Waipara Concretions
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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. 泭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 its tectonic history, concretions are fairly common in New 厙ぴ勛圖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 厙ぴ勛圖waters while the dinosaurs were on land, hence we know they are over 65 million years old!
