Titahi Bay Rocky Coast

BY JULIAN THOMSON (GEOLOGIST)
Accessibility: MODERATE
Sea Cave at Titahi Bay Rocky Coast, Photo J.Thomson @ GNS Science
A good place to explore coastal erosion features.
Sea Arch at Titahi Bay Rocky Coast, J.Thomson @ GNS Science
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.
Titahi Bay Rocky Coast, Photo J.Thomson @ GNS Science
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.
Directions/Advisory

From Porirua follow the signs to Titahi Bay and continue until you reach the ramp at the right (north) end of Titahi Bay beach. Turn right here onto Richard St, then first left, then right. There is a parking area on the left at the start of Terrace Road.

These cliffs are relatively unstable. Avoid standing under them unnecessarily or scrambling over them. Sturdy footwear advisable.

Accessibility: MODERATE

The area of this GeoTrip is a short (5 minute) walk from the parking area. Walk down to the flat rocky area (wave-cut platform) below the road and follow a vague path to the right (north). There is a cave passage leading through to the next bay (mind your head!). This is the best place to see the erosion features locally.

Features
Sedimentary Rock Deformation Landform
Geological Age
Holocene.
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present