Gooch's Beach Concretions

These spherical rocks are known as concretions. What are concretions??
Concretions can be made of a few types of minerals (e.g. iron oxide, silica, calcite) but these ones have been formed by dolomite (calcium magnesium carbonate) that formed over several million years after the mudstone they are formed in had been deposited and buried by overlying sediment. Concretions are fairly common in New Zealand marine mudstones. Concretions usually start around a piece of wood, fossil or a shell and grow radially outwards by the precipitation of mineral cement. 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.
These particular concretions were formed during the Late Cretaceous (>65 million years ago!) in the Herring Formation when New Zealand sediments were accumulating in the ocean off the coast of the super continent of Gondwana.

See if you can spot the "growth rings" of the dolomite cement inside a broken concretion! It will look like onion skin layers within the sphere. Some of the concretions have harder centres which will have eroded slower than the outside rings.