Travelling from Auckland or Whangaparaoa to Tiritiri Matangi, the typical 'whitish' cliffs of the Auckland area shine out in the sun. These are the familiar 'Waitemata Group' rocks. They are made up of mainly alternating layers of sandstone and mudstone, interspersed irregularly with thick beds of volcanic debris flows. The Waitematas were laid down in a submarine basin between 22 and 18 million years ago. During that time, volcanic activity began to the west of Auckland, the only remnants being the Waitakere hills. The Waitemata basin was then uplifting about 17 to 15 million years ago and erosion began.
 
In contrast to the Waitemata Group rocks, the cliffs on Tiritiri Matangi are darker and less clearly stratified. These rocks are much older, part of what is called the 'Greywacke Basement' of New Zealand. These rocks are also mainly mudstones and sandstones which were laid down on the deep ocean floor between 270 and 150 million years ago. They were compressed, fractured and folded in higher temperatures and pressures as they were buried beneath many kilometres of rock. Subsequently, about 100 million years ago, during the early cretaceous period, these rocks were uplifted and began eroding. They later formed part of the basement of what was to become the New Zealand continent. The greywackes, of which Tiritiri Matangi is a part, are known as the 'Waipapa Terrain'.

Greywackes underlie the whole of Auckland. Faulting and uplift to the east of the city have exposed these on Tiritiri Matangi, as well as on Motutapu, Waiheke, and in the Hunua Ranges which are uplifted Waipapa greywacke. Most of the Waitematas which overlay Tiritiri Matangi have been eroded off since the basin was uplifted. They are now only seen on the north east tip, unnoticeable to most visitors.   Helen Holzer