Geography Cat’s Gran & Granpf sent him this beautiful postcard of the Moeraki Boulders, on the east coast of South Island, New Zealand.
The boulders are calcite concretions, they began forming around 60 million years ago in what was then ocean sediment. They took around 4 million years to grow into the rock spheres you see today. The boulders are released as the surrounding bedrock is weathered or eroded away. The ones here in Moeraki are up to 2m in diameter and 7 tonnes in weight.
This clip explains their formation purrfectly:
Concretion boulders can also be found in other places around the world: