The Northern Region
Just as the great Alpine fault divides east from west, the Hope fault seems to mark off the north as something distinct from the rest of the South Island. Following the line of the Hope tributary of the Waiau River, and the middle Waiau itself to Hanmer, it swings round north-east along the line of the inland valley route to Kaikoura. All the country north of it is mountainous, penetrated by deep and narrow fault-angle valleys with isolated pockets of coastal lowland about its ragged margin. These are the centres of almost all the closer settlement, especially in Golden Bay (Takaka), Tasman Bay (Nelson and Motueka), the Wairau Plain (Blenheim), and the lower Awatere (Seddon). Along much of the north-east coast, both road and railway cling to the base of the mountains, with the only considerable pocket of valley lowland at Kaikoura.
The north-west, containing the Mt. Arthur (5,834 ft) plateau and the Tasman Mountains, is particularly rugged and inaccessible. The narrow valleys of its Takaka, Anatoki, Aorere, Heaphy, and Karamea Rivers give ready access only to the fringes of this highland granite country.
The mountains of the north-east stand out more sharply as unit ranges because they are separate crust blocks bounded by the faults that guide the main rivers. Most spectacular are the two Kaikoura Ranges, with Tapuaenuku rising to 9,465 ft. Most prominent of the valleys, too, is that of the Wairau along the line that takes the Alpine fault itself north-east to the sea. With the Waimea fault (at Nelson), this encloses the triangular zone containing the Marlborough Sounds country of hard-rock hills (greywacke and schist). Local subsidence of this whole zone has drowned the intricate pattern of its valleys to make Pelorus and Queen Charlotte Sounds with their many branching inlets. This typical ria coast has become a very popular holiday resort.
In the west, the catchment of the Buller is well known for its local diversity of scene, especially for the glacial lakes of Rotoroa and Rotoiti at its head and for the several gorges it has cut across a succession of mountain ridges. But the high country of the rest of the South Island north seems to be not so widely known because so much of it is difficult of access. Main roads to Westport cross it only by the Lewis Pass route from Canterbury, by the Wairau-Buller route from Blenheim, and by the Tasman Bay lowland – Buller valley route from Nelson. The main highway from Christchurch to Nelson clings to the coast for most of the way.
The Midland Zone
A section across the South Island from, say, Christchurch to Hokitika would show how the central hard-rock highland is compressed to a narrow waist and bordered by the piedmont lowlands of Westland and the Canterbury Plain. Its front ranges rise from the inner edge of the Canterbury Plain like a wall. In North and South Canterbury, however, the inner highland opens out into range and basin forms with hard-rock ridges rising above weak-rock downlands, or enclosing basin plains and valley plains made of the ubiquitous greywacke gravel.
In the landscapes of the midland zone these quite distinctive units are recognisable at once. The hard-rock highland, 50 miles or more wide, rises gently west to the main axis of the Southern Alps to be cut off abruptly by the great Alpine fault. The Canterbury Plain is an apron of shingle spread out in front of the highland as huge coalescing shingle fans built by the big rivers that cross it. In the downlands of North and South Canterbury are discontinuous patches of the weaker (Tertiary) rocks, above which lower ranges of the hard-rock understructure stand out in bold contrast. Banks Peninsula is an entirely separate unit, an elliptical dome of basic volcanic rocks (mainly andesites and basalts) erupted from two centres at Lyttelton and Akaroa, to be since tied to the mainland plain by a neck of low swampy country. West of the Alpine highland is the piedmont lowland of Westland, reaching north into the wide open valley of the Grey River behind the coastal Paparoa Range. Each of these topographic units has its own characteristic landscape features.
With the exception of a schistose core of the Southern Alps proper, the Canterbury high country is made of tough greywacke and more friable argillites. This is the stuff of the screes of waste that mantle the bare tops of the ranges and pour down into the valley bottoms to be taken away by the rivers in flood to make the bordering plain. The size of the plain and the thickness of its gravels (over 2,000 ft in the old oil bore at Chertsey) is some measure of the amount of erosion. Narrow branching valleys (locally known as gorges) have been cut deep in the highland. Thus the high country is a maze of ridges and valleys; a peculiar uniformity of level of ridge summits suggests a possibility that the whole complex of ranges has been made by the carving up of an old plateau.
