Annually, a multitude of travellers congregate at the coastal bluffs flanking Victoria’s Great Ocean Road, their gazes fixed upon the iconic Twelve Apostles.
These majestic limestone formations, soaring up to 70 metres from the tempestuous Southern Ocean, stand as some of Australia’s most identifiable natural monuments.
However, despite their widespread recognition, the genesis of these landmarks remained largely enigmatic. This is no longer the case.
In groundbreaking scientific inquiry, recently disseminated within the Australian Journal of Earth Sciences, my associates and I have definitively elucidated this question, a narrative interwoven with primordial oceans, dynamic tectonic shifts, and a profound geological transformation commencing eons ago.
A Glimpse into Geologic Epochs
The limestone composing the Apostles encapsulates an extraordinary repository of millions of years of Earth’s history, particularly concerning its climatic evolution. Yet, it has historically attracted rather limited scientific scrutiny.
Each distinct stratum was meticulously deposited in shallow marine environments during the Miocene epoch. This era in planetary history, characterized by a significant climatic transition from warmer to cooler conditions, spanned an approximate timeframe from 23 million to 5 million years past.
Every alteration observed between successive sediment layers signifies a modification in the prevailing local environmental parameters, such as fluctuations in temperature, chemical composition, or oceanic currents.
My research collaborators and I undertook an exhaustive mapping exercise of the coastal cliffs and isolated sea stacks, employing high-resolution digital imaging techniques in conjunction with conventional field investigations and sample collection. Furthermore, we meticulously analysed fossilized microscopic marine organisms, specifically foraminifera, embedded within the geological strata. My calculations indicate that one of these formidable stacks harbours an astonishing approximately 760 trillion such microfossils.
Consequently, we were empowered to interpret the sedimentary layers with a precision akin to reading the annual growth rings of a tree.
This intensive investigation has afforded us the most accurate chronological determinations to date for the geological formation of the Apostles’ limestone. Our comprehensive fossil analysis reveals that the most ancient limestone strata date back approximately 14 million years, with the most recent layers being around 8.6 million years old.
Beneath the limestone, readily observable at the beach level to the east of the Apostles, lies an older stratum of yielding, dark-hued material designated as the Gellibrand Marl. This sediment was deposited at the base of deeper, warmer oceanic waters approximately 14 million to 15 million years ago.
Superimposed upon the marl, constituting the principal mass of the cliffs and the stacks themselves, is the Port Campbell Limestone. This geological formation originated in shallower, cooler aquatic conditions, accumulating over subsequent millions of years.
From approximately 14.1 to 13.8 million years ago, our fossil record meticulously documents a period when global temperatures were significantly elevated compared to contemporary levels. The strata from this epoch serve as a natural chronicle of elevated thermal conditions and sea levels, preserved with remarkable fidelity along the Victorian coastline.
Tectonic Activity, Inclination, and Thrust Faults
Thus, how did calcareous rock, formed beneath the sea’s surface, come to stand tens of metres above the ocean? The explanatory key lies in the fundamental principles of plate tectonics.
As the Australian continental plate progressively migrated northward following its separation from Antarctica, evolving stresses within the Earth’s crust exerted compressional forces upon the region, oriented predominantly in a northwest–southeast vector.
Commencing around 8.6 million years ago, this compressive stress induced a buckling and subsequent uplift of the submerged limestone, physically raising it from the sea. This geological process did not result in the perfectly vertical ascent of the strata.
Upon close examination of the present-day cliffs, a perceptible inclination of the once-horizontal layers, amounting to a few degrees, becomes evident. Furthermore, minor geological faults are discernible within the cliff faces, serving as residual indicators of ancient seismic events attributable to the same tectonic compressional forces.
The Cliffs: Geologically Contemporary
Our most unexpected revelation: while the underlying geological material dates back millions of years, the striking coastal topography we observe today is, from a geological perspective, remarkably recent in its current configuration.
The prominent sea stacks and cliff faces attained their present morphology only within the last few thousand years. This occurred after sea levels experienced a substantial rise of approximately 125 metres, a consequence of the conclusion of the last glacial period, which concluded roughly 20,000–23,000 years ago.
As the ocean once again advanced inland, wave action commenced its persistent erosion of the exposed limestone, a process exacerbated by prior weakening from tectonic stresses. The rock fractured and yielded to the relentless forces of erosion, successively forming headlands, then natural arches, which ultimately succumbed to collapse, leaving isolated stacks standing sentinel in the surf.
This dynamic geomorphological process continues unabated. When the Twelve Apostles were assigned their appellation—a designation imbued with a degree of poetic license—in the early 20th century, only seven or eight sea stacks were present (depending on classification criteria).
A significant collapse occurred in 2005, followed by the erosion of another in 2009, resulting in a generally accepted current count of seven. The unceasing power of oceanic waves guarantees that further structural failures are inevitable, underscoring the imperative to pursue ongoing research while these natural archives remain accessible.
A Vital Climatic Archive
The most exhilarating aspect of this research extends beyond our current discoveries; it encompasses the vast repositories of data yet to be deciphered within these geological formations.
We are currently engaged in the intricate task of reconstructing, with fine detail, the temporal dynamics of climatic shifts, sea-level fluctuations, and oceanic conditions across the vast expanse of millions of years of Earth’s history.
At a juncture where humanity confronts critical questions regarding our planet’s climate trajectory, the Twelve Apostles present an unparalleled repository of information, offering profound insights into past climatic conditions and potential future directions.
