by the outrunning spurs of the Southern Alps, having, as I have pointed out previously, by their disintegration, offered the material for the present configuration and other physical features of those plains. In their greater bulk they consist of the accumulation of post-pliocene torrents. Having had their glacier sources much nearer to Banks' Peninsula than in present times, the latter were able to throw the boulders, shingle, sand, and ooze, carried along with them, not only in greater masses but also on steeper slopes than the present rivers crossing them can do, for reasons given in the geological portion. These post-pliocene deposits of huge rivers, have covered with an almost uniform gradient the palæozoic, volcanic, or tertiary rocks, composing here the former sea bottom.

On my arrival in Canterbury, I was informed that the Canterbury plains had only such a slight gradient, that at the foot of the mountains they barely reached an altitude of 300 feet, and I was there fere somewhat surprised when the barometer revealed to me the fact, that this was a popular error, and that the upper portion of these plains was at least 1500 feet above the sea-level, having thus a fall of about 45 feet in the mile, or 1 in 117. Owing to causes, fully to be explained in the geological portion of this Report, the large rivers which built up their huge shingle-fans side by side, afterwards lowered their beds in course of time, so that at the intersection of the fans with the present river-bed, new shallow fans were, and are still, being formed. Below this line of intersection, some of the finest and most fertile agricultural land in Canterbury is situated. Although the Canterbury plains terminate at the sea coast near Double Corner, they continue across the middle Waipara for ten miles further north, being first about four miles broad and gradually getting narrower, ending at the Omihi saddle, which leads into the Waikari, a tributary of the page 233Hurunui. South of Timaru, a belt of fine alluvial land, of an average breadth of seven miles, which may be considered the southern continuation of the Canterbury plains proper, reaches to the Waitaki. It consists of plateau-like downs, having an altitude of about 600 feet near the foot of the ranges, by which it is bounded on the west. Gradually it falls to about 100 feet a few miles from the coast, where it forms a line of small inland bluffs. Numerous creeks have cut broad channels through the incoherent strata, of which these downs are composed. From their eastern termination to the sea coast, a belt of low lying, rich land, with a series of lagoons close to the sea, is situated, also extensively used for agricultural purposes. A glance at the Geological Map attached to this Report, will show us that these downs are simply the lower portion of the old Opihi valley, and that before that river broke through the small palasozoic ranges, the large river issuing from the extension of the Waitaki-Opihi glacier in the Great Glacier period, followed a southerly course to the Pacific Ocean. This river, breaking through the huge moraine across the Opihi valley below the present gorge, followed a south-by-east course, first receiving the Tengawai, then the Pareora, and reaching the sea coast after its junction with the waters of the Otaio. In this broad opening, the rich alluvial lands of the upper Otaio, Pareora, and Tengawai are situated. North of the Opihi gorge, this plain continues for 12 miles in a northerly direction, being at one time after the retreat of the great glacier, without doubt, a lake, which was gradually filled up by the alluvial deposits of the Opihi and Opuha branches, and contains now some very good agricultural land.

Near the mouth of the Waitaki, the shallow alluvial fan of that river is partly preserved, continuing about eighteen miles inland and having, on the Canterbury side, an average breadth of three miles. West of the township of Waimate, following the small Waimate Creek, and after passing through the Coast range, we reach the Waihao plains. It is evident that, before the Waihao river had cut its present channel, a lake was here situated, having gradually been filled up by the deposits brought down the Waihao. The last remnant of this lake is still to be found on the flat, now drained by the Waimate Creek. There is also some fine agricultural land in this basin. Ascending the Waitaki, and crossing the small gorge at the junction of the Hakataramea with that river, we reach the Hakataramea plains. Here on the western side of the river for nearly twenty miles, older alluvial deposits, along which the river has excavated its present bed, have been preserved. They are, on an average, about three miles broad and are all fit for the plough.

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The Mackenzie plains are, with the exception of the Canterbury plains, the largest in the two provinces under review. However owing to their great altitude, sloping from 3000 feet to about 1300 feet above the sea level, the lower portion only is available for agricultural purposes. Great have been the changes which took place before this portion of the country could assume its present configuration. The upper portion of these plains is formed by morainic accumulations forming large ridges running parallel to the course of the rivers above the lakes, and afterwards encircling the latter to a breadth of several miles. Below them, alluvial beds deposited by the huge torrents issuing from the glaciers, form shallow fans into which the rivers, as they laid their beds lower, have excavated a series of terraces on both sides. Ice-worn hills and small ranges rise here and there amongst the morainic and fluviatile beds, often with enormous erratic blocks on their summits and slopes. I wish finally, (omitting a number of smaller plains or basins on the eastern side), to allude to the Waikari-Hurunui plains which, with the Waiau-ua plains, form a large basin about thirty miles long and six miles broad. They are also the former bed of a large inland lake, in course of time, filled up by alluvial deposits brought into it by a number of rivers, and of which the deltaic deposits (or shingle-fans) can still be traced. Here also, some fine agricultural land is met with.

In Westland, along the coast, a fringe of more or less level land is situated, of which the triangular area stretching into the Grey plains across the Arnold river, is the largest and most important. It generally consists of older alluvium in which the richest of the Westland Goldfields are situated, and of younger morainic accumulations and alluvium formed by the rivers once issuing from the extended glaciers. It forms a table-land rising gradually to 800 feet, above which a number of hills, consisting of tertiary strata, rise a few hundred feet more. It is generally densely wooded, so that to prepare it for agricultural or pastoral purposes, a great deal of work will be required. Thus it does not possess the same advantages with which the downs and plains on the eastern side of the Southern Alps are endowed, where in most cases the virgin soil can be ploughed at once without any further preparation. We have, however, to except some small area north of the Taramakau to Lake Brunner, the so-called Pakihi or Paddock, and a portion of the Hokitika plains, where some good grass land is met with.

A low alluvial plain stretches from the Tauperikaka, eight miles north of the River Haast, to Jackson's Bay, with a total length of thirty page 235miles and an average breadth of four miles. It is covered with dense forests from which, in course of time, a large quantity of valuable timber will be extracted for exportation. Everywhere amongst the valleys, descending from the Southern Alps along the West Coast, a great deal of land will in years to come, be used by the settlers for farm purposes, principally when the mining industry on coal, gold and other precious metals has become more developed, and a larger permanent population has settled in that fine district.