Waipara Concretions

Waipara Concretions

When one sees large spherical boulders, the first landmark to spring to mind are the famous Moeraki Boulders in northern Otago, right on the beach next to State Highway 1.  These impressive balls of rock are so spherical that many people think they cannot be natural. But they are!

The Waipara Concretions have formed geologically in much the same way as the Moeraki Boulders. They are made of calcite (calcium carbonate) that formed over several million years after mudstone had been deposited and had been buried to about 500 m by overlying sediment, at the bottom of the ocean. Since our continent was submerged for quite a significant portion of it’s tectonic history, concretions are fairly common in New Zealand mudstones but not easily accessible to view like at Moeraki. Concretions usually start around a piece of wood, bone 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. The Waipara Gorge in North Canterbury is special for the concretions forming around fossilised bone of the marine reptile groups mosasaur and plesiosaur, which were swimming around in New Zealand waters while the dinosaurs were on land, hence we know they are over 65 million years old!

A large concretion sits in the Middle Waipara River after eroding out of the Cretaceous-aged mudstones of the Conway Formation. Kate Pedley (2010).

The largest of our concretions has a 2 m diameter and weighs over 6 tonnes! One of our other concretions broke open when we were trying to install it in the garden – this was a happy accident, because it meant we could see what was inside! Excitingly, it was revealed that the rock contains many fossiled bones of a mosasaur skeleton, preserved here for you to discover.

Bone when fossilised has a kind of fine honeycomb texture – it can be hard to spot at first as it’s a similar colour to the rock. Also check out the complete plesiosaur paddle skeleton located in another mudstone boulder on Level 2 of the Ernest Rutherford building, outside our geology teaching labs.

Fossilised bone under the microscope – petrophotography by Kamen Engel, Thor Photography, UCNZ School of Earth and Environment graduate. (2025)

These are cleaned up fossil vertebrate bones from a mosasaur in our palaeontology research lab, like the one in our open concretion – can you find the vertebrate bone preserved in the rock? Also see if you can spot some of the long, thin rib bones! Jaden Cameron (2025).

These concretions have been provided to the garden by Mike Holden, former landowner up at Waipara.