COASTLINES AND HARBOURS
The outline of the mountainous islands of New Zealand reflects the complexity of their structure and later geological history. The coast, as usually defined, is a zone of indeterminate width adjacent to the sea; the line where land and water meet is better defined as the coastline or shoreline. The kind of country against which the sea comes to rest in New Zealand shows remarkable diversity in short distances, the most striking contrasts being those between mudstone uplands readily trimmed off into long lines of cliffs (as in the North Island east and west), and the hard rock highlands, especially where mountain ranges run end on into the sea (as in Cook Strait, eastern Bay of Plenty, and the South Island south-west). Elsewhere long stretches of very simple shoreline mark the edges of alluvial plains built out into the sea (as in Canterbury, Westland, and Manawatu-Horowhenua).
Moreover, New Zealand is a heterogeneous assemblage of crust blocks that have not been moved by the young mountain-making processes to the same extent in all parts of the country. As some have moved upwards and others downwards in late geological time, contiguous parts of the coast may show quite different and unrelated features. The Marlborough Sounds, for example, seem to be contained in a unit area depressed to drown a complex system of branching river valleys, while adjacent areas affected by upward movement have relatively simple shorelines. So a special and striking feature of the outline of New Zealand is its diversity; even small unit areas may display considerable local variety. In spite of all this diversity, however, there are few good natural harbours with ready access to the more closely settled and productive parts of the country.
In the narrow Auckland peninsula, the north-east to south-west grain of the country is, in general, parallel to the shoreline which has been affected by extensive submergence. This has produced a system of branching bays and inlets now occupied by tidal mudflats and bounded by bold headlands. The western coastline has been profoundly modified by the building of sand beaches and spits, this having the general effect of making entry to the inlets very difficult. Inlets of the east coast, not so affected by alongshore sand drift, are deeper and more widely open, giving much better and more accessible sites for making good harbours for ships. The contrast in kind between east and west is well displayed by the Waitemata and Manukau Harbours; on the former Auckland has grown as New Zealand's major port; the latter remains inaccessible and undeveloped. The other main centres of harbour development on the Auckland east coast are at Whangarei and Tauranga. Access to the latter (at Mt. Maunganui) is by a narrow gap in the beach barrier built right across the front of a wide and shallow bay.
From East Cape to Cape Palliser the coastline is, for the most part, a line of cliffs made by the rapid trimming back of the spur ends in a recently elevated weak-rock hill country. This kind of coast is never easy of access from the sea; the main centres of local settlement on it are the valley plains at Napier, Wairoa, and Gisborne. At Napier and Gisborne artificial harbours have been made.
Similarly most of the North Island to the west ends at the sea in cliffs (except for the dune-fringed plain of Manawatu-Horowhenua), and the coastline is unbroken by any major inlet from Porirua to Kawhia. Rivermouth ports, as at Wanganui, Patea, and Waitara, cannot compete with the main western harbour at New Plymouth, built at a point where remnant stumps of a volcano much older than Egmont project into the sea.
New Zealand's main central harbour of Wellington has been made in Port Nicholson. Here local down-warping of the mountain spurs that run end on into Cook Strait has made a wide and sheltered arm of the sea, but a serious disadvantage of this as a natural harbour site is the difficulty of access to the hinterland.
In the South Island the Cook Strait coastline transects the grain of the country. In the drowned sector of the Marlborough Sounds is the port of Picton, but its local hinterland is limited by the topography. Its main role is one of a route terminal for the South Island main trunk road and railway, and for the new Cook Strait steamer service. The heart of the South Island in the north is rather at Nelson and the Tasman Bay lowland. The margin of this is a complex of the cliffs of the Moutere Hills and the sand and shingle beaches of the Waimea Plain enclosing extensive lagoons. Here the port of Nelson has been built to service the north coast settlements. This north coast is one of abrupt contrasts between the rugged ends of the mountain ranges, the edges of the bayhead plains, and the long curving sandbank of Farewell Spit partly enclosing Golden Bay.
Despite all the diversity of land on the West Coast, alongshore drift of shingle and sand with the strong northerly current makes the shoreline remarkably even all the way from Martins Bay to the Farewell Spit. At the river mouths, too, off-shore bars make entry from the sea difficult and uncertain.
Fiordland, in the far south-west, is a distinctive topographic unit with its long, narrow arms of the sea filling deep trough-shaped valleys that are a legacy of the ice ages. Here are natural anchorages in plenty, but flat land at the heads of the fiords is so limited, and access to any productive hinterland so difficult, that there no good sites for the making of ports.
In the rest of the southern South Island, the coastal country changes abruptly in short distances, one of the most spectacular shorelines being that between the mouths of the Mataura and Clutha Rivers. But on all this southern coast the only two harbour sites of any significance are those at Bluff and Dunedin. Behind the isolated hard rock mass of Bluff Hill and the sandbank enclosing the adjacent shallow estuary, the harbour of Bluff has lately been rebuilt. Its hinterland is the whole rich province of Southland.
The Otago Harbour is a long, narrow arm of the sea made by the tying of a rocky island to the mainland by the sand bank on which South Dunedin is built. Port facilities are at Dunedin itself at the head of the inlet, and Port Chalmers some nine miles nearer the entrance.
On all the rest of the east coast of the South Island, the only other extensive sheltered waters are in the embayments of Banks Peninsula. Elsewhere the coastline is remarkably uniform though the kind of coastal country shows much variety. The main harbour is that of Lyttelton but it is not a very good natural site for a port; access to the neighbouring plains is not easy, and making the port itself has involved the building of costly breakwaters.
On the open east coast are ports at Oamaru and Timaru. The former is sheltered somewhat in the lee of Cape Wanbrow, but making the latter has required the building of a big breakwater. This has locally halted the alongshore drift of shingle and behind it the popular sandy beach of Caroline Bay steadily grows. The front of the Canterbury Plain has been trimmed off by the sea, and the shingle, drifting north along the shore, makes the big barrier enclosing Lake Ellesmere. North of Banks Peninsula sand accumulates in the sheltered Pegasus Bay; here on a dune-fringed shoreline extending for some 30 miles almost to the Waipara River, are several popular beach resorts.
On the north-east coast of the South Island are long straight lines of weak-rock cliffs, but occasionally (notably near Kaikoura) hard rock hills plunge steeply into the sea; here main road and railway alike cling precariously to the very edge of the land. In general, however, this is a coastline of recent elevation unbroken by any deep inlets.