One of the chief charms of the New Zealand scene is its infinite variety. Such level lowlands as exist are small in area; contrasts between coastal plain, valley plain, basin plain, and bordering hard-rock mountains are abrupt. High mountains make up most of the South Island area – hard-rock highland often stark and bare or mantled in permanent snow. By contrast, most of the North Island is weak-rock hill country mainly made of the ubiquitous mudstone locally known as papa. In this, however, thick tilted bands of harder sandstone or limestone may stand out as higher ridges in the upland that has been rapidly wasting away. From Cook Strait to the Bay of Plenty a hard-rock mountain core dominates the North Island scene, forming an effective barrier between east and west; the only low level gap across it is at the gorge cut by the Manawatu River near Palmerston North.
A peculiar and special feature of the North Island is the volcanic country of the interior. Here volcanoes have erupted on a vast scale, Lake Taupo being the centre of some of the most violently explosive of these, and the Taupo ash showers the greatest of those that mantle much of this volcanic interior. Here, too, are the largest North Island lakes and, in a line from Ruapehu to White Island, most of the still active volcanoes, hot springs, geysers, and other hydrothermal phenomena.
The most spectacular mountains are in the South Island; high mountains, deep and narrow valleys, swift rivers, and glacial lakes large and small give infinite variety to the scene. It is in this high country that ice has left its special mark in glacial troughs, arêtes and horns, fiords and, above all, the noble southern lakes. There is little weak-rock hill country in the South Island; the lowlands are mainly bordering plains, basin plains, and valley plains almost always built of shingle brought down from the wasting mountains by the turbulent rivers that were fed by melt waters as the ice of the recent past rapidly disappeared. Of these the most extensive are the plains of Canterbury and Southland.
As the ice sheets waxed and waned and still active mountain-making forces raised or lowered the land margin more in some places than others, sea-level changes also marked the recent past. A legacy of all this is a great variety of coastal forms, especially the deep fiords of the far south-west, the sounds of Marlborough, the down-warped basin of Wellington Harbour, the branching bays of North Auckland and the deeply embayed Banks Peninsula – all testify to local drowning of the land margin. Elsewhere the sea comes to rest against the base of mountains parallel to the shore or of high cliffs cut in the bordering mudstone uplands. Here long sweeps of coastline may be straight and simple; it is where hard-rock mountains run end on into the sea as in Cook Strait and parts of the Bay of Plenty that the coastal scene is more spectacular.
Weak-rock coasts have been cut back very rapidly by wave attack leaving long lines of steep cliffs, and sand drifting alongshore has piled up locally in belts of dunes. The North Island west coast shows all this very well. The big western salient of Taranaki would have been trimmed away long since had it not been held by the hard-rock base of Mt. Egmont; but for this, points now deep in the Wanganui hill country might well be on a long sweep of coastline backed by high mudstone cliffs. Taranaki owes its very existence to the volcanic cone of Egmont that dominates the local scene.