The Twelve Apostles are made of limestone, specifically a formation known as the Port Campbell Limestone, which dates to the Miocene epoch roughly 10 to 20 million years ago. These towering sea stacks along Victoria’s southern coast are carved from a rock that was once a shallow seabed teeming with marine life, and the fossils of those creatures are literally what the limestone is built from. The rock type explains almost everything about the Apostles: why they look the way they do, why they erode so quickly, and why some have already collapsed.
The Port Campbell Limestone
The rock forming the Twelve Apostles belongs to a geological unit called the Port Campbell Limestone, found in the Otway Basin along the southeastern coast of Australia. It is a sedimentary rock, meaning it was laid down grain by grain over millions of years on a relatively shallow continental shelf. Geologically, the formation is classified as a temperate carbonate grainstone, a type of limestone composed almost entirely of calcium carbonate fragments cemented together. The “temperate” label matters because it distinguishes this limestone from the tropical reef limestones most people picture. There were no coral reefs here. Instead, the rock was built from the skeletal remains of cooler-water marine organisms that thrived in seas more similar to today’s Southern Ocean than to a Caribbean lagoon.
The stacks themselves reach up to about 45 metres above sea level, which gives a sense of how thick the original limestone deposit was before erosion began sculpting it into cliffs, arches, and isolated pillars.1Journal of Coastal Research. The Drowned Apostles: The Longevity of Sea Stacks over Eustatic Cycles The Port Campbell Limestone extends well beyond the Apostles themselves, forming the dramatic coastal cliffs of Port Campbell National Park and continuing inland beneath paddocks and farmland for some distance.
What the Rock Is Actually Made Of
If you could look at a thin slice of one of the Apostles under a microscope, you would see a jumble of tiny fossil fragments packed together like gravel. The dominant components are the shells and skeletons of organisms that lived on and near the ancient seafloor: bottom-dwelling foraminifera (single-celled creatures that build intricate calcium carbonate shells), bryozoans (colonial animals sometimes called “moss animals” that build lacy, branching structures), brachiopods (shellfish that look superficially like clams but are unrelated), and echinoids (sea urchins and their relatives). Mixed in with these are planktonic foraminifera, the shells of tiny organisms that floated in the water column and rained down onto the seabed after death.2Sedimentology. Marine‐derived dolomite in the shallowly buried temperate Port Campbell Limestone (Miocene), Otway Basin, Australia
This composition is what makes the Port Campbell Limestone a “grainstone” rather than, say, a mudstone or a chalk. The individual grains are sand-sized or larger fossil fragments rather than fine lime mud. In practical terms, the rock has a grainy, somewhat porous texture. When you see close-up photographs of the Apostles’ cliff faces, the rough, pitted surface you notice is partly a reflection of this grain structure: as the softer cement between fossil fragments dissolves or washes away, the rock develops an uneven, almost honeycomb-like surface.
Because the grains are sand-sized carbonate particles, the rock can also be described as a calcarenite. That term comes from “calci-” (calcium carbonate) and “arena” (sand), and it simply means a limestone whose building blocks are sand-grain-sized. You will sometimes see the Apostles described as calcarenite rather than limestone in geological literature. Both terms are correct; calcarenite is just more specific about the grain size.
From Seafloor to Cliff Face
The Port Campbell Limestone was deposited during the Miocene, a geological epoch spanning roughly 23 to 5 million years ago. At that time, the area now occupied by Victoria’s coast was submerged under a cool, relatively shallow sea. Marine organisms lived and died in enormous numbers on that continental shelf, and their skeletal debris accumulated layer upon layer on the seafloor. Over millions of years, the weight of overlying sediment and the slow chemical action of mineral-rich water cemented these grains into solid rock.
The limestone was never deeply buried. Studies of the formation describe it as having been buried to less than about 350 metres before it was uplifted and exposed at the surface.2Sedimentology. Marine‐derived dolomite in the shallowly buried temperate Port Campbell Limestone (Miocene), Otway Basin, Australia That shallow burial is significant. Rocks that have been pushed deep underground and subjected to intense pressure tend to become very hard and densely cemented. The Port Campbell Limestone never went through that process. It remains a relatively soft, porous rock, easy for water, wind, and salt to penetrate and break apart. This is one reason why the coastline here erodes so dramatically compared to coastlines made of harder rock like granite or basalt.
Eventually, tectonic uplift and falling sea levels brought the limestone above the waterline. The sea that had deposited the rock now set about destroying it. Waves began attacking the newly exposed cliffs, and the long, slow process of carving sea stacks began.
How Waves Carve Sea Stacks from Limestone
The Twelve Apostles did not start as isolated pillars. They started as part of the mainland cliff. The process that created them follows a well-understood sequence familiar to coastal geologists around the world, but it plays out with particular speed in soft limestone.
