Discovered: The Drowned Apostles
Wednesday, Mar 9, 2016, 01:53 AM | Source: Pursuit
By David M. Kennedy
Scientists have discovered five more of the iconic Twelve Apostles, the spectacular limestone range of columns that stand off Australia's southern coast – but the hitch is they are 50 metres under water.
In a surprise geological find, a University of Melbourne PhD student has identified a range of "drowned" limestone stacks in front of a submerged ancient coastal cliff about 6km offshore of the Twelve Apostles on Victoria's southern coast.
The discovery was made as researcher Rhiannon Bezore was analysing new sonar data collected as part of a project to survey potential reef habitats for sea-life such as crayfish and abalone.
"We are calling them the Drowned Apostles because if you had stood on this ancient cliff face over 20,000 years ago they would have looked largely the same as the current Twelve Apostles," says University of Melbourne coastal geographer David Kennedy, who is supervising Ms Bezore's PhD work.
It is the first time such stacks have been found preserved below the sea. And they simply shouldn't be there.
Ordinarily such structures should have been completely eroded before being submerged by rising sea levels. According to Associate Professor Kennedy, the most likely explanation is that around 20,000 years ago, at the end of the last ice age, sea levels rose so fast as the ice melted that the stacks were simply swamped in place.
Unlike their famous surface cousins, the tops of the "Drowned Apostles" are flattened, suggesting the softer rock at the top was eroded away quickly by the rising waters, effectively shaving off their tops.
"Sea levels probably rose at the end of the last ice age so quickly that the sea just ran across the top of these things without knocking them over," says Associate Professor Kennedy, from the School of Geography.
"It is amazing that they survived."
When Associate Professor Kennedy says the sea level rise was rapid he is talking in geological terms and not in terms of sudden catastrophic waves. Nevertheless he says the find suggests that sea levels back then rose by about five-to-ten times the current rate of 2.3mm a year along the Victorian coast, driven by the sheer size of the ice melt.
"When we go into an ice age it is a very slow freezing process, but when it decides to melt it generally does it quite quickly," he said.
He says the sea level rise would have been readily noticeable within a generation to Aborigines living along the original coastline.
The "drowned" stacks are just like the Twelve Apostles, consisting of limestone that had been carved out by erosion from softer surrounding rock.
These things would have been big stacks just sitting out in an open coastal plain beyond a dry cliff.
"If you had stood on that ancient cliff line they would have looked similar to the current Apostles, but when sea levels rose the sea went straight across the top of them," Associate Professor Kennedy says.
In the process their height was worn down by erosion well below the height of the Twelve Apostles, suggesting that the Drowned Apostles had stood for much longer under eroding conditions before they were submerged still intact. They now stand up to nearly 7m tall compared to the Twelve Apostles that range from 30m to 67m tall.
Ms Bezore, a Californian who came to Australia to work with Associate Professor Kennedy after completing a masters degree at University California Santa Cruz, had been going through the sonar data comparing the existing coastline with the ancient submerged coast as a way to model the location of potential sea-life habitats. When she stumbled across the submerged limestone stacks she recognised them immediately, but doubted what she was seeing.
"I was pretty surprised to see them," Ms Bezore says. "We had to check with each other on what we were seeing because no one has seen stacks submerged at this sea level before."
The similarity to the Twelve Apostles was obvious. "It was so close by and of a similar shape and form that there has to be some correlation here," says Ms Bezore.
The sonar data had been collected using advanced multi-beam sonar technology bolted to the bottom of Deakin University's $650,000 research vessel the "Yolla" as part of a project to map the ocean floor along Victoria's coast line. Deakin marine scientist Dr Daniel Ierodiaconou, who is also Ms Bezore's co-supervisor, has led the mapping project. The data was collected for Parks Victoria and the Port Campbell Marine Park.
"As the drowned Apostles are found in the same geological setting as the current Twelve Apostles, it is reasonable to assume that they were formed under the same geomorphic processes, some 60,000 years apart," the researchers say in the journal article. "Were it not for the relatively quick submergence of the stacks, they likely would have continued to erode at a similar rate as seen with the modern sea stacks until they collapsed."
Associate Professor Kennedy said the discovery would add a new point of interest to visiting the Twelve Apostles, allowing people to imagine what the coastline would have been like over 20,000 years ago.
The Twelve Apostles have long been a tourist attraction along the cliffs of Victoria's "Ship Wreck Coast." The stacks were originally named the Sow and Piglets, with nearby Mutton Bird Island the "Sow," but the formation was renamed the Twelve Apostles in 1922, even though there were only eight of them. On July 3, 2005 one of the tallest of the Apostles at about 45 metres high suddenly collapsed leaving seven remaining. While some count eight Apostles by adding in a much smaller stack, Park Victoria is adamant there are just seven.
"To know there are more Drowned Apostles offshore I think will add a whole new dimension to the experience," says Associate Professor Kennedy.
And with five new apostles under the sea to go with those remaining above the sea, it means we now really do have 12 Apostles.
Banner Image: Tourism Victoria/James Lauritz