Kupe's Sail, Ngā-Rā-o-Kupe

This impressive outcrop is a tilted triangle of sandstone, with abundant fragments of barnacle, brachiopod and bryozoan fossils. There are also trace fossils of burrows which formed at the same time during the middle to late Miocene (~15 million years ago). They were very well bedded and compacted on the ocean floor and this is why you can see the very flat surface (now tilted up) that remains today. The presence of these particular fossils shows that the sediment was deposited in a shallow marine environment close to an ancient shoreline.

Heading towards Mangatoetoe Stream from the car park along the road you will pass a sea cave which has been eroded out of hardened muddy sandstone that is a part of the rock sequence here. Further along (west) you will be able to walk up or alongside the stream which will take you behind the 'Sail'. If you continue to follow the stream to the entrance of the deep cut gorge you can see where the sandstone lies unconformably on the much older basement rocks. These continue north to underlie the Aorangi Range.

These sail shaped outcrops formed due to a reverse fault which ruptured approximately 5-10 million years ago. This left a fissure which was filled by the muddy sands and fossils to form the shelly sandstone. Later this was uplifted and tilted to the west (outlined in the process diagram above).

According to Māori legend, while camped near Mangatoetoe stream Kupe who was an early Maori explorer challenged his companion Ngake to see who could make a sail the quickest. Kupe won the challenge and completed his well before Ngake. Afterward they left the sails where they still remain. Today, Nga-Ra-o-Kupe points high into the wind, as if the wind has taken hold.

A good place to have a close look at the rocks is next to the car park by the reef. You may find some intact fossils but mostly they have been fragmented. The types of fossils, along with rough sorting, layering and cross bedding, suggests that they were deposited in turbulent (shallow) water. The types of fossils you may find include:trace fossils (burrows), large barnacles and maybe a fossilised echinoid or two.
If you have time to explore the stream to the west of the main cliff (starting about 200m further west along the road), it will lead you into a gorge, crossing an unconformity (time gap) into much older greywacke sediments. See if you can find the unconformity.