Story: West Coast region
Page 2 – Geology and landscapes
Radio Scenicland, a West Coast radio station, was aptly named because the landscapes of the region are spectacular – from high peaks and low-altitude glaciers to stony beaches within a few kilometres. Mountains, including New Zealand’s highest peaks, are in sight almost everywhere. Glaciers have moulded many of the landscapes, and are responsible for many of the lakes and the bumpy moraine country.
Grinding tectonic plates
The West Coast region lies along the boundary between the Pacific and Australian crustal plates, which are slowly grinding against each other. This stress causes rising mountains, earthquakes and landslides.
The plate boundary through the South Island is marked by the Alpine Fault. This huge feature, visible from space, forms the western edge of the Southern Alps. Land on the eastern side of the fault is rising at the rate of about a centimetre a year, making it one of the most rapidly rising mountain ranges in the world. At the same time, the rocks on each side of the fault are gradually sliding apart. Those on the western side of the Alpine Fault have moved northwards about 480 kilometres compared to those on the eastern side, so that there is a sudden change in rock type across the fault.
West Coast bottom
Blue-grey mudstone or muddy sandstone is widespread in some parts of the West Coast, and is locally called ‘blue bottom’. Gold miners quickly discovered that gold was found only in near-surface gravels, and it was not worth digging any further once the blue-grey mudstone bottom was reached.
Rocks east of the Alpine Fault
Greywacke – hard, grey muddy sandstone with bands of shale, 200–240 million years old – makes up much of the Southern Alps. Close to the Alpine Fault, where uplift has been greatest, the deeply buried greywacke has been heated and metamorphosed into schist. Pounamu (greenstone or jade) comes mainly from a small area of schist found north-east of Hokitika.
Because the mountains are being rapidly uplifted and eroded, the dominant boulders in rivers and on the beaches are greywacke, with a smaller amount of schist.
Rocks west of the Alpine Fault
The rocks on the western side of the Alpine Fault are more varied. The oldest rock type is greenish-grey greywacke, about 480 million years old, known as Greenland Group. Pale-coloured varieties of granite have intruded into this rock.
These older rocks were overlain by layers of softer sedimentary rocks, including sandstone, mudstone, limestone and conglomerate. Coal-bearing rocks (known as ‘coal measures’) are widespread, and coal has been mined at a number of locations. A widespread layer of limestone, now partly eroded away, forms distinctive karst (eroding limestone) landforms such as subterranean caves and the Pancake Rocks at Punakaiki.
Goodletite – ruby rock
Rare boulders of an unusual greenish-grey rock containing crystals of ruby and sapphire have been found in glacial gravels near Hokitika. The rubies are not high enough quality to be regarded as gems, but the rock is prized by collectors and is on sale in Hokitika. Although originally discovered by miners, the rock is named after William Goodlet from the Otago School of Mines.
Effects of glaciation
Although the rocks on the West Coast are tens to hundreds of millions of years old, the landscape is much more recent, recording the impact of glaciers and erosion over the last 500,000 years. During this period there have been six major ice ages, when glaciers filled many of the main valleys, extending out past the present coastline south of the Taramakau valley.
The last glacial period – known as the Ōtira glaciation – finished about 14,000 years ago. As the climate warmed and the glaciers retreated, rivers flowed in the previously ice-filled valleys. Post-glacial river floodplains are some of the most productive soils in the West Coast today.
Glaciers and gold
Rivers and glaciers carried fine gold from the Southern Alps to the sea, and concentrated it in some locations. One of the richest areas of alluvial gold ever found was near Kumara, where river action concentrated gold at the margins of older moraines (glacial mud and gravel).