Titahi Bay Rocky Coast

Coastal erosion due to wave action has sculpted this area into a variety of classic erosion features. The greywacke rocks consist of stronger sandstone layers alternating with weaker siltstone. They have been badly deformed over their long tectonic history and faults are additional lines of weakness in the rocks. Waves crashing into the cliffs therefore erode the softer material more easily to create a variety of hollows, gullies and caves. Because the cliffs get undercut by wave action, and the rocks are very fractured, the area is prone to occasional rockfalls. Ultimately the waves carve out a flat (or relatively flat) surface called a wave-cut platform, shore platform, marine terrace or marine abrasion surface. (Take your pick!) The eroded material is moved along the coast by currents, to be deposited in a more sheltered bay as beach sand.
The big Wairarapa earthquake in 1855 has lifted the whole area by almost a metre, so some recent erosion features (e.g. caves) are now stranded at a level above the reach of the sea. Mana Island, visible in the distance, has a flat top. This is a marine terrace that has been uplifted (and slightly tilted) by tectonic uplift (repeated earthquakes) over tens of thousands of years. Similar uplifted terraces can be found in many places around the Wellington coast.

In the cliffs here you can see folds, faults (one fault runs through the sea cave you have just walked through), some rather eroded sea stacks and a nice example of an arch. Note how the sea caves are now beyond reach of the waves due to uplift from the 1855 Wairarapa earthquake.