‘A history of the Earth’: Twelve Apostles revealed to be as old as 14m years

Microscopic fossils embedded in limestone have helped reveal the true age of Victoria’s Twelve Apostles, as 8.6 to 14m years old.

The conclave of giant golden pillars is visited by 2.8 million tourists each year, a highlight for those travelling along the Great Ocean Road south-west of Melbourne.

But where visitors see rocks rising out of the Southern Ocean, geologists see layers of history, said A/Prof Stephen Gallagher, from the University of Melbourne. “We see layers, we see time, we see a history of the Earth”.

Tectonic plate movements over millions of years have lifted and tilted the layers of underlying Gellibrand marl (15m years old) and the Apostles’ yellow and grey Port Campbell limestone, rocks formed since the mid Miocene, he said. They are topped by a more recent layer of red-brown soil called Hesse clay.

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“The tectonic movements didn’t push up the Apostles perfectly straight. Instead, they forced layers to tilt and break along the way. If you look closely at the cliffs around the Twelve Apostles today, you can see the limestone layers are not flat but are, in fact, tilted by a few degrees. Small fault lines can also be seen, which are records of ancient earthquakes,” he said.

The rocks might be ancient, but the seven or eight sea stacks – originally known as the “Sow and Piglets” on nautical maps – formed more recently as the cliffs eroded.

“You go to the look out platforms and you see this fantastic vista in front of you with millions of years of strata, and millions of years of a story. And then those sea stacks are the tiny last minute in that story, the last few thousand,” Gallagher said.

“Because only 20,000 years ago, you could walk to Tasmania because Bass Strait was a lake – you could walk about another 70km offshore from the present Twelve Apostles and you would still be on land.”

The geology of the Twelve Apostles has now been painstakingly described and precisely dated by scientists using photographs, digital mapping, samples, measurements of gamma radiation and analysis of minuscule popcorn-like fossils extracted from the rock. The results are published in the Australian Journal of Earth Sciences.

The fossils, tiny critters that evolved, lived and went extinct at particular times, were used to establish exact ages for each layer of rock.