Moeraki Boulders / Kaihinaki
Interesting facts
- The concretions grew close to the ancient sea floor while between 10 and 50 metres of marine mud slowly piled up above them.
- The calcite cement makes up only about 10 to 20 percent of the outer rim even in the larger boulders; the rest is mud, silt and clay.
- Besides calcite, the cracks inside the boulders contain small amounts of dolomite and quartz.
- Similar but even bigger concretions, the Koutu Boulders on Hokianga Harbour in the North Island, reach about three metres across.
- The Katiki Boulders at Shag Point, 19 km to the south, include disc-shaped forms, and some contain bones of mosasaurs and plesiosaurs.
- Comparable septarian concretions are found far away in England's Kimmeridge Clay and Oxford Clay.
- Walter Mantell's 1848 sketch of the beach, showing far more boulders than today, is held by the Alexander Turnbull Library in Wellington.
- The boulders weather out of Palaeocene mudstone belonging to a rock unit named after the area, the Moeraki Formation.
Why it is worth it
On Koekohe Beach, on the Otago coast of New Zealand's South Island, a scattering of grey stone spheres lies at the edge of the surf as if a game of giant marbles had been abandoned. Some sit alone, some in clusters; some are half buried in the sand, others have split open to show a hollow, crystal-lined heart. They are the Moeraki Boulders, officially Moeraki Boulders / Kaihinaki, and on a still morning, with the sea washing around them and the low sun picking out the net-like pattern of veins on their skins, they are one of the most quietly surreal sights in the country.
What looks like the work of a sculptor is the work of chemistry. The boulders are septarian concretions: balls of mud, fine silt and clay that were cemented by calcite while they lay in the soft sediment of the sea floor during the Palaeocene, around 60 million years ago. Because calcium spread into them evenly from all sides, they grew into near-perfect spheres; the largest, around two metres across, are thought to have taken several million years to form. Later, cracks — the septaria — opened inside them and slowly filled with brown and then yellow calcite, which is what gives the weathered surfaces their tortoiseshell pattern. The cliffs behind the beach are made of the same mudstone, and as the waves cut them back, new boulders are released and roll onto the sand.
Most of the boulders measure between half a metre and just over two metres in diameter, and they fall into two distinct size groups. The stretch of beach where they lie is protected as a scientific reserve. Walter Mantell sketched the beach in 1848, when the boulders were more numerous than today. Māori tradition gives them a different origin: they are the eel baskets, gourds and kūmara washed ashore from the wreck of the great canoe Āraiteuru, whose hull lies petrified as the reef off Shag Point to the south and whose fishing nets survive as the veined patterns on the stones.
The boulders are best seen around low tide, when more of the beach is exposed and you can walk among them, and at sunrise, when the light comes straight off the Pacific and the wet spheres shine.
Getting there
The boulders lie on Koekohe Beach between Moeraki and Hampden, about 30 km south of Oamaru in a straight line and some 80 km north of Dunedin by road. The nearest airport is Dunedin International Airport (DUD), about 81 km away. The beach is reached by a short turn-off from State Highway 1.
On these lists
From the encyclopaedia
The Moeraki Boulders are unusually large spherical boulders lying along a stretch of Koekohe Beach on the wave-cut Otago coast of New Zealand between Moeraki and Hampden. They occur scattered either as isolated or clusters of boulders within a stretch of beach where they have been protected in a scientific reserve. These boulders are grey-coloured septarian concretions, which have been exhumed from the mudstone and bedrock enclosing them and concentrated on the beach by coastal erosion. Especially in recent years, the boulders have been a popular tourist attraction.
Wikipedia · CC BY-SA 4.0
-45.34528, 170.82606per Wikidata