The collection of offshore rock pillars rise abruptly from the water like a line of giant soldiers protecting the coastline from the extreme force of the Southern Ocean. Some are lightly vegetated, while others are bare of any greenery. Measuring up to 45 metres in height, they are pounded constantly by the relentless wind and waves that crash into them in explosions of whitewater. As the sun sets, it paints them in rich orange and purple light; when it dips below the horizon, they transform into foreboding silhouettes against the bruised dusk sky.
Located on Kirrae Whurrong Country, off the shore of Port Campbell National Park by the Great Ocean Road in Victoria, the Twelve Apostles – as these rock pillars are most commonly known – are an iconic natural landmark in Australia and around the world. Their name, however, hasn’t always been the same: early European charts, for example, refer to them as the Sow and Piglets – the ‘sow’ being Mutton Bird Island and ‘piglets’ being the surrounding rock pillars to the east.
This name persisted until 1922, when the pillars were renamed to “The Apostles” in a bid to attract more tourists to the region. Eventually, they were renamed to the Twelve Apostles, despite there only being nine pillars visible from the Twelve Apostles viewpoint at the time. Since then, two rock pillars have collapsed, leaving only seven visible from the viewpoint.
Although these pillars now stand in the sea, all of them were once connected to the heath-carpeted cliff line of the mainland. Their separation from it speaks to the landscape-shaping power of the harsh and extreme conditions of the Southern Ocean. But to understand how it happened first requires understanding the basics of the geology of the coastline.
This stretch of coast – which is also known as Shipwreck Coast due to the number of seafaring vessels that have met their demise there – is comprised primarily of Port Campbell Limestone that was deposited under the sea roughly 15 to 20 million years ago during the Mid-Late Miocene period. This limestone isn’t consistent in durability: some parts that contain silt depositions are much softer than others that have a higher concentration of calcium carbonate, provided by the skeletons and shells of millions and millions of ancient marine creatures.
The limestone is also interspersed with layers of softer mudstone and calcareous clays. Generally speaking, these layers are located towards the bottom of the strata and are the first to be eroded by the extreme forces of waves, wind and rain. The different rate of erosion undermines the stability of the land; it leads to overhangs, sea caves, and arches, which in turn collapse to create the majestic pillars of rocks like the Twelve Apostles.
This process is still occurring now; evidence indicates that the Port Campbell Coast is eroding at a rate of roughly two centimetres per year. Geologists like Dr Eric Bird, who have extensively studied the limestone plain in the region, surmise the evolution of a rock stack from headland to arch to stack and eventual collapse can happen can take place in just 800 to 1800 years in this region.
In geological terms, this is a very short amount of time, but it is – for the most part –difficult to visually detect the changes in the landscape during a human lifespan. Every now and then, however, the impermanence of this landscape is visible for all to see in a sudden moment of drama – like, for instance, in July 2005, when one of the Twelve Apostles – a 50-metre-tall pillar – collapsed into the sea.
There are between 20 and 30 other similar limestone pillars scattered up and down this stretch of coastline, and new ones will continue to be formed as the wind and ocean continues to carve away at the cliff. But in March 2016, University of Melbourne PhD Student Rhiannon Bezore also discovered five “drowned” apostles in front of a submerged ancient coastal cliff six kilometres offshore while analyzing new sonar data collected as part of a project to survey potential reef habitats.
This was the first time such stacks have been found preserved below the sea. Like their terrestrial counterparts, they consist of limestone that had been carved out by erosion from softer surrounding rock.
20,000 years ago, at the end of the last ice age, these underwater pillars would have resembled their terrestrial counterparts. But as the ice rapidly melted, sea levels rose so quickly that the pillars were simply swamped in place. Instead of knocking them over, the ocean shaved off their tops, leaving them flattened; they now stand up to seven metres tall and are, on average, sixty metres below sea level.
These drowned apostles now support a thriving reef system. They also allow us to imagine what the coastline would have been like over 20,000 years ago – and what the future may hold for natural landmarks like the Twelve Apostles as sea levels continues to rapidly rise as a result of Anthropogenic climate change.
One of the most spectacular coastal drives in the world, the Great Ocean Road is the perfect road trip: with no shortage of natural wonders, rugged coastline, stunning national parks and friendly towns to explore. Whatever the season, the Great Ocean Road welcomes you.