Moeraki Boulders/Kaihinaki, Spherical concretions on Koekohe Beach, Otago, New Zealand

Moeraki Boulders are round rock spheres at Koekohe Beach in the Otago region, scattered along a stretch of roughly one hundred meters near the waterline. The largest among them reach diameters up to two meters and rest partly free on the sand, partly half in the water.

The boulders are septarian concretions that grew in Paleocene marine mud of the Moeraki Formation, near the seafloor surface. The largest, about 2 metres across, took an estimated 4 to 5.5 million years to form while 10 to 50 metres of mud accumulated above them. After the concretions formed, radiating cracks called septaria opened and were filled with brown calcite, then yellow calcite spar, and rarely thin layers of dolomite and quartz. Coastal erosion of the enclosing mudstone later exhumed and concentrated them on Koekohe Beach.

Māori tradition holds that the boulders are eel baskets, calabashes and kūmara washed ashore from the wreck of the canoe Āraiteuru; the raised veining is said to be the remains of its fishing nets. The rocky shoals off Shag Point are called the petrified hull, and a nearby promontory the captain's body.

A short walk from the parking area leads down to the beach and offers easy access to the rocks. The tides determine how close you can get to the stones, so a visit at low tide is especially worthwhile.

Many of the stones show cracks filled with yellow calcite crystal that form a pattern reminiscent of turtle shells. A few specimens are completely broken and reveal their hollow interior, once lined with minerals.