Geological Walks on Motutapu

• The Northern Coastal
• The Southern Coastal
  

A walk on Motutapu reveals part of the story of Auckland's geological past; it provides a glimpse of some of the major geological events which produced the rocks underlying Auckland City and The islands of the inner Hauraki Gulf.

The greywackes - Rocks of the oldest period - 150 million to 200 million years - The basement rocks

Greywacke forms the foundation for much of present day New Zealand. The piece of continental crust which now comprises greater New Zealand (75% which remains below sea level) only came into existence as a result of ancient earth movements around 100 million years ago which buried, squeezed and heated the greywackes then raised them out of the sea as a mountain range attached to the edge of Gondwanaland. These basement rocks were originally mud and sandstone (called agillite and greywacke when consolidated).

Later, after being worn down by erosion and separated from the rest of Gondwanaland by continental drift, the greywackes became the basis of modern New Zealand. Greywacke forms most of Motutapu (and an area to the south western end of Motuihe). One can visualise the old greywacke basement rocks dipping south-westward beneath Auckland City and the Manukau lowlands in the distance. To the east the basement is exposed on Motutapu, Motuihe and Waiheke islands. To the south it forms the uplifted Hunua Ranges. Everywhere  else in the region,  the basement is overlain by the Waitemata sediments.

The Waitemata Group - Rocks of the middle period - 22 million to 15 million years

The greywackes seem totally different from the predominant Waitemata Group (the familiar yellowish coloured layered rocks which form most of the cliffs around the Waitemata harbour) yet they are essentially similar sedimentary rocks. The differences between the Waitemata and greywacke relate to the extent to which they were:

• Buried in the earth's crust (1-2 km for Waitemata, tens of kms for greywacke)
• Squeezed (scarcely at all for Waitemata, intensely for greywacke)
• Heated (hardly at all for Waitemata, 200c for greywacke)

Waitemata Group rocks are sediments somewhat hardened to form sandstones and mudstones but still normally soft enough to be easily scratched by your fingernail.

As you can imagined, much geological phenomena can be seen on these following island walks.

The Northern Coast Walk

Important - only attempt the coastal section within 3 hours of low tide

This walk takes you along the coast from Islington Bay to Administration Bay and reveals all the main features of the rocks of the middle and oldest periods. The early part of the walk is along the base of cliffs formed of well bedded sandstone and mudstone of the Waitemata sediments.

Small faults displacing sedimentary beds by anything from a few centimetres to several meters are readily seen and a close look may turn up a few small fossils

Further along are outcrops of basal Waitemata conglomerate resting against dark grey stacks of greywacke.

Behind the beach a little further along it is possible to see sections of ash which came from the eruptions of Rangitoto.

A climax to the walk is provided by the spectacular colours and complex folds of cherts at Administration Bay. This is one of the best places to view chert. You can see the complexly folded red chert exposed on the shore platform and in a quarry behind the beach.

What is chert?

Here and there you among the basement rocks of Motutapu you come across other rock types beside's greywacke and agillite. The commonest one is chert. Chert is a very fine grained sedimentary rock composed of silica. Chert on Motutapu is commonly red, less commonly green or grey and rarely cream. The red variety is widely used around Auckland as a paving aggregate. The different colours are due to the presence of iron (red and green) in different states of oxidation, and manganese (black). The cherts are typically well bedded and at some localities display very well the complex folding characteristics of basement greywacke (e.g. Administration Bay)

Return to Islington Bay, Rangitoto, via the inland Walk Motutapu track (MOEC) Track. Refer to the Walk Motutapu map for alternate routes to Home Bay via Northern Junction or Billy Goat point

The Southern Coast Walk

Important - Please note that the return walk along the coast is long and strenuous and can only be carried out at low tide

If arriving at the island via Rangitoto, continue along the causeway beyond Islington Bay and take the track to Home Bay (90 minutes). From Home Bay, start from the southern end of the beach (near the camp ground facilities) and head off around the coastal rocks, rounding Emu point to arrive back at Islington Bay (3-4 hours).

On this walk you can see exposures of chert in the greywacke basement between Home Bay and Otahuhu Point, the greywacke/Waitemata sediments contact, and excellent examples of a comparatively rare phenomenon - sandstone dykes. These are formed when sand has been squeezed up through the overlying Waitemata sediments by the weight of overlying sediment