What Is a Sea Stack and How Are They Formed?

A sea stack is a column or pillar of rock that stands isolated in the sea, separated from the nearby coastline by the erosive power of waves. These dramatic landforms typically begin as part of a rocky headland and are gradually carved free as waves exploit weaknesses in the rock over decades to centuries. The process is more complex than simple wave-battering, and the classic textbook sequence of cave to arch to stack turns out to be just one of several pathways that produce these formations.

How Waves Turn Headlands Into Isolated Pillars

Coastal headlands are promontories of rock that jut out into the sea, and they receive more wave energy than the sheltered bays on either side. Waves concentrate their force on headlands because of refraction: as a wave approaches the coastline, it slows down over shallower water and bends to wrap around the protruding rock. This means the headland’s flanks get hit from multiple directions, and over time the rock erodes faster along those flanks than along the straight stretches of coast nearby.

When waves find a zone of weakness on a headland, they begin to carve inward. This can start as a notch at the base of the cliff, a shallow cave, or a channel cutting along one side. If erosion proceeds far enough to cut entirely through the headland, the seaward portion is severed from the mainland and becomes a sea stack. The newly isolated pillar then stands alone in the surf, continuing to erode on all sides until it eventually collapses or is worn down to a low-lying stump at or below the waterline.

Why Rock Fractures Steer the Process

Waves alone are not especially efficient at chewing through solid, intact rock. What makes sea stack formation possible in most locations is the presence of structural weaknesses in the rock itself: joints, faults, bedding planes, and fractures. These pre-existing cracks give waves a way in. Water surges into a joint, compresses the air trapped inside, and the repeated hydraulic pressure gradually wedges the crack wider. At Hopewell Rocks in New Brunswick, Canada, most of the stacks, arch-tunnels, and caves result from the dissection of the rock mass along prominent, well-spaced joint planes.¹ Where joints are closely spaced, the rock breaks apart too easily and erodes into a uniform cliff face rather than leaving isolated pillars. Where joints are too rare, waves struggle to gain a foothold. The sweet spot for stack formation is rock that has well-defined planes of weakness separated by wide enough intervals that substantial blocks of intact rock remain between them.

The type of rock matters too, but not in the simple “hard rock lasts, soft rock doesn’t” way you might expect. Stacks form in limestone, sandstone, basalt, chalk, and conglomerate. What they all share when stacks develop is some combination of structural integrity between fractures and a pattern of joints that channels erosion into discrete corridors. A headland made of very soft, rapidly crumbling material is actually less likely to produce a classic stack because it erodes too uniformly, retreating as a broad front rather than being selectively carved.

The Cave-Arch-Stack Sequence and Its Alternatives

The most widely taught formation story goes like this: waves carve a cave into the headland, the cave grows through the headland to form an arch, and eventually the arch’s roof collapses, leaving the seaward portion standing alone as a stack. This sequence does happen. But research into actual coastal sites shows that it is only one of several routes to stack formation, and probably not the most common one.

At La Jolla, California, historical photographs spanning more than a century allowed researchers to trace the origins of local arches and stacks. They found that each feature had a somewhat different cause, rather than all following a single process like erosion on two sides of a projecting point. The only arch still standing in the area at the time of the study was apparently the result of coastal caves being intersected by an embayment formed through combined land and sea erosion, not the simple two-sided erosion model.² Some arches formed when surge channels undercut hard layers sitting on top of softer material. Others were products of tectonic uplift exposing irregular coastlines to wave attack. The point is that stacks can be born from a variety of starting conditions, not just the tidy cave-arch collapse narrative.

Another pathway involves the gradual narrowing of a channel between the headland and a resistant block of rock, without an arch stage at all. Waves erode inward along parallel joints on either side of the headland’s flanks, and the middle portion is simply cut free when the two erosion fronts meet. Modeling work has shown that this type of alongshore-directed erosion, accelerated by beach sediment acting as an abrasive, can cut a channel through a headland and isolate a stack without any arch needing to form or collapse.³

Beach Sand as Both Chisel and Shield

One of the less intuitive factors in sea stack formation is beach sediment. Sand and gravel can play opposite roles depending on the width and position of the beach relative to the cliff.

A wide beach in front of a cliff actually protects it. Waves break on the sand, dissipating their energy before reaching the rock. This is why long sandy beaches rarely have stacks forming along their length. But a narrow beach, particularly one curving along the flank of a headland, does the opposite. Waves pick up sand grains and hurl them against the cliff face, and this abrasion accelerates erosion. The sand becomes a grinding tool. Along the flanks of headlands where pocket beaches tend to be narrow and curved, this abrasion can drive preferential erosion that eventually cuts through the headland and separates the outer rock as a sea stack.³

This dual nature of beaches creates a feedback loop. As the cliff erodes, it produces sediment. If it produces too much sediment too quickly, the beach grows wider and starts to protect the cliff, slowing further erosion. Numerical modeling suggests that when sediment input from cliff retreat is too high, such as when cliffs are made of sand-rich material that crumbles easily, stack formation is actually discouraged because the growing beach smothers the erosive process before a stack can be isolated.³

Why Headland Shape and Wave Direction Matter

Not every headland produces a stack. The initial shape of the headland turns out to be a surprisingly strong predictor. Headlands that are elongated, sticking out much farther into the sea than they are wide at their base, are more likely to be cut through and produce stacks. Rounder headlands that are much wider at their base than they are long tend to resist being severed because there is simply more rock for waves to chew through.³

Wave direction plays a role as well. Under a symmetrical wave climate, where waves arrive equally from the left and right, the erosion on both flanks of a headland is roughly balanced, and the resulting stacks tend to be larger. A highly asymmetrical wave climate, where most waves come from one direction, decreases stack size and can even prevent stack formation entirely through the uneven sediment dynamics it creates.³ Coastlines facing the open ocean with consistent swell from a single dominant direction may produce fewer stacks than coastlines that receive wave energy from multiple directions.

The Mushroom Stacks of Hopewell Rocks

Some of the most photographed sea stacks in North America stand at Hopewell Rocks on the Bay of Fundy, where the world’s most extreme tides expose the full height of the formations twice a day. These stacks have a distinctive mushroom shape: wider at the top than at the base, with deep notches carved into the rock at tide level. The notches are responsible for that top-heavy silhouette, and they form at predictable elevations. The deepest part of most notches sits a little below the mean high tide level, though some notches, especially on the landward side, extend a meter or two below it.¹

Interestingly, the notches are not always deeper on the seaward side, which is where you might expect the strongest wave attack. There is no consistent relationship between notch depth on the seaward and landward sides. However, notches on the seaward side do tend to sit at a higher elevation than those on the landward side. This likely reflects the way waves pile up and reach higher on the exposed face compared to the more sheltered rear of the stack.

Old photographs suggest these particular stacks may have developed within just the last 100 to 250 years, which is remarkably fast by geological standards.¹ The Bay of Fundy’s extreme tidal range, which can exceed 15 meters, likely accelerates the process by exposing rock to wave attack across a much larger vertical zone than most coastlines experience. Stack shape and notch depth change in a fairly predictable manner over time as the stacks become increasingly isolated from the retreating cliff behind them.

How Long Stacks Survive

Once a stack is formed, it is living on borrowed time. Cut off from the mainland, it no longer receives any structural support from the cliff behind it, and it is exposed to wave erosion on all sides. Most stacks are geologically short-lived features, lasting centuries to a few thousand years before collapsing. Some go much faster. The famous London Bridge stack along Australia’s Great Ocean Road lost its connecting arch in 1990, and one of the Twelve Apostles, also in Victoria, collapsed in 2005. Both events happened suddenly and without warning.

The collapse pattern tends to follow a recognizable sequence. First, notching at the base narrows the stack’s footprint, creating an increasingly top-heavy profile. The center of gravity shifts, stress concentrates in the thinning neck, and eventually the structure fails. After collapse, what remains is a stump, sometimes called a stack base, that may persist at or below tide level for a while longer before it too is worn smooth. The progression from tall stack to stump to submerged reef to nothing is the final chapter of a landform whose entire existence is one of active destruction.

When Stacks Threaten Buildings

The same erosive processes that carve stacks can pose real problems when cliffs are topped by buildings or infrastructure. Geophysical surveys at the historic Pointe du Hoc site on the Normandy coast, where U.S. Rangers scaled cliffs during D-Day, found that the highest hazard for mass movement was associated with marine caverns at the base of the cliff positioned at the point of strongest wave attack. Researchers concluded that these caverns occupy the future site of development of a sea arch that would threaten the observation post building above.⁴ In other words, the same process that creates a scenic sea stack in an uninhabited setting can be a structural emergency when it happens beneath a heritage site or a coastal road.

Monitoring cliff erosion to predict when the next arch or stack might form (or collapse) is an active area of coastal engineering. Techniques include ground-penetrating radar, resistivity imaging to map hidden cavities inside cliffs, and repeated lidar scanning to track how quickly cliff faces are retreating. The challenge is that erosion does not happen gradually and uniformly. Long periods of apparent stability are punctuated by sudden block failures, making prediction difficult even with good monitoring data.

Miniature Ecosystems Perched Above the Waves

Sea stacks are more than scenic rock pillars. Because they are isolated from the mainland, they function as tiny islands, and that isolation creates unusual ecological conditions. Mainland predators like rats, cats, and foxes often cannot reach them, which makes stack summits valuable refuges for nesting seabirds. Many stacks around the world serve as important breeding colonies for species that need predator-free nesting sites.

Plants colonize stack summits too, and their communities shift over time. A study tracking vegetation on a small rock stack off the Coromandel Peninsula in New Zealand found significant change over a 45-year observation period. The stack, a steep-sided column of rock with a gently sloped summit of roughly 80 square meters reaching about 20 meters above sea level, saw total vegetation cover on its summit increase from about 63% to 77%. Fourteen new species arrived during that period, and the vegetation shifted from being dominated by herbs and grasses to shrubs with the beginnings of forest development, including pioneering native trees.⁵ The researchers described the stack as an example of a historically rare ecosystem type in New Zealand, a category that extends to similar formations worldwide.

This ecological role raises a conservation tension. Sea stacks are temporary by their geological nature, constantly being worn down and destined to collapse. Protecting the species that depend on them means accepting that the habitat itself has a finite lifespan and planning for what happens when it disappears. For seabird colonies, the loss of a key nesting stack can have population-level effects if alternative predator-free sites are not available nearby.

Famous Stacks and What Makes Each One Different

The Old Man of Hoy in Scotland’s Orkney Islands is one of the tallest sea stacks in the British Isles, rising about 137 meters from the sea. It is carved from Old Red Sandstone sitting on top of a basalt base, and the difference in erosion rates between the two rock types contributes to its distinctive narrow-waisted profile. The basalt resists erosion better than the sandstone above, so the base is undercut less rapidly than the upper column. First recorded by maps in the 18th century, the Old Man is thought to have separated from the cliff within the last few hundred years, and climbing it has become a celebrated mountaineering challenge.

The Twelve Apostles along Victoria’s coast in southern Australia are limestone stacks carved from the high cliffs of the Port Campbell Embayment. Despite the name, there were never twelve of them standing at the same time; the count has fluctuated as old stacks collapse and new ones are occasionally isolated by ongoing erosion. Their limestone is relatively soft and porous, which means they erode faster than stacks made of harder rock, and some are visibly thinner or more undercut with each passing decade. The loss of one stack in 2005 was a reminder that these formations are transient even on a human timescale.

Elsewhere, Kicker Rock off the coast of San Cristóbal in the Galápagos is a volcanic tuff stack that rises vertically from the ocean floor and is split by a narrow channel. The Needles off the Isle of Wight are chalk stacks, bright white against the sea, formed as the chalk cliffs retreat. Each of these sites tells a slightly different geological story, shaped by local rock type, wave conditions, and tectonic history, but all share the fundamental mechanism of selective coastal erosion isolating a resistant remnant of a once-continuous landmass.

Common Misconceptions Worth Clearing Up

One persistent misconception is that sea stacks always form through an arch collapse. As the La Jolla research and the abrasion modeling both demonstrate, stacks can be born through several different mechanisms, and the arch stage is not a requirement. The cave-arch-stack sequence is a useful teaching model, but treating it as the universal rule overstates how orderly coastline erosion really is.

Another common misunderstanding is that harder rock automatically produces more stacks. Hardness matters, but structural characteristics like joint spacing and orientation are at least as important. A headland of moderately soft rock with widely spaced vertical joints can produce impressive stacks, while a headland of very hard rock with no exploitable weaknesses might erode so slowly and uniformly that no individual pillar is ever isolated.

People also tend to think of stacks as ancient features, imagining they must take millions of years to form. The evidence from Hopewell Rocks, where stacks may have developed in as little as a century, shows that the timescale is far shorter than most people assume. Under the right conditions, particularly with large tidal ranges, aggressive wave climates, and well-jointed rock, a stack can go from connected headland to freestanding pillar within a few human lifetimes. Equally, a stack that looks permanent can collapse overnight. The geology is fast enough to be genuinely surprising.