The Moeraki Boulders are a cluster of large, spherical boulders scattered along Koekohe Beach on the Otago coast of New Zealand's South Island. They formed as concretions within a Paleocene-age marine mudstone roughly 60 million years ago, growing slowly outward from a central nucleus as calcite cemented the surrounding sediment in roughly concentric layers, with some reaching more than 2 meters in diameter. Ongoing erosion of the surrounding, much softer mudstone cliff has gradually exposed and released the harder, more resistant concretions onto the beach below, and where some boulders have cracked open, they reveal internal veins of calcite crystals in a pattern geologists call septarian.
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