Ward Beach Boulders

These brown spherical rocks are concretions within Late Cretaceous (>65 million years ago!) marine sediments. The rocks here contain a variety of siltstones, sandstones, limestones, conglomerates and turbities (underwater landslide deposits) typical of the marine setting at that time. They accumulated in the ocean off the coast of the super-continent of Gondwana.
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 within a mudstone that 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 as the surrounding material gets weathered away.

You can easily spot the concretions here as their distinctive brown colour contrasts quite nicely against the grey mudstone in which they sit. Why do you think they have a different colour?
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 softer centres which will have eroded faster than the outside rings.