The limestone stacks known as the Twelve Apostles along Victoria’s Great Ocean Road attract millions of visitors yearly. Despite their iconic status, their geological origins remained poorly understood, with the last specialized study dating back to 1944. Associate Professor Stephen Gallagher of the University of Melbourne explained that limited access and expertise hindered research.

A new study published in the Australian Journal of Earth Sciences uncovered a far more intricate story spanning millions of years. The formations consist of Miocene-era limestone layers deposited between 23 and 5 million years ago, reflecting shifts from warm deep seas to cooler shallow waters. Researchers combined fieldwork, sampling, and high-resolution digital mapping to reconstruct these environments. Microfossils called foraminifera—preserved in staggering numbers—provided detailed records of ancient ocean temperatures and chemistry.
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Analysis revealed precise timelines: the oldest limestone dates back 14.1 million years, while the youngest is about 8.6 million years old. Beneath lies the Gellibrand Marl, formed in deeper, warmer seas, overlain by the Port Campbell Limestone deposited in cooler, shallower conditions. Remarkably, the formations preserve evidence of a globally warmer climate between 14.1 and 13.8 million years ago, with higher CO₂ levels comparable to future projections.

Tectonic forces lifted these marine deposits above sea level after Australia’s drift northward from Antarctica. The uplift tilted and fractured layers, later exposed to waves and erosion that sculpted the dramatic coastal landscape. Despite appearing horizontal, the strata are slightly inclined and faulted, direct evidence of ancient seismic activity.
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Surprisingly, the striking pillars are geologically young. During the last Ice Age 20–23,000 years ago, sea levels were 125 meters lower, and the coastline lay 50 km south. Rising seas after glaciation eroded weakened limestone, forming arches that collapsed into isolated stacks. Several have already fallen in recent decades, leaving seven main Apostles today, with others vulnerable to ongoing wave action.

Researchers emphasize these formations are temporary in geological terms. Future collapses and new pillars are expected, while climate change and rising seas may accelerate coastal changes. Gallagher concluded: “We can use these layers as time capsules to study past climates and sea levels. By looking into the past, we glimpse our future.”