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Moeraki Boulders.
NZ
The boulders are grey-coloured septarian concretions, which have been eroded out by wave action from the cliffs of soft, black mudstone that back the beach. In places, partially exposed concretions can be seen in the cliffs. They originally formed on the sea floor when the mudstone was accumulating during the early Tertiary period some 60 million years ago. The largest concretions are traversed by cracks, filled by yellow calcite. In some the upper part is worn away; only a shell remains, looking like discarded segments of orange peel. The concretions weigh several tons and are up to 12 ft in circumference. Original public domain image from Flickr
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