DURDLE Door is one of Dorset's most iconic landmarks, a natural limestone arch stretching out to sea along the Jurassic Coast World Heritage Site.

Tens of thousands of people, from around the world, flock to see the stone arch every year.

Durdle Door. Picture: Lewis Johnston

Durdle Door. Picture: Lewis Johnston

But what they may not appreciate is that one day, Durdle Door is doomed to collapse into the sea, claimed by the same forces that helped sculpt the natural structure over the ages.

Read more: Jurassic Coast named as one of Seven Natural Wonders of the UK

The Daily Echo spoke to Sam Scriven, head of heritage and conservation at The Jurassic Coast Trust, to find out how the arch was formed, and, ultimately, what its fate will be.

So just how was Durdle Door formed?

According to Sam: "Like a lot of things in geology the explanation is always to do with some kind of process that just takes time.

"You have to try to unpick the process in little bits, so there is a bit of a story to tell to understand Durdle Door."

In the simplest terms, Durdle Door is a big slab of rock which has been punched through by the sea.

""So let's take the slab of rock first, " said Sam. "That is an important part of the story, it is where we start."

The raw materials

The rock that makes up Durdle Door is Portland Stone, the same hard rock they quarry on Portland further along the coast.

Dorset - Home of the Jurassic Coast

Dorset - Home of the Jurassic Coast

It is a kind of limestone laid down at the end of the Jurassic period, about 145 to150 million years ago, when Dorset had a climate more like the Bahamas.

So this brings us to the seabed.

"But the seabed is not stuck on in the sir like that, it is not vertical...it is flat," said Sam.

And this is the next chapter in the story, which took place about 30 million years ago

Sam said: "This is when far, far to the south of us the African tectonic plate and the European tectonic plate collided, and that formed the Alps."

"Huge pressures buckled the Earth's surface to form a mountain chain

"Those pressures rippled through the Earth's surface and affected the rocks in what we know as southern England."

So the rocks around Durdle Door and Lulworth Cove that should be laying horizontal, have actually been tilted – so they are almost standing up on end.

The work of the sea

Sam said: "We've told the story of how we got the raw material with which the sea is going to work – the sculptor coming along now to shape this hard limestone."

Different types of rock will be affected in different ways by the sea.

"Amongst the Portland Stone, either side of it (Durdle Door), you have softer rock," said Sam. "You see that at Lulworth Cove as well."

At Lulworth the cliffs are Portland Stone, which have taken the full battering of the sea.

But, as Sam explains, a small gap was once formed by a river. Once the sea got in it carved out those softer rocks behind the limestone and washed them all away.

Sam said: "When it comes to Durdle Door, that explains why you have the limestone stuck out on its own in the sea, because the rocks behind it have been almost washed away.

"When people come down in the summer, they trudge down those clay soft bits to get to the beach."

How did the arch take shape?

So we have this large slab of rock being slowly eroded by the sea.

And one of the ways this happens is that a notch is formed at the base of the cliff, which becomes a cave over time.

Durdle Door Picture: Echo Camera Club/Tom Taylor

Durdle Door Picture: Echo Camera Club/Tom Taylor

"Eventually the cave goes all the way through that hard stone and the cave becomes an arch," said Sam.

What is the end of the story?

Sam said: "Durdle Door is the process halfway finished.

"Ultimately what will happen is that the arch will collapse and leave a stack."

Indeed, we see an example of this at Old Harry Rocks.

Old Harry Rocks

Old Harry Rocks

"There are some chalk stacks there and the same kind of process would have happened there," said Sam. "There would have been an arch – it just didn't last as long."

Sam says the eventual collapse of Durdle Door will likely be "a very sudden event."

He explained: "In terms of when it is going to happen, it is very, very hard to predict. But I think it will happen one day when we get a big storm.

"There will be some terrific storm and it will be just one storm too many for Durdle Door.

"It might not necessarily be during that storm that the arch will collapse, but some kind of weakness will form which will begin the journey.

"Obviously, we hope very much that nobody will get hurt when that happens.

"It could happen anytime, but that is true for any part of the natural coastline.

"The sea will keep on sculpting it (the natural coastline) like it has for thousands of years.

"Aren't we lucky because it has created all this amazing natural beauty."

So with rockfalls in mind

"We come back to that point that these things are unpredictable, that is our starting point," said Sam.

"First of all people should take seriously the hazards that exist in this special environment. You are going into a natural environment, make an effort to understand what the hazards are.

"One of those hazards is the sea – it is very silly to go out on the beach in a storm.

"It is not very sensible to walk right along the cliff face, or linger or sit below the cliff.

Landslip at West Bay, Dorset

Landslip at West Bay, Dorset

"Our advice is that if you are going to explore the beach, go at low tide, certainly not in a storm, and stay away from the cliff face itself.

"Don't linger, just keep walking, keep moving."

Read More: Dramatic cliff fall sparks 'stay away' warning

And at the cliff top?

Sam said: "What we see a lot of these days is people standing on the cliff edge trying to get that perfect photo.

"This is a very dangerous place to go, you are at the edge of a cliff."

He reminds readers that the Jurassic Coast path is never right at the cliff edge, and that is for a reason.

What can you do to help?

Sam is one of the team at The Jurassic Coast Trust

It is the charity charged with looking after the World Heritage Site.

They work closely with other organisations, including the Lulworth Estate – in which Durdle Door is situated.

Read more: The shocking impact of 'hit and run' visitors on Durdle Door

"We would always encourage that everyone has respect for this place," said Sam. " It is a very special place.

"We would always encourage people to try to help the landowners down at Lulworth as much as they can by following the signage, following the rules and showing respect to the staff and businesses that are there.

"In particular when it comes to litter - take all you rubbish home with you."

Litter left at Durdle Door following huge crowds in 2020

Litter left at Durdle Door following huge crowds in 2020