Regionally, the geological structures may control drainage patterns and the general physiography of mountain ranges. More locally rock type, fracture and joint patterns will influence resistance to erosion, thereby influencing the development of landforms and the general morphology of the mountains. The hard granites and gneisses of Fiordland have retained the imprint of the last ice age, with extremely steep valley sides over 1000 m (3300') high, and hanging valleys. Here The Elephant, Milford Sound. Photo NC.

Greywacke and argillites: Torlesse Supergroup
These are tough sedimentary rocks mostly made of layers of sandstones and siltstones. The 'greywacke suite' may also include basaltic and spilitic lava, tuff, jasper and chert. Limestone is rare. There are local variations in both origin and composition, induration, and metamorphism. Close to the boundary with the Haast Schist Group the rocks are among the most resistant in the Southern Alps (Mt Cook).
However the rocks have been intensely fractured, and are easily desaggregated by erosion to form extensive debris slopes (the 'rotten rocks' famous among New Zealand mountaineers).
The rocks of the Torless Supergroup are marine sediments of detritic origin, deposited on the western margin of the New Zealand landmass. Ages range from permian to lower cretaceous.

Haast Schist Group
These are the metamorphosed equivalents of the Torlesse Supergroup, with gradations in the degree of metamorphism. Five sub-zones have been defined based on mineral associations of chlorite, biotite and garnet.
Schistosity may have a strong influence on landforms. The platey structure of the rock encourages disintegration, and these rocks are more prone to weathering and erosion than greywackes.
Schistsity also favours the formation of landslides, often slow and involving whole mountain sides.

Granites and gneiss
These rock formations are the most resistant to erosion in New Zealand. In Fiordland they have retained the intense and precipitous features left by the ice ages. The granites of the Darran Range offer some of the best rock climbing in the country.

Paleozoic sedimentary rocks
These formations are mostly located in the mountains of North-west Nelson and include a variety of ancient sedimentary rocks: limestones, marbles, greywackes, and other. They are erosion resistant and result in a variety of landforms, including large areas of karst, deep-cut valleys, gorges and scarps.

Volcanic rocks
The history of volcanic eruptions in New Zealand has left a variety of volcanic rocks in the landscape, from basalts to andesites and dacites to rhyolites. Some of New Zealand's highest and most distinctive mountains are young volcanoes, such as Ruapehu and Taranaki.
Although many of these rocks are intrinsically hard and erosion-resistant, volcanoes are usually eroded quickly once activity has stopped. This is particularly the case with strato-volcanoes such as Ruapehu and Taranaki, which are made of a succession of layers of varying hardness and weak cohesion.
Softer volcanic rocks are extensively present in the central North Island, where layers of ignimbrites, tuffs and pumice resulting from huge eruptions of the Taupo volcano in the last twenty five thousand years have mantled the landscape over thousands of square kilometres.