The Auld Acquaintance Cairn, a man-made pile of stones located along the Scottish-English border, is a symbol of British unity attracting supporters from across the country to contribute to the growing hill. (Griff Witte and Ashleigh Joplin/The Washington Post)

It began in July with a single stone placed along a bend in the River Sark, the muddy trickle in a sea of green fields where Scotland and England meet — and where they could diverge if Scots choose to break from Britain in Thursday’s independence vote.

As the polls have hardened into a dead heat, the river bend has become a pilgrimage site for those who want to save the United Kingdom. And that single stone has evolved into a 9-foot-tall, 350-ton monument to a country that may cease to exist as the world has known it for three centuries.

“It’s difficult to explain how moving it is,” said Christine Bethune, a 63-year-old library assistant from Edinburgh who fought back tears after adding a stone, as all pilgrims do. Hers was round and smooth, with red and green striations.

“If you broke it, it wouldn’t be the same stone,” she said quietly.

Building a pile of rocks may seem an unusual way to try to salvage the union at the heart of the United Kingdom. But the collection of tens of thousands of stones from all corners of Britain — many daubed in the red, white and blue of the Union Jack — has become a growing emblem of the country’s shared history. It also has struck a deep emotional chord that otherwise has been lacking from the unionist campaign.

Decorated stones form the Auld Acquaintance Cairn, a monument on the English border near Gretna, Scotland. (Matt Dunham/AP)

As Scottish nationalists have charted a vision for an egalitarian and just society apart from the austerity-minded English, the unionist camp has tried to scare Scots into shunning independence by focusing on all that could go wrong: the uncertain currency, the diminishing oil, the border that may need policing.

The stones — known as the Auld Acquaintance Cairn, from a poem by favorite Scottish son Robert Burns — send an altogether different message, one that highlights the common bonds between two groups that retain their own identities, but whose lives have become intimately entwined.

“When a relationship breaks, when one part of your country is trying to get a divorce, it doesn’t work to just say, ‘You’re never going to be able to survive on your own. You’re going to be too poor. You’re going to come running back,’ ” said Rory Stewart, a Parliament member who represents the English side of the border. He helped to lay the first stone in July and has overseen the cairn’s construction ever since. “You have to say, ‘I love you.’ ”

It’s an apt message for this quiet little border area, which in the 18th and 19th centuries was a destination for runaway brides and grooms who were too young to tie the knot in their native England, but who found looser marriage laws just across the River Sark, in Scotland.

Even then, Scotland and England had different legal and education systems, as they do today. But ever since the Acts of Union in 1707, the two nations have been peaceful partners following centuries of warfare.

The merger has been so complete that it has muddled the picture of who is English and who is Scottish. Among Scotland’s 5.3 million residents are a half-million people who were born in England, and who will be eligible to vote Thursday. But the 750,000 Scots who live elsewhere in the United Kingdom — in England, Wales or Northern Ireland — will be out of luck, with no voice in the future of their native land.

At the cairn, many of those who come to lay stones have identities on both sides of the border. The stones themselves come from all over — some have been carried across rough seas from remote Scottish isles, others mailed from the far reaches of England. A fragment of the Berlin Wall, and a bit of rubble from a Glasgow house bombed during the Blitz have been added to the pile.

On Sept. 18, voters in Scotland decide whether or not to end their 307-year union with England and become the newest independent nation in the world. Truