Looking south west over the eroded anticline, Tora Coast. K.Amai / VUW
Well-bedded layers of sandstone from the Late Cretaceous Glenburn Formation are well exposed along the shoreline to the south of Pukemuri Stream. The formation is about 60 metres thick here. The beds along the shoreline are tilted (or dip) towards the west. But if you look offshore at low tide, you can see that the beds are tilted towards the east. The beds were originally formed as horizontal layers of sand and silt on the seafloor, probably as part of an ancient submarine fan system. They have since been buried and hardened. Tectonic forces have then uplifted and folded them into a large inverted U-shaped fold called an anticline. If you continue walking south along the beach, you can find the hinge or axis of the fold where the layers are still horizontal.
Over time, the erosional force of the ocean has created the wave-cut platform, exposing the internal structure of the large anticline. As you follow the anticline north along the coast, increasingly younger sedimentary rocks are exposed. This indicates that the anticline axis is tilted to the northeast.
The folding is a response to the pressure of plate collision along the Hikurangi subduction zone. This plate boundary (the interface between the colliding Pacific and Australian Plates) extends below the southern North Island at a shallow angle, from the sea floor 50 kms offshore of Tora. Below Tora, the boundary is approximately 20 kms deep. Like many folds and faults in Wairarapa, the Tora Coastal Anticline runs northeast-southwest, which is parallel to the subduction zone and at right angles to the forces generated by the subduction process.