Mountains and hills
The Southern Alps are made of greywacke that is between 230 and 170 million years old. They are being uplifted by the movement, along the Alpine Fault, of the Pacific tectonic plate under the Australian Plate. While this is a relatively rapid process – among the fastest in the world – erosion keeps the height of the peaks at around 3,000 metres.
In 1862, explorer Julius Haast wrote that ‘the majestic forms of Mt Cook, Mt Haidinger, and many other wild, craggy peaks, covered with snow and ice, rose in undescribable grandeur before us and whilst their summits were gilded by the last rays of the sun, the broad valley of the Tasman River, with its numerous meandering channels, was already enveloped in deep purple shade.’ 1
Aoraki/Mt Cook, New Zealand’s highest mountain (now 3,724 metres), lies in South Canterbury. In 1991 the top of the mountain collapsed and roared down its eastern face to the valley floor, taking 10 metres off its height. It later lost another 30 metres.
Glaciers shaped the landscape of inland South Canterbury, gouging out wide valleys, depressions and basins. There have been at least four major glacier advances in the last 250,000 years. At their greatest extent, glaciers covered the Mackenzie Basin, which is now filled with moraines and outwash gravels. Lakes Tekapo, Pūkaki and Ōhau occupy valleys excavated by glaciers. The last major advance ended about 15,000 years ago. Since a minor advance in the late 1800s, the remaining glaciers have been retreating.
In 1889, in a quarry near Timaru, bones of the giant, flightless moa bird were discovered in clay more than 23 metres below the surface. Above the bones were 14 metres of lava that had baked the clay, and above the lava 9 metres of more recent windblown soil (loess).
The interior ranges and hills – between the Mackenzie Basin and downlands – are composed of sedimentary rocks (greywacke) that are between 300 and 150 million years old.
The downlands – rolling hill country – of South Canterbury are made up of younger sedimentary rocks, mainly limestone, which contain minor coal deposits. A small pocket of altered limestone at Kakahu forms the oldest exposed rocks in Canterbury.
During the periods of glaciation in the past 250,000 years, loess (a fine, windblown silt) was deposited over the sedimentary and volcanic rocks of much of lowland South Canterbury. In 1891 amateur scientist John Hardcastle was the first person to recognise that the loess deposits indicated periods of cold climate.
A mere 2–2.5 million years ago, lava erupted from a source near Mt Horrible and reached the coast at Timaru, forming the reefs that later provided shelter for vessels. A smaller area of basalt near Geraldine comes from an unknown source.
South of the Rangitātā River, the downlands edge closer to the coast, with the Canterbury Plains ending at Washdyke. A narrow coastal plain extends south of Timaru and widens just north of the Waitaki River. The plains are formed by sediments carried by rivers from the Southern Alps and interior ranges, and deposited as fans.
The downlands reach the coast only at Timaru, where there are low cliffs. The Canterbury and lower Waitaki plains, and the narrow plain south of Timaru, end on shingle beaches. Caroline Bay is an artificially created sandy beach. Many of the coastal lagoons and swamps have been drained, but the largest, the Wainono Lagoon, is still extensive.
Slime on the move
South Canterbury’s rivers are endangered by the spread of didymo, a brown algae also known as ‘rock snot’. First discovered in 2004, it spreads rapidly, choking rivers and reducing food supplies for fish and birds. Forming in the lower reaches of the Waitaki, by 2006 it had reached one of its tributaries, the Ahuriri River.
South Canterbury has two types of river. The boundary rivers, Waitaki (209 kilometres long) and Rangitātā (121 kilometres), flow from glaciers in the Southern Alps. The other rivers drain from the ranges east of the Mackenzie Country and are less than 100 kilometres long. They are exposed to heavy rain from the south and east and cause South Canterbury’s worst floods.
In the southern part of the region, the Hakataramea River flows south to join the Waitaki near Kurow. It is separated from both lowland South Canterbury and the Mackenzie Country by ranges of the same older sedimentary rocks as the Southern Alps.