JEREMY WALKER / SCIENCE PHOTO LIBRARY JEREMY WALKER / SCIENCE PHOTO LIBRARY
Moeraki boulders, New Zealand. These huge spherical stones are septarian concretions, rock nodules formed in a sediment. These are thought to form when material precipitates around a small nucleus object, which can be anything but is often organic material, such as a leaf or part of an animal. These boulders are thought to be some 60 million years old. They were buried in the sand of the beach, but have been exposed by wave action over the millennia.
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