The Seven Sisters of Sussex Are a Massive, Crumbling Plankton Graveyard

The chalk has lots of stories to tell.

The Seven Sisters are a series of iconic chalk cliffs guarding the English Channel in Sussex. It is a beautiful, peaceful landscape.

But travel back 85 million years, and East Sussex was the floor of a tropical sea, buried under an endless “blizzard of death.” This marine “snow” consisted of the calcium-rich skeletons of microscopic algae called coccolithophores. They drifted down for eons, piling up and eventually crushing into pure white rock: chalk.

But it took the violent collision of tectonic plates (the same geological shove that built the Alps) to rip this seabed upward and expose it to the sky.

Big, Tall, and Stunning

It is hard to wrap your head around the scale of the Seven Sisters. The cliffs peak at around 500 feet (150 meters) high, a sheer wall of calcium carbonate. All of this, plus the massive amount that has already eroded away, was formed by microscopic organisms.

About 80 to 90 million years ago, during the Late Cretaceous period, this part of the world was a warm, shallow sea, teeming with life. The most abundant life form was the coccolithophore, a single-celled phytoplankton that covered itself in tiny limestone plates.

When these creatures died, they sank. They drifted down to the ocean floor, piling up layer upon layer, century upon century. It took roughly 10,000 years to accumulate just one meter of this chalk ooze. When you look at a 150-meter cliff, you are looking at millions of years of snowfall.

The pressure of the water above eventually compressed this sludge into rock, then the weight of the rock itself added even more pressure.

But that’s not the end of the geology of the Seven Sisters of Sussex. If you look closely, you’ll also see jagged black lines running horizontally through the white chalk. It’s almost like someone took a giant marker pen to the landscape.

This is flint.

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While the chalk is made of calcium (from the algae), the flint is made of silica. Back in that ancient ocean, sponges and other siliceous organisms lived alongside the plankton. When they died, their silica dissolved into the sediment, collecting in the burrows of ancient sea creatures. Over millions of years, it hardened into the glassy, razor-sharp nodules we see today.

Early humans loved this stuff. It was the Swiss Army Knife of the Stone Age—perfect for arrows, axes, and starting fires.

The Great Retreat

That history is impressive, but here’s the bad news: The Seven Sisters are fading away.

Unlike the White Cliffs of Dover, which are protected by sea defenses, the Seven Sisters are allowed to erode naturally. This policy is called “managed retreat,” which is a polite way of saying “letting nature take its course.”

And nature doesn’t like these structures.

For thousands of years, the erosion was slow—about 2 to 6 centimeters a year. But recent studies from Imperial College London suggest that in the last 200 years, that rate has accelerated ten-fold. We are now losing anywhere from 22 to 60 centimeters of cliff every single year. In bad years, fueled by heavy storms, huge chunks of the cliff face simply shear off, sending thousands of tons of rock crashing into the surf.

Chalk is a soft rock. If it stopped eroding, it would quickly be covered in vegetation, turning green and brown. The constant cleaving of the cliff face exposes fresh rock, keeping the walls that brilliant, dazzling white. They are beautiful because they are being eroded.

The acceleration in erosion isn’t a coincidence. Rising sea levels and increasingly severe storms driven by a warming atmosphere are battering the base of these cliffs with more energy than before. The water eats the toe of the cliff, gravity does the rest, and the land retreats.

So if you get a chance, enjoy them. Who knows how long they’ll be around for?

This artile was originally published in 2015 and has been edited with additional information.