Nelson’s limestones, marbles, granites, mudstones and ultramafic rocks are geologically part of rock groups found west of the Alpine Fault. These are quite diverse and complex compared to the greywacke mountains of the Southern Alps, east of the fault.
At Red Hill in the Richmond Range, there is an outcrop of ultramafic rocks, which have a striking red colour and are rich in minerals containing iron and magnesium. They form poor soils and support little vegetation. Similar rocks are found on the southern West Coast, some 480 km away – movement on the Alpine Fault has displaced them. The coarse golden-sand beaches of the Abel Tasman National Park are derived from granite.
Nelson is largely mountainous. The only extensive areas of flattish land are the Waimea Plains, the floodplains of the Motueka, Riwaka, Tākaka and Aorere rivers, and narrow coastal strips between Nelson and Motueka in Tasman Bay and Farewell Spit and Pōhara in Golden Bay.
South-east of Nelson city the Richmond Range rises to over 1,700 metres. Between these mountains and the western Arthur Range is a 25-kilometre-wide basin of lower hills, which reach 500–600 metres inland and 200–300 metres near the coast. This basin, known to geologists as the Moutere Depression, was formed by faulting. It was filled with gravel by rivers flowing northwards from the Spenser Mountains between 2.8 million and 500,000 years ago (the Buller River originally flowed north into Tasman Bay). The Motueka and Waimea rivers and their tributaries have cut into these gravel hills, creating many small valleys.
Tunnelling to Tākaka
A tunnel through Tākaka hill was first mooted in 1912. In the early 1990s a large white semicircle appeared on the rock face above Eureka Bend on the Golden Bay side of the hill – graffiti marking a proposed tunnel entrance. A 1992 investigation estimated that the cost of a 4.4-km tunnel would be uneconomic, at $200–435 million. The tunnel was never built.
West of the Motueka River are the Arthur Range and the Tasman Mountains. Mountains (typically 1,500 metres high) incised by rivers extend to the West Coast, making up Kahurangi National Park. Farewell Spit, New Zealand’s largest sandspit, stretches for 32 kilometres, protecting Golden Bay from Tasman Sea swells. On the western coast is Whanganui Inlet, a river system which was drowned when sea levels rose after the last glaciations. The Tākaka River valley separates the Tasman Mountains from the Arthur and Pikikiruna ranges (Tākaka hill) to the east. South of Mt Arthur, Mt Owen has New Zealand’s longest cave, Bulmer Cavern, which has been explored to over 50 kilometres in length.
Near Murchison, north of the Buller River, are huge marble and limestone outcrops in an earthquake-shattered landscape. In Kahurangi National Park many lakes have been formed by earthquake-triggered landslides blocking rivers. The Matiri Range, north of Murchison, has two spectacular plateaus – a rare New Zealand landscape. The Thousand Acres Plateau and nearby Hundred Acre Plateau (both considerably smaller than their names suggest) are unusual tussock-covered benches 800 metres above the surrounding valley floors.
Inland, the upper catchment of the Buller River dominates. Flattish land is found along the river and in tributary valleys, with settlements, farmland and roads along these narrow corridors. Nelson Lakes National Park – one of three in the region – contains Lake Rotoiti (the source of the Buller River), Lake Rotoroa, and mountains rising to over 2,000 metres in the south. These large lakes are flooded depressions carved by past glaciers, although glaciation here was less severe than further south in the higher Southern Alps.