The Moeraki Boulders are large and spherical boulders found on the Koekohe Beach on the wave cut Otago coast between Moeraki and Hampden.
Local Māori legends explain the boulders as the remains of eel baskets, calabashes, and kumara washed ashore from the wreck of Arai-te-uru, a large sailing canoe. In more recent times they have become a popular tourist attraction, often described and pictured in numerous web pages and tourist guides.
The Moeraki Boulders are concretions created by the cementation of mudstone. The main body of the boulders started forming in what was then marine mud, near the surface of the sea floor. They gain their shape through calcium diffusion as opposed to a fluid flow, and the larger boulders (2 metres in diameter) are estimated to have taken 4 to 5.5 million years to grow while 10 to 50 metres of marine mud accumulated on the seafloor above them. The cracks known as septaria are created by quartz, dolomite and brown & yellow calcites being laid by fresh groundwater. Tidal erosion of the mudstone bedrock frequently exposes new boulders.