“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 metres in diameter.”
Read more about the boulders here ( includes map)
Maori legend says the boulders are remains of calabashes, kumaras and eel baskets that washed ashore after the legendary canoe, the Araiteuru was wrecked at nearby Matakaea (Shag Point).