Moeraki Boulders

These impressive balls of rock are so spherical that many people think they cannot be natural. But they are. They are exposed along the beach between Moeraki and Hampden. Some of them can be seen partially eroded out of the mudstone cliff at the back of the beach. This mudstone is called the Abbotsford Formation (named after a suburb in Dunedin) and is widespread around the area. The spheres are called concretions by geologists. They are made of calcite (calcium carbonate) that formed over several million years after the mudstone had been deposited and had been buried to about 500 m by overlying sediment. Formation temperatures of 25-35 deg C have been calculated for the Moeraki Boulders. Concretions are fairly common in New Zealand mudstones but the perfect shape of the Moeraki ones sets them apart. Concretions usually start around a piece of wood or a shell and grow radially outwards by the precipitation of calcite cement within the rock. Because concretions are much harder and denser than the surrounding mudstone, they tend to remain after the surrounding rock has been removed by erosion. This means that the boulders become concentrated in places along the shore.

The boulders are aesthetically pleasing to observe in their own right, and make for great photography especially at dawn or dusk.
You will notice that the concretions are often split, This occurred because they originated at depth where the pressure was high. As they came towards the surface due to erosion of the overlying material, they cracked whilst still within the mudstone. Often the cracks were then filled with more carbonate cement to give rise to the segmented appearance of some of the boulders.