A Coastline in Motion The sea stacks known as the Twelve Apostles are temporary features in a long geological story. These pillars are composed of Port Campbell Limestone, a rock deposited in a cool, shallow marine environment between 15 and 5 million years ago, during the Miocene epoch.%%CITE_1%%%%CITE_2%% The limestone is primarily made of the fossilized remains of marine organisms, including seashells and microscopic plankton, compacted over millions of years.%%CITE_1%%%%CITE_3%% The cliffs and the stacks are actively and visibly eroding at an average rate of about two centimeters per year.%%CITE_1%%
The process of their creation is also the process of their destruction. The powerful waves of the Southern Ocean attack weaker points in the limestone cliffs, forming caves. As erosion continues, these caves break through headlands to become arches. Eventually, the arches collapse, leaving isolated rock stacks up to 50 meters high. This entire cycle, from a solid cliff face to a collapsed stack, can take between 800 and 1800 years. At 9:18 AM on July 3, 2005, tourists watched as one of the nine stacks, a 50-meter-tall pillar, shuddered and collapsed into a pile of rubble within seconds. Another smaller stack collapsed in 2009, leaving the seven rock stacks that are visible today.
Reading the Rocks
Scientists monitor this coastline to understand the mechanics of coastal erosion. The Port Campbell Limestone is not uniform. It consists of layers of harder limestone alternating with softer, clay-based mudstone, which erodes more quickly. This differential erosion rate is what undercuts the base of the cliffs and stacks, leading to collapses. The horizontal layers seen in the cliffs are not flat but are tilted between one and three degrees due to tectonic forces from the Pacific Plate interacting with the Australasian Plate.
A surprising discovery has added another dimension to this location's history. In 2016, researchers using sonar mapping found five more limestone stacks 6 kilometers offshore, submerged under 50 meters of water. Nicknamed the "Drowned Apostles," these stacks are believed to have formed through the same erosional processes as the modern apostles, but around 60,000 years earlier. They were likely inundated by a rapid sea-level rise at the end of the last ice age, approximately 20,000 years ago. The tops of these 7-meter-tall drowned stacks are flattened, show the swift marine transgression that preserved them.