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Motutapu and Rangitoto, although ‘cheek by jowl’ present islands of great contrast. They are the smooth and the rough; the oldest and newest in the Hauraki Gulf.
Motutapu is a Jurassic park, consisting of hard greywacke sedimentary rock laid down 175 million years ago and overlaid with more recent soft Waitemata Group sediments.
There are tightly folded beds of red and green chert at Administration Bay. A walk from the Causeway to Administration Bay showcases the tops of early Miocene greywacke stacks, sticking up through Waitemata beds. Deep-water barnacles from 20 million years ago are found fossilised in siltstone around the base of a stack.
When Rangitoto erupted from the sea about 600 years ago, Motutapu was cloaked in layers of volcanic ash or tephra, with the deepest layers up to three metres in the north of the island because of the prevailing wind at the time. Farm road cuttings reveal the friable dark ash, which filled stream valleys and smoothed the hilltops compared with the rubbly surface and stream free island of Rangitoto – our ‘Hawaiian’ volcano.
