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<\/p>\nVictoria’s stunning sea stacks known as the Twelve Apostles were carved off coastal cliffs by waves and erosion in only the past few thousand years.<\/p>\n
But the geological history of the iconic monoliths, which tower up to 45 metres in height, and of the adjacent coastline goes back millions of years.<\/p>\n
It’s easy to see in the region’s exposed limestone faces, with about 40 kilometres of continuous cliffs that rise to 80m, that ancient marine sediments have been laid on top of each other over time.<\/p>\n
Now, a team of geoscientists has narrowed down when the different layers formed over a 16-million-year period, by analysing micro-fossils of sea creatures.<\/p>\n
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A clear layering of sediments can be seen in the sheer cliff faces of the Port Campbell coastal region. (Supplied: Stephen Gallagher)<\/p>\n
Their study, published in Australian Journal of Earth Sciences<\/a>, provides the most complete picture of the Apostle’s geological history so far by combining the findings of previously unpublished research from the 1960s with contemporary fieldwork.<\/p>\n “That is really exciting for two reasons,” lead author and micropalaeontologist Stephen Gallagher from the University of Melbourne said.<\/p>\n “Suddenly we know exactly where in Earth history these layers are and we can get a climate record,” he said.<\/p>\n “Also, we now know when the whole of the landscape of limestones went from under the sea to land 8.6 million years ago.”<\/p>\n What are the Twelve Apostles made of?<\/p>\n The study focused on a 17km stretch from a rock archway, known as The Arch, to Clifton Beach just to the south-east of the Apostles.<\/p>\n Besides shallow clays and sands on the top, most of the cliffs and rocks in this region are made from limestone \u2014 which is mostly calcium carbonate remnants of ancient sea life.<\/p>\n And in some areas you can also see an older layer of marl \u2014 made from clay and calcium carbonate \u2014 above sea level.<\/p>\n Both the limestone and the marl contain microfossils of ancient sea creatures.<\/p>\n Microfossils can be found within the limestone layers of the Twelve Apostles and the region’s coastal cliffs. (Supplied: Stephen Gallagher\/University of Melbourne)<\/p>\n Dr Gallagher said the largest Apostle contained about 760 trillion microfossils.<\/p>\n “Almost every grain in these limestones is a fossil, basically,” he said.<\/p>\n “But you can’t see them because they’re either microscopic or they’ve been smashed up.”<\/p>\n The time periods when many planktonic microfossils \u2014 tiny single-celled organisms with little carbonate shells \u2014 existed are well established.<\/p>\n So the identification of each species helped the study’s researchers work out when each layer of rock formed.<\/p>\n When did the different layers form?<\/p>\n The rocks that make up the Apostles originally formed in the ocean after Australia made a clean break from Antarctica about 30 million years ago.<\/p>\n As the Australian landmass started drifting northward, from the South Pole, the first layer of the Apostles was forming on a deep sea bed.<\/p>\n Shelly material and clay deposited from about 16 million to 14.1 million years ago became what is known as Gellibrand marl.<\/p>\n Parks Victoria say there are actually just seven limestone stacks on Victoria’s southern coast which make up the Twelve Apostles. (Pixabay: xiSerge<\/a>)<\/p>\n Then the ocean became shallower and a grey limestone layer, the colour coming from some still-present clay, formed up until about 11.6 million years ago.<\/p>\n As the water level became even shallower, a yellow limestone started to deposit.<\/p>\n Around the same time, the continental plate Australia sits on was colliding with the Pacific Plate to the north.<\/p>\n The tectonic shift caused new features, such as the Otway Ranges, to pop up, Dr Gallagher said.\u00a0<\/p>\n University of Melbourne associate professor Stephen Gallagher using a Jacob’s staff to measure the thickness of cliff layers. (Supplied: Mark Cuthell)<\/p>\n This geological activity continued to 8.6 million years ago \u2014 indicated by the shallowest layer dated from the yellow limestone \u2014 which was when the landmass that would eventually become the Apostles rose from the ocean.<\/p>\n “You [then] had this soil formed on top called the Hesse clay, but there was also sand dunes and some gravel,” Dr Gallagher said.<\/p>\n That landmass extended about 40\u201350km further south than the coastline today.<\/p>\n Curtin University geologist Milo Barham, who was not involved in the study, said the research made a useful contribution by integrating detailed coastal mapping and understanding of the age relationships and rock types along one of Australia’s most iconic coastlines.\u00a0<\/p>\n How did we get the sea stacks that make up the Apostles?<\/p>\n The seven sea stacks known as the Apostles were only formed by erosion in the last 20,000 years or so as ice sheets melted at the end of the last glacial maximum and sea levels rose.<\/p>\n![\"Close-up](\"https:\/\/www.newsbeep.com\/au\/wp-content\/uploads\/2026\/04\/028c0333c0242d502a54539bd9a36a5c.jpeg\")
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The Twelve Apostles before the 2005 collapse of a sea stack \/ and after. (Reuters: Parks Victoria)<\/p>\n![\"Large](\"https:\/\/www.newsbeep.com\/au\/wp-content\/uploads\/2026\/04\/1136e28568381922df255a790eac146e.jpeg\")
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