Big grey spheres lying on Koekohe Beach on the Otago coast as though somebody rolled them there. About a third are half a metre to a metre across and most of the rest are 1.5 to 2.2 metres. They are concretions: they grew inside Paleocene seafloor mudstone, mud and fine silt and clay cemented together by calcite, and the two-metre ones are reckoned to have taken four to five and a half million years to reach that size while sediment piled up over them. Coastal erosion is now cutting the cliff back and letting them out onto the beach. Crack one and you find septaria, radial fractures lined with brown and yellow calcite. Māori tradition holds them to be debris from the wreck of the canoe Āraiteuru, the netting patterns being what is left of the fishing nets. Protected scientific reserve, open beach, best at low tide.
Koekohe Beach, Otago, New Zealand
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