Waves pound the base of a cliff relentlessly, concentrating their force at and just above the waterline. In limestone, the rock is soluble in seawater (especially slightly acidic water), so chemical dissolution works alongside the physical hammering. Over time, a notch forms at the cliff base. The notch deepens until the rock above it can no longer support its own weight, and a slab collapses. The cliff retreats, and the cycle repeats.
Where the cliff juts out as a headland, waves can attack from both sides. They exploit cracks, joints, and bedding planes in the rock, hollowing out caves. When caves on opposite sides of a headland break through to each other, a sea arch forms. Eventually the arch’s roof becomes too thin and collapses, leaving an isolated pillar of rock standing offshore: a sea stack. That is what each of the Apostles is, a remnant headland whose arch has long since fallen.
The speed of this process at Port Campbell is striking. The cliffs here retreat at a rate estimated at roughly one to two centimetres per year on average, though local collapses can be sudden and dramatic. One of the Apostles collapsed entirely in July 2005, falling into a pile of rubble in a matter of seconds. There was no warning. One moment a 50-metre-tall pillar of rock stood in the surf; the next it was a low mound of debris being washed away by waves.
Salt Weathering and Why Bedding Planes Matter
Waves are only part of the story. Above the waterline, the Apostles are also attacked by salt weathering, a process that can be surprisingly destructive to porous limestone. Seawater soaks into the rock through its pores and tiny fractures. When it evaporates, it leaves behind salt crystals, mostly sodium chloride. As those crystals grow, they exert pressure on the surrounding rock from the inside, prying grains apart and gradually weakening the stone.
Research on calcarenite, the same rock type as the Port Campbell Limestone, shows that the orientation of the rock’s internal layering has a major effect on how salt weathering plays out. Samples cut so that their bedding planes were perpendicular to the exposed surface accumulated significantly more salt than samples with bedding planes running parallel to the surface. In one set of experiments, perpendicular-oriented samples showed halite (salt crystal) concentrations of roughly 4.5 to 5.2 percent, while parallel-oriented samples accumulated only about 2.7 to 4.2 percent.3Journal of Building Material Science. Salt Weathering in Anisotropic Calcarenite: Bedding-plane Controls on Sodium Chloride Precipitation Patterns The bedding planes act as pathways that either help or hinder water and salt movement through the rock, depending on their angle relative to the surface.
For the Apostles, this means that different faces of the same stack can weather at different rates depending on how the internal bedding layers happen to intersect the exposed cliff face. Some faces are riddled with cavities, overhangs, and flaking surfaces, while others on the same stack appear relatively smooth. The rock is the same, but its internal architecture interacts differently with the elements depending on orientation. This is one reason the stacks develop their characteristically uneven, sculptured shapes rather than eroding uniformly.
How Many Apostles Remain
Despite the name, there were never twelve stacks visible at once in the modern era. The formation was originally called the “Sow and Piglets” until it was renamed for tourism purposes in the 1950s. At the time of renaming, there were nine stacks. After the 2005 collapse, eight remain.1Journal of Coastal Research. The Drowned Apostles: The Longevity of Sea Stacks over Eustatic Cycles New stacks are in the process of being created, as arches and headlands along the same stretch of coast continue to erode, but that process takes thousands of years. In the meantime, the existing stacks continue to shrink. Each storm, each salt crystal, each wave notch brings them closer to collapse.
The 2005 collapse made international news partly because it happened in broad daylight with tourists watching from the nearby viewing platform. Nobody was hurt, but the event served as a vivid reminder that these formations exist on a geological clock that occasionally ticks in human-visible increments. Park authorities have long warned that more collapses are inevitable and that the viewing platforms are set well back from the cliff edge for good reason.
Why Limestone and Not Something Harder
Visitors sometimes wonder why this stretch of coast has such dramatic stacks and arches when other Australian coastlines do not. The answer is almost entirely about rock type. Limestone, especially a porous, lightly cemented grainstone like the Port Campbell Limestone, is one of the most erosion-prone rock types that a coastline can be made of. It is soluble in slightly acidic water, it is physically soft enough for waves to abrade, and its porosity makes it vulnerable to salt weathering from the inside out.
Compare this to a coastline made of granite or basalt. Those rocks are far harder, far less porous, and essentially insoluble in seawater. A granite coast erodes too, but on a timescale orders of magnitude slower. You get rocky shores and boulders, not towering stacks and delicate arches. The spectacular scenery at Port Campbell is, paradoxically, a product of weakness. The rock is soft enough to be sculpted but hard enough to stand for a few thousand years before it falls. That sweet spot of erodibility is what produces sea stacks, and it is a defining characteristic of limestone coasts worldwide.
It also helps that the Port Campbell Limestone is relatively uniform. Some coastal formations involve alternating layers of hard and soft rock, which erode in steps and ledges. The Port Campbell Limestone is fairly consistent in composition from top to bottom, so erosion produces smooth, vertical cliff faces and tall, columnar stacks rather than terraced or stepped profiles. The consistency of the grainstone gives the Apostles their characteristic sheer-sided appearance.
Submerged Stacks and Ancient Coastlines
The stacks you see today are not the only Apostles that have ever existed. During past ice ages, when sea levels were much lower than today, the coastline sat kilometres further out to sea than its present position. Waves carved stacks and arches from the Port Campbell Limestone at those lower shorelines too. When sea levels rose again at the end of each ice age, those earlier stacks were drowned. Bathymetric surveys of the seafloor offshore from the current Apostles have revealed the stumps and bases of older, submerged sea stacks sitting on the continental shelf, sometimes called the “drowned Apostles.”1Journal of Coastal Research. The Drowned Apostles: The Longevity of Sea Stacks over Eustatic Cycles
These submerged remnants tell geologists something interesting about how long sea stacks can survive. A stack standing in shallow water after being drowned by rising seas is no longer being attacked at its base by breaking waves in the same way. It still weathers and dissolves, but the primary erosive force, wave impact at the waterline notch, is now distributed across a different part of the rock or reduced by deeper water. Some of these drowned stacks appear to have persisted for tens of thousands of years or longer beneath the surface, far outlasting what they would have managed if they had stayed exposed to surf-zone conditions.
The existence of multiple generations of stacks, both modern and drowned, also tells us that the Port Campbell Limestone has been producing sea stacks repeatedly through many cycles of rising and falling sea levels. The current Twelve Apostles are simply the latest generation in a process that has been going on for hundreds of thousands of years. Future generations of stacks are already being roughed out in the arches and headlands visible along the coast today.
Fossils You Can Actually See
Because the Port Campbell Limestone is made of compressed fossil fragments, the rock is full of visible remains if you know what to look for. Fallen boulders and rubble at the base of the cliffs sometimes show cross-sections of bryozoan colonies, the circular outlines of echinoid spines, and the chambered shells of foraminifera. You cannot collect fossils from the national park (it is protected), but simply looking at loose rocks on the beach reveals the biological origins of the stone.
The bryozoan content is particularly characteristic. Bryozoan-rich limestones are a hallmark of temperate-water carbonate deposits in the Southern Hemisphere, and the Port Campbell Limestone is one of the best-known examples.2Sedimentology. Marine‐derived dolomite in the shallowly buried temperate Port Campbell Limestone (Miocene), Otway Basin, Australia These organisms built intricate, branching or net-like calcium carbonate skeletons that, when broken up and cemented together, create a rock with a distinctive porous texture. If you have ever seen a piece of coral rock and noticed the tiny holes left by individual coral polyps, bryozoan limestone has a similar feel, though the structures are finer and more irregular.
Brachiopod shells, which look like small, ridged clam shells, are another common find. Though brachiopods are rare in most modern oceans, they were abundant in the Miocene seas that deposited this limestone. Their presence in the rock is a reminder that the marine ecosystem off southern Australia looked quite different 15 million years ago, even though the water temperatures were broadly similar to today’s.
How the Rock Relates to the Famous Golden Colour
One of the most photographed features of the Twelve Apostles is their warm, golden-to-ochre colour, especially at sunrise and sunset when the light hits the cliff faces at a low angle. The colour is not intrinsic to pure limestone, which tends to be white or pale grey. Instead, the golden tones come from trace amounts of iron oxide within the rock. As groundwater and rainwater percolate through the limestone over millennia, they oxidise tiny amounts of iron present in the sediment, staining the rock yellow, orange, and brown. The staining is typically strongest near the surface, where exposure to air accelerates oxidation, while freshly broken rock deeper inside a stack tends to be paler.
Salt weathering contributes to this effect indirectly. As salt crystals grow and spall off surface layers of rock, fresh surfaces are continuously exposed to air and water, which then stain them in turn. The result is that the Apostles maintain their golden appearance even as their surfaces are constantly being renewed by erosion. A freshly collapsed face may look startlingly pale compared to the weathered surfaces around it, but within a few years it too develops the characteristic warm patina.
The interplay of rock colour, surface texture, and the ever-changing light over the Southern Ocean is what makes the Apostles so visually dramatic. But all of it traces back to the rock type: a porous, iron-bearing, fossil-rich limestone that is soft enough to sculpt, open enough to stain, and weak enough to eventually fall.