These boulders are classed as septarian concretions, and were formed in ancient sea floor sediments.
For the oyster, this core is an irritating grain of sand.
For the boulders, it was a fossil shell, bone fragment, or piece of wood.
The original mudstone seabed has since been uplifted to form coastal cliffs.
Similar boulders occur at Shag Point, and the nearby swimming beach of Katiki.
[Text viaNew Zealand’s Information web connection]
