Much of the east coast of the North Island is characterised by rocky shore platforms, most of which consist of alternating layers of sandstone and mudstone. These sedimentary rocks are quite young, geologically speaking, and originate from sea floor sediments that have been compressed and then uplifted over millions of years to form the bedrock that underlies most of this part of the North Island.
As well as being uplifted, these sandstones and mudstones are often tilted, buckled and otherwise deformed by tremendous tectonic forces so that their layers may reach the surface at an angle, or be curved or warped, or be offset from each other along small faults or fractures. It is also common for these rocks to be subjected to compression forces from different directions at various times as they are being uplifted, so that sets of stress fractures running in completely different directions can be seen in the same rock layers when they are exposed. This appears to be the case in this example of a shore platform on Mahia Peninsula, where two different sets of fractures run almost at right angles to each other, creating a “tiled” effect.