Ice has left its mark by straightening valleys, smoothing their walls, and leaving scattered remnants of old moraines and glacial lakes. The big rivers Hurunui, Waimakariri, Rakaia, Ashburton, and Rangitata drain this high country by a maze of streams that have carved out deep narrow valleys. Ice filled them all but a few thousand years ago; floods of water from its rapid melting spread coarse gravel over their floors and out on to the bordering plain. This country, clothed in tussock on the front ranges and beech forest inland, is the land of the high-country sheep runs. Easily accessible only by the valleys of the major rivers, it has become a mountain playground.
The Canterbury Plain, 120 miles long and some 40 miles in greatest width, is a vast apron of greywacke gravel. The rivers, which made it out of the waste from the valleys they carved in the highland, now flow across it in terraced trenches, each let down into its own big alluvial fan. Its surface rises fairly steeply inland, at a grade of 30 to 45 ft or so per mile, to heights of over 1,200 ft at the base of the bordering ranges. Here are Canterbury's richest farmlands, varying much in quality according to the thickness of the finer silts that cover the raw river gravels.
North and south the plain merges into downland country. The South Canterbury downlands, reaching from Geraldine to Oamaru across the Waitaki River, seem to be the remains of some older surface planed out in front of the hard-rock ranges Those of North Canterbury, however, are of a more distinctive range and basin type. The biggest of the basins is that of the Waiau-Hurunui about Culverden. The rivers here flow east to the sea across a series of hard-rock ranges, leaving the basin floors filled with greywacke gravels. Remnants of Tertiary rocks with prominent bands of harder limestones lend further variety to the North Canterbury scene.
Banks Peninsula stands apart as a volcanic pile, built up on a base of the same greywacke as makes the central highland. Its twin volcanoes (at Akaroa and Lyttelton) built up a dome-shaped mass with rather gentle outer slopes and a maximum height of 3,014 ft (Mt. Herbert). Ages of stream erosion have left a radial pattern of deep valleys that have since been drowned by the sea to make a coast of intricate outline. The biggest of the drowned valleys are those made from the breaching of the old craters of Akaroa and Lyttelton.
The piedmont lowland of Westland is a complex terrain. Granite ridges west of the Alpine fault, dumps of moraine spread right out to the sea by glaciers of the recent past, and re-sorted gravels of the terraced river valleys are all distinguishable in it. Most of the lowland is cutover forest country; its primitive cover was rain forest with open patches of pakihi land and extensive swampy zones. North of Greymouth is the coastal Paparoa Range, mantled on parts of its crest and along its flanks by the younger (Tertiary) beds that contain much of the West Coast coal. Inland of the coastal range is the wide open valley lowland of the Grey River.
The Otago Range and Basin Country
The pattern of the mountains of Central Otago and inland South Canterbury is best seen from the air, or from the summit of such a range as the Benger near Roxburgh or the Dunstan near Cromwell. Heights are very uniform and skylines very even; the whole mosaic of mountains appears to have grown out of the raising, warping, and faulting of the ancient plateau. Individual ranges are simple blocks enclosing wide open basin plains or long narrow fault-angle valley plains. The simplest form is the tilted block with one gentle back slope, a steep scarp face, and a curiously even summit line. Occasionally a range (e.g., the Rock and Pillar) may have both its faces as steep scarp slopes because it is bounded by faults on both sides, taking the form of a horst.
The rocks of Central Otago are the monotonously uniform schists, laminated in structure and flaking away on exposure to weather. But scattered tors of tougher material may give local surfaces a very rugged aspect. The schist passes into greywacke north of the Waitaki River.
The largest of the inland basins are the Mackenzie in the upper catchment of the Waitaki, and the Maniototo drained by the Taieri. Indeed, almost all the true range and basin country is contained within the catchments of the Waitaki, Clutha, Taieri, and Shag Rivers. Their valleys are the main routes of access to Central Otago, but the traveller by road, confined to the valley floors, sees little of the pattern of the mountains. The 12-mile-long gorge, cut by the Clutha across the Dunstan Range between Cromwell and Clyde, is typical of several of its kind in this country – of mountain ranges that have been raised as independently moving crust blocks across the paths of rivers. Below the gorge the basin floor, too, has obviously had something of a complex history. High terraces near Alexandra are remnants of an older, higher basin floor; the history of the basin seems to have been one of successive filling and re-excavation by the river. This would be connected with the waxing and waning of floods from the melting of the ice that once filled the upper valleys about the lakes Wakatipu, Wanaka, and Hawea.
The Mackenzie basin, especially, shows marks of the glaciation of the recent past. In it are the big glacial lakes of Ohau, Pukaki, and Tekapo; on its floor are moulds of moraine, just as the retreating ice has left them. Elsewhere the waste has been respread and re-sorted by melt water floods, leaving the present lake outlet streams entrenched in stretches of more level plain. Standing above the basin floor, too, are hard-rock mounds and ridges (e.g., Mt. John at Tekapo) scraped smooth by the ice that rode over them but a few thousand years ago. The present Tasman Glacier, about the base of Mt. Cook, is but a puny remnant of the ice that once filled this biggest of our intermontane basins.
The range and basin country is topographically unique. The pattern of some of its characteristics forms and the story of their making was first clearly described in Cotton's classic paper on the region. The country is unique, too, for its semi-arid climate. Its primitive tussock cover has been much depleted since European settlers took it over. Today some of this country looks like a desert, though with control of the rabbit infestation the tussock and grass cover seems to be slowly coming back.
For long the focal point of interest in this mountainous country of the far south-west has been Milford Sound. Access to it is by way of the Milford Track from the head of Lake Te Anau, by the Eglinton Valley – Homer Tunnel road, or by sea from the west. By whichever of these routes he takes, the visitor sees little more than the valleys in which he is confined. These are deep, narrow troughs with very steep sides and flat floors that end abruptly inland in high rock walls. This is the typical form of mountain valleys shaped by ice.
The whole sweep of the Fiordland terrain is seen only from the air, and modern aerial photographs show its maze of mountain ridges to be remarkably uniform in height. It seems to be an ancient plateau rising abruptly from the Waiau Valley in the east and from the Tasman Sea in the west. The sea is very deep close inshore and the western spur ends are trimmed off to a straight line by the Alpine fault.
So Fiordland seems to be a very distinct topographic unit, a high plateau in which deep valleys have been carved and later shaped by ice. The “sounds” are true fiords and would be more properly called such; they are glacial valleys, shaped and deepened by ice scour, and drowned by the sea as the ice melted away. The narrow arms of the Lakes Te Anau, Manapouri, Monowai, Hauroko, and Poteriteri that penetrate the eastern edge of the plateau are identical in form; they might aptly be called fresh-water fiords.
Geologically this country is one of the oldest parts of New Zealand. It is made mainly of coarse gneisses and intrusive igneous rocks with some very ancient sedimentary beds (Ordovician) and a few Tertiary deposits on the margin, notably about Preservation Inlet. Stewart Island is made of similar old hard rocks, and Foveaux Strait, which separates it from the mainland, is little more than 100 ft deep. Stewart Island is not so high as Fiordland, its highest point (Mt. Anglem) being only 3,214 ft.
Variety of landscape forms in Southland reflects a somewhat complex structure; it is here that the main mountain axis swings round to the south-east. The mountains, made mainly of greywackes and similar old, hard sedimentary rocks with occasional outcrops of intrusive volcanics are, however, not very high. There are some extensive but scattered areas of the younger Tertiary beds, too, in undulating hills and downland. In these are found the valuable Southland coals and limestones. But it is the large alluvial plains made by the Mataura, Oreti, Aparima, and Waiau Rivers that comprise the productive heart of this rich province.
Specially prominent in the west are the Takitimu and Longwood Ranges, and the Hokonuis stand out as lower hard-rock hills above the valley plain of the middle Mataura River. In the south-east again, in the triangle between the lower Clutha and Mataura Rivers and the coast, is a zone of rugged hard-rock hills. The road from Fortrose to the Catlins reveals an attractive variety of coastal scene where the sea comes to rest against the base of these.
The plains of Southland are much the most significant features of the local landscape. They vary in kind and in quality from the stony stretches in the upper basins of the Oreti and Aparima to the rich alluvial lowlands in the middle and lower courses of these and the Mataura Rivers. Here are some of the richest farmlands of New Zealand. Yet again, however, they change to the poor Awarua swamplands just east of Invercargill. From Balclutha to Gore a wide belt of gently undulating lowland takes the main road and railway routes north. Here, too, is a landscape of rich farmland.