Balls of stone

On a beach in New Zealand, large spherical stones lie quietly in the sand, some of them 60 million years old. How did they get there?


Nature has a habit of surprising you. The moment you think you have seen it all, it throws at you something special, something remarkable, that you never thought it was capable of producing. These things may take a variety of shapes and forms. Such as the strange, alien-like, ice cream cone-ish stone balls at Koekohe Beach, on the south-eastern coast of New Zealand’s South Island.

Some have jokingly labelled them ‘dinosaur eggs’. And in fact, the oldest stones hail from as far back as 60 million years; around the time when the last dinosaurs wandered the earth. (Though apart from that, there is rather little evidence to support this theory.) The oldest stones weigh up to seven tonnes, and have a diameter of three metres. There is legislative protection against moving or damaging them, though even if you tried, you would not stand a chance.

Alongside the dinosaur joke, there are other, more mythical, theories as to how these stones ended up on the beach. One Māori legend claims they were part of a large canoe called Āraiteuru, which was wrecked outside the South Island, at a nearby coastal destination called Shag Point, 13 kilometres (eight miles) south of where the stones lie today. What now decorates the beach, the legend says, are calabashes from the wreck that have turned to stone.


Like that legend, the popular name of these stones is influenced by local connections. They are called the ‘Moeraki Boulders’, dubbed after the nearby village of Moeraki. The village name translates to ‘A place to rest by day’, which refers to its history as an old Māori resting place. Being located near the coast, the village has a tradition of whaling and sealing. However, those industries have faded, and the boulders are easily its most famous attraction. Tourists do arrive to see them, and cafés and restaurants are placed strategically near the beach. Other nearby sights are bird colonies and a sanctuary of yellow-eyed penguins.

The scientific theory behind the stones’ arrival is, one may say, somewhat more realistic than dinosaur eggs and old Māori tales. Academics believe they were formed in ancient sea floor sediments, roughly 60 million years ago – during the geological time epoch referred to as Paleocene. As such, the material they are made of is called Paleocene marine mudstone. These boulders are septarian concretions, meaning they have formed gradually around specific objects. Examples can include small shells, bone fragments and pieces of wood. Many liken this formation to the creation of a pearl.

In the process of such a concretion, the sedimentary rock forming around the object endures a change. The spaces between its particles are filled by minerals, which have the practical effect of cement. It hardens the rock and makes it more resistant. This is crucial in terms of explaining how the boulders came to see the light of day.


After the boulders were formed, the seabed of the Paleocene epoch was lifted from the ground around 15 million years ago. After this happened, the boulders were caught inside the mudstone that formed the cliffs next to the ocean. Then, over millennia of erosion – during which rain, wind and waves gradually tore down the rock – these boulders gradually appeared from the cliffs. The reason is that their ‘concrete’ effect made them more solid, enabling them to resist the weather conditions that bring the gradual erosion. This is in contrast to the mudstone rock it was surrounded with, which is soft and easy to tear down.

As such, the boulders have been excavated by mother earth herself, one by one. Even today, you can see some of them being in the process of escaping: some are halfway caught inside the cliff; halfway free. Among those already out in the open, some boulders lie in clusters, while others rest in isolation. Some are whole and round, others are cracked open. They may all look bizarre, but as it happens, science does have a logical explanation of how they came there. And, knowing how nature works, that is not something we should take for granted.

Photos: HTU, ian woolcock, Khoroshunova Olga [all via].