Turakirae Head storm ridge, J.Thomson / GNS Science
Turakirae Head sits at the southern tip of the Rimutaka Range, to the east of Wellington. It is a scientific reserve featuring fascinating coastal landforms, giant scattered boulders and a seal colony. It is popular with mountain bikers, walkers, climbers and geologists.
During the frequent storms along this coast, large and powerful waves heap up stones from the gravel beach into long ridges called ‘storm ridges’. Further, larger storms rework this material, pushing the ridge further up the beach. This leaves a record of how far the waves have reached. Later, smaller storms might add additional, smaller ridges on the seaward side. The seaward face of the ridge is usually steepest, with an area called the swash zone in front of it. The landward side of the ridge is a more low-angled slope onto which floating material like wood and seaweed gets washed by big waves overtopping the ridge.
This process of formation of gravel ridges is different from sandy beaches which usually get flattened by storm waves, with the sand being dragged out to sea, rather than pushed further up the beach. A good supply of gravel for this process at Turakirae comes from some large slips that have brought greywacke fragments down to the beach from the nearby hill slopes.
Storm ridges can become large features that develop and remain in place for many decades or centuries.
At Turakirae there are several storm ridges running parallel to each other along the beach, separated by many tens of metres. They are easily noticed as they look like trackways, with little vegetation on them. They show up particularly well from aerial photos. They are evidence of a sequence of earthquakes that have raised the Rimutaka Range and its coastal storm ridges over the last six thousand years. The most recent uplift occurred when the Wairarapa Fault ruptured in 1855, raising the area by up to 6 metres. This pushed the sea out beyond the pre-1855 swash zone and caused the initiation of the latest storm ridge that has been forming amongst the boulders over the last 160 years.
The uplifted (pre 1855) beach has many abandoned rock pools and overhanging boulders. Scientists have identified over 100 species of molluscs and other sea creatures that were stranded as the sea retreated.
As you move further up from the 160+ year old ridge, the next one has been dated at about 2300 years and the one beyond that at approximately 5000 years old.