Moeraki Boulders at sunrise, North Otago

The Moeraki Boulders are a well-known geological oddity on the North Otago coast, their giant rounded forms giving the impression that they are anything but a natural feature. Over the years many people have tried to explain their presence with colourful descriptions ranging from them being left behind by aliens through to them being the results of volcanic activity, though the reality is rather more sedate.

A clue can be gained from a look at the nearby cliff face, where more of these boulders can be seen emerging from the surrounding soft mudstone as it is eroded back by the sea. These boulders have in fact been formed “in situ”, where for some reason the surrounding mudstone particles have been bound together into gigantic spherical shapes by a calcite-rich mineral. Some theorise that each boulder contains a nucleus of calcareous material, perhaps a fossilised shell, from which calcite has gradually leached out over time to cement together the surrounding mudstone. No such nucleus has ever been seen in any of the boulders that the sea has broken open, however – so their exact origin remains something of a mystery.