A little fishing village on the coast south of Oamaru has become famous for a geological wonder on its windswept sands.
The Moeraki Boulders are huge spherical stones that are scattered over the sandy beaches, but they are not like ordinary round boulders that have been shaped by rivers and pounding seas. These boulders are classed as septarian concretions, and were formed in ancient sea floor sediments. They were created by a process similar to the formation of oyster pearls, where layers of material cover a central nucleus or core. For the oyster, this core is an irritating grain of sand. For the boulders, it was a fossil shell, bone fragment, or piece of wood. Lime minerals in the sea accumulated on the core over time, and the concretion grew into perfectly spherical shapes up to three meters in diameter.
The original mudstone seabed has since been uplifted to form coastal cliffs. Erosion of the cliffs has released the three tonne captive boulders, which now lie in a haphazard jumble across the beach. Further erosion in the atmosphere has exposed a network of veins, which gives the boulders the appearance of turtle shells. Similar boulders occur at Shag Point, and the nearby swimming beach of Katiki. In Hawke’s Bay in the North Island, scientists have found that the central core of similar boulders contained perfectly preserved skeletons of turtles, sea snails and extinct reptiles, such as plesiosaurs.
Local legends explained the boulders as the remains of eel baskets, calabashes, and kumara washed ashore from the wreck of an Arai-te-uru, a large sailing canoe. This legend tells of the rocky shoals that extend seaward from Shag Point as being the petrified hull of this wreck and a nearby rocky promontory as being the body of the canoe's captain.
Moeraki makes a fascinating stopover point, both for the dramatic coastal scenery and the curious geological phenomenon on the beaches.