Moeraki Boulders

The Māoris were the first to find the strangely spherical Moeraki Boulders of Koekohe Beach and believed that they could be the eel baskets, calabashes and kumara washed ashore from the wreck of Arai-te-uru.  Their legend has it that the Arai-te-uru was a massive canoe that transported the first ancestors the Ngāi Tahu people to the South Island of New Zealand.  Later Western explorers whimsically speculated that the boulders might be fossilised dinosaur eggs especially as some seemed intact while others appeared to have hatched. Geologists had the last word though and demonstrated that these formations were actually formed around 60 million years ago from ancient sea sediments.

moeraki boulders broken open

Hollow interior of a Moeraki Boulder.

The Moeraki Boulders can be as large as three metres in diameter and are to be found on the Otago coast of New Zealand between Moeraki and Hampden now part of a scientific preserve. The boulders are made from of mud, fine silt and clay, cemented by calcite. The degree of cementation varies from being quite weak at the interior to quite hard on its outside shell.

Cross section of the remains of a Moeraki Boulder

Cross section of the remains of a Moeraki Boulder showing the calcite septaria.

Each boulder is covered in a web of cracks known as septaria that radiate outwards from the hollow interior.  It was these hollows that led some people to speculate that the boulders were in fact Sea Dragon Eggs. Geologist are not sure what causes these septaria but they are often filled with multiple calcite deposits and sometimes an even a very thin layer of quartz.

The Moeraki Boulders don’t originate on the beach but have been eroded out of the sandy cliffs and rolled together by action of the tides.

Moeraki Boulders in a Sand Pond

Tidal action creates a beautiful sand pool around a lone Moeraki Boulder.