ZEPHYR / SCIENCE PHOTO LIBRARY ZEPHYR / SCIENCE PHOTO LIBRARY
Moeraki boulders, New Zealand. These boulders are examples of structures known as septarian concretions, a form of hard sedimentary rock that is usually spherical in shape. As the surrounding, softer, rock is eroded away, the concretions are left behind, as seen here. It is thought that these concretions formed some 60 million years ago. They weigh several tonnes and can be 3 metres across. They are found along the shoreline of Koekohe Beach, near Hampden, in the south-east of New Zealand's South Island.
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