This rock type started out as sediments laid down under an unrecognisable ocean off embryonic Australia two hundred million years or more ago. Subsequently it was transformed by stupendous squeezing and heating into the metamorphic schist of today. In New Zealand, Central Otago bears a mother-lode of this rock.
A byproduct of its formation is gold – hence the region’s history of sluicing, mining and dredging. Sluicing pursued the fragments of gold released into water courses by natural erosion; the early miners dug adits and shafts to access gold-bearing quartz reefs; and huge floating dredges churned through river beds and adjacent terraces chasing the gold remnants. The Otago gold rushes, which followed those of California and Victoria in Australia were of world significance, and many nationalities arrived to try their luck.
The pattern of settlement today, including the arterial roading, was largely determined by gold, with towns like Queenstown, Arrowtown, Alexandra and Cromwell, and villages like St Bathans, Ophir, and Naseby all built on backs of the gold diggers.
In the 1860s and 1870s, Otago took in Southland, Fiordland and Stewart Island. These days it is smaller but a large region nonetheless, the second largest in New Zealand, and Central Otago is its hinterland, spreading from Taieri Ridge and the Strath Taieri west as far as the Southern Alps, and from the Hawkdun, Ida and Kakanui Ranges in the north to the Old Man, Old Woman and Umbrella Ranges in the south.
These mountains might look old, rounded and worn down by a rigorous climate. In geological timescale, though, they are surprisingly young, just five million years old in the case of the Hawkdun and Old Man groups, and a mere three million years in the case of the block-faulted central ranges (Rock and Pillar, Rough, Dunstan, Pisa and so on), which form a parallel pattern emanating from the Alps. Picture them as land waves, steep-sided on their eastern flanks, where faults are controlling their uplift. On their western side, they slope back more gently to an intervening valley.
The whole shooting match – the ranges and adjacent basins – are still going up, although only by a millimetre or two a year.
Whereas the mountains are relatively young, there are surfaces in Central Otago that go back a very long way – to a time, say, 20 million years ago, when the landscape was low-lying and a vast wetland of lakes and swamps and meandering rivers filled an area stretching from present-day Roxburgh to St Bathans. ‘Lake Manuherikia’ was subtropical, the home of crocodilians, snakes and eucalyptus trees, all now extinct. It had the look of Queensland. Did ancient mammals also once live here?
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