Auckland: The tide goes out

By John Roughan

As Auckland merges to create a supercity, the Herald looks back at how Auckland has changed over the years. Click here to view the full series.

Aucklanders live on mountains and high plateaus. If the ocean level was suddenly to drop as far as it did in the last ice age, we would find ourselves looking down on a vast lower landscape of hills and valleys, rivers and distant plains.

The Waitemata would be reduced to a river far down in the channel of the present harbour.

The river would be snaking its way around a high North Head and past the Motutapu hills, crossing the spot where Rangitoto has erupted, and onward across the floor of a dry Hauraki Gulf to meet the ocean somewhere beyond Great Barrier.

The last ice age was 20,000 years ago, not very long in geological time. Temperatures were about 5 degrees cooler on average - as cold as Invercargill today - and with much greater quantities of water frozen in snow and glaciers, the world's sea level was 120-130m lower than it is now.

The North Island, South Island and Stewart Island were connected by land and New Zealand was larger by 50 per cent.

The landscape of Auckland looked quite different. It was not covered in ice but it was colder and drier, swept by stronger winds that caused more erosion, made slopes less stable and sent larger quantities of silt down rivers to build plains.

Auckland and the north were more fortunate than the rest of the country. The region did not lose its trees. The ancient kauri and other podocarps that do not grow much further south than Auckland, somehow survived.

When the Earth began to warm up again, about 15,000 years ago, the sea level gradually rose, plains of silt became sediment and sand was washed back on to the coast to form dune banks, spits and beaches.

On Auckland's east coast the sand often formed partial barriers, creating the distinctive estuaries and lowlands of Orewa, Wenderholm, Omaha and the like.

Where barriers did not form, the rising sea simply drowned the valleys of rivers and streams creating the Hauraki Gulf and the Waitemata and Manukau harbours with their islands and jagged coasts that we know as today's inlets and bays.

The sea reached its present level about 7200 years ago. Ever since then waves have been battering the soft sedimentary rock that underlies most of the Auckland region.

The battering has worn away the slopes of sediment to leave cliffs that are still being eroded at a rate of 1-5cm a year. At low tide on many parts of Auckland's coast rocky platforms are visible that show how far the coastline has been worn back in not much more than 7000 years.

References: A Field Guide to Auckland by Ewen Cameron, Bruce Hayward and Graeme Murdoch, Godwit, 2008.
Ghosts of Gondwana by George Gibbs, Craig Potton Publishing, 2008.

- NZ Herald

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