South Bay Concretions

These semi-spherical and elliptical 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 more dense 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 while dinosaurs roamed the land.

The concretions stand out here because they are very large and smooth, in contrast to the rough, thinly bedded (and slightly darker coloured) mudstone in which they have formed. How many can you see just by standing in one spot?
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 even look like petrified wood and have very interesting swirling patterns in the rock. Some of these particular concretions have softer centres which will have eroded faster than the outside rings, once broken, leaving a ring structure.
These concretions are the continuation of the same unit (the Herring Formation) which stretches from Gooch's Beach (see link for GeoTrip) on the northern side of the Kaikōura Peninsula through to South Bay. How do these ones compare to what you see over at that northern site? What is different about their size, shape and inside layers? What do you think might have caused these differences? (HINT: think about time taken to form in the sediment, pressure/weight of overlying sediment...). Can you see how the long axis of the concretions lines up with the fine bedding in the mudstones - why would that be?