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Fossilised Forest

Below the surface of Titahi Bay Beach lies an ancient forest...

 

Fossilised Forest

by Judy Tuck

At low tide at Titahi Bay Beach you can see the remains of a 96,000+ year old Pleistocene era forest which grew here before the sea level rose after the last interglacial period (or Ice Age, 150,000-70,000 years ago). The forest was a mix of rimu, totara and kahikatea and most likely nikau palms, tree ferns, sedge, flax and raupo and the size of the stumps indicates the trees grew to a large size. Amazingly, the fossilizing process has preserved the normal characteristics of wood.

The forest fossilized in situ, meaning it was not transported there by other means. The trees grew in a swampy environment during the last warm Inter-glacial period, then the climate warmed and seawater flooded the vegetation and caused sediment to accumulate around the remaining larger vegetation types, promoting preservation. The broken off tree stumps on the beach today may have been buried by many metres of sediment with the sea level rising considerably higher than today.

Sea level rises since fossilisation has eroded the coastline and uncovered the fossil beds. The fossil trees sit in old gravelly, silt and peat beds that are around 10 metres deep and are underlain by a deeper greywacke basement - the same rock as in the headlands of the Bay.

The tree stumps along the entire beach are only clearly exposed for extended periods about once every ten years. Conserving the fossil forest is important as it provides clues as to what the climate was like and how forests grew 100,000 years ago. The best time to see the fossil forest is at low tide during spring when northwest storm events have scoured sand from the beach.

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