The Northern Peninsula
This part of New Zealand is very different from the rest of the country; its “grain” is north-west to south-east for it belongs to the complex island arc that comes in from the direction of New Guinea and New Caledonia. Its broad base, or root, is buried deep beneath the volcanic material of the inland plateau but, narrow and very ragged in outline to the north, it is almost severed in two at the site of Auckland city. Within its wider base are the lesser peninsula of Coromandel and the open valley plains of the Waikato-Waipa and Waihou-Piako river systems. These alluvial plains are the productive heart of the peninsula.
Variety in a short distance is the keynote of the peninsula terrain, and surface forms reflect the nature of the local rocks of which they are made. Rocks of the hard greywacke understructure stand out above residual patches of weaker beds of the Tertiary cover in scattered ridges of steep and rugged aspect, but these are only of moderate height, indeed, no part of the peninsula territory is as high as 3,000 ft. Scattered patches of volcanic rock, too, give local variety of surface forms as in Coromandel Peninsula, Pirongia, the Auckland city area, and throughout the narrow north. North of Auckland city the only alluvial plains of considerable size are those about Ruawai and Kaitaia.
In the north-west, however, and in the narrow neck, or tombolo, that ties distant Reinga to the mainland, coastal forms are made of older sands, raised, consolidated, and eroded. Newer sands, still on the move, pile up in coastal dunes in many places and the only effective way of stabilising these is by the planting of marram grass and trees. Alongshore drift of sand has made this west coast remarkably straight by comparison with the east with its wide branching bays and rocky headlands.
In the wider base of the peninsula there are strong topographic contrasts between the rugged hills of the west, the terraced lowlands of the river valleys, and the Kaimai-Coromandel ranges in the east. The valley plains are mainly made of pumice sands and silts, with extensive beds of peat. In all the peninsula there is no sharper contrast than that between the Kaimai-Coromandel upland and the low Hauraki Plains from which it rises abruptly in a steep fault scarp. This remarkable plain, only a little above the surface of the sea, has been made by the filling of a once much larger Firth of Thames on the floor of a fault-bounded trough or graben.
The Mountain Axis and East Coast Hills
By contrast with the northern peninsula, this eastern part of the North Island is aligned north-east to south-west. It carries the mountain structures of the South Island from Cook Strait to the Bay of Plenty, and the ranges of its high, hard-rock core dominate the North Island scene. These ranges are a complex of faulted blocks, the pattern of faulting becoming extremely complex in the north-east. A convenient, if perhaps somewhat arbitrary, western boundary of this zone might be set at the line of the Kaingaroa fault.
This is the North Island high country; though its mountain summits rarely exceed 5,000 ft, they stand out boldly above the weak-rock hills about their base. The weak rocks, especially displayed in a wide belt of hilly country on the eastern flanks of the axis, are mainly mudstones. But interbedded with them are harder bands of sandstone and limestone which were tilted when the whole of this eastern zone, caught up in the mountain making, was warped and broken by faults, mainly with a north-easterly trend.
Thus the whole pattern of the resulting landscape is closely related to the structure of the country. Rapid erosion of the uplifted land has revealed the hard-rock core and found the harder bands in the covering beds. On these an array of scarpland forms has developed. Long ridges with gentle backslopes facing the sea and steeper scarp slopes inland give a special character to the coastal hill country north of Hawke Bay. Rivers now flow in deep terraced trenches across this country from the higher mountains of the axis that has been exposed by rapid erosion.
From Hawke Bay to Palliser Bay an almost continuous valley lowland lies in front of the Ruahine, Tararua, and Rimutaka Ranges; a lowland drained (and enlarged to its present form) by the Tukituki, Upper Manawatu, and Ruamahanga River systems. Here are the most extensive of the lowland plains of the North Island east. To the east of this lowland again is a wide belt of hill country where local ridges of the hard rock of the understructure outcrop above the eroded cover to give a ragged surface in a land of only moderate height.
Recent studies in the Wellington area show that surfaces on which the weak rock cover was laid and since completely stripped away are still clearly recognisable. The story of the raising, warping, faulting, and eroding that have made the present landscape becomes very clear. One of the most spectacular and best preserved of the great faults that blocked out the pattern of the present surface is the Wellington fault itself, which defines the western edge of the Hutt Valley.
The North Island West
Most of the North Island west of the main divide is of difficult terrain but rather simple structure; when seen from the air, from a high point like Mt. Egmont, or even from Mt. Messenger – which is merely a ridge summit on the main highway between two local river catchments – the Taranaki-Wanganui inland shows a very even skyline. An old land surface seems to rise inland in two main tiers or steps. A maze of steep ridges with their sharp crests rising only gently inland is all that remains of an old plateau form. It has been cut to pieces by all the branching tributaries of the master streams like the Mokau, Waitara, Wanganui, and Rangitikei that drain it. The valleys are deep and narrow, the ridges steep and closely spaced with little summit area. Access to this country in the days of its primitive forest cover was most difficult. Even now the valleys of the master streams are its only easy routeways. Such is the wide open terraced valley of the Rangitikei which takes railway and main highway north.
The rocks of the region are mainly mudstones and sandy beds, but locally there are beds of harder sandstone and extensive thick layers of limestone. One such is the Te Kuiti limestone in which the celebrated Waitomo Caves represent solution effects characteristic of this kind of country. This broad zone of sedimentary rocks seems to have been raised with little disturbance of their nearly horizontal bedding, though there are some major faults and inland places where the hard rock of the understructure comes to or near the surface. Such a weak rock upland in a region of abundant rain would be rapidly and closely dissected by a system of closely spaced branching streams. Where the master streams from the upland cross the coastal zone of lesser height, stretching all the way from the Rangitikei to New Plymouth, valleys are not so closely spaced, but are wide, open, and terraced. Such are the lower valleys of the Rangitikei and Wanganui and the lesser streams parallel to these. Wanganui city is sited on the valley floor of such a channel cut in this mudstone country.
The Manawatu alluvial plain is merely a newer, lower plain built out in front of this rapidly wasting upland. It merges southward into the Horowhenua coast plain, a complex of shingle fans from the bordering mountains, and coastal dunes. The steep and difficult terrain of the west has therefore been made out of the rapid destruction of a two-tiered surface and the building of a new plain at a lower level.
Mt. Egmont, standing distinct and apart, dominates the Taranaki scene. As noted already, Taranaki may be said to owe all it is and all it has to its protection by the hard rock base of this western volcano.
The Volcanic Plateau
The belt of recent volcanic activity with all its associated hydrothermal phenomena is only a narrow strip running north-east from Ruapehu to White Island. The noble mountain of Ruapehu (9,175 ft) and the lesser masses of Tongariro (6,517 ft), Ngauruhoe (7,515 ft), Tarawera, and White Island have piled up round local vents; but other eruptions of a violently explosive kind have spread showers of “ash” and sheets of the harder ignimbrite far and wide, blotting out the features of the former surface. Main centres of these are about Taupo and Rotorua where groups of lakes, large and small, fill hollows in the floor of a big trough-like sag in the plateau surface. Here the country has been much warped and faulted, caught up in the mountain making that raised the eastern ranges.
Most of the rocks of the region are acidic tuffs made of consolidated volcanic ash, mainly pumice. Of special importance are the ignimbrites that seem to have spread out over long distances from vents not now visible, levelling out extensive surfaces like that of the Kaingaroa Plain. It is thought that they must have travelled close to the ground in dense, hot turbulent clouds which, on cooling, consolidated into tough, light coloured rocks. By contrast, many of the more localised ash showers have left thick layers of loose material quickly cut up by deep and terraced stream channels, especially in the upper catchments of the Waikato and Rangitaiki.
So warping, faulting, rapid erosion, together with the spasmodic activity that builds volcanic forms anew, all give diversity of surface form to this inland plateau which forms the heart of the North Island, upwards of 2,000 ft above the sea. Above this level, of course, stand the volcanic mountains. The most spectacular of these is Ruapehu; any traveller by the main north road from Taihape to Waiouru can see the landscape change abruptly as the mudstone country of the upper Rangitikei passes beneath the volcanics of its broad base. It dominates the North Island inland.
The volcanic plateau today is of special interest for its recent and spectacular settlement and development. Long thought to be more or less worthless on account of its mineral deficiency and porous pumice soils, it has been the site of large-scale plantings of exotic forests. Moreover, new methods and new fertilisers have made grassland farming possible. All this, together with the harnessing of the Waikato River water and the geothermal steam of Wairakei for generation of electric power, has brought new prosperity to country that only a few decades ago was mostly a scrub-covered wilderness.