Achilles Point - Geology

Geology

Going east, past Ladies Bay and Gentleman's Bay, we round the West Tamaki Head into the Tamaki Estuary. It is here we find an excellent example of the grey coloured 'Parnell Grit' accessible from Karaka Bay. 'Parnell Grit', is a brittle sedimentary rock containing fragments of scoria, andesite, and pumice.

At low tide there is an interesting walk from Ladies Bay along the base of the cliffs to St Helier's Bay. The rocky shoreline overlooks the Waitamata Harbour which itself occupies a drowned valley system cut in marine sediments of Miocene Age (15–25 million years ago). Generally, around Auckland these soft Miocene sediments are alternating Waitamata series sandstone and mudstone with more massive beds of sandstone sometimes with minor occurrences of limestone. The concretions that can be easily seen, are formed by the action of ground water with dissolved minerals (mainly calcium carbonate of which limestone is made) percolating through the layers. Calcium carbonate is an important component of concrete and fills in the pore spaces between grains of sediment thereby cementing the grains together and making them more durable against erosion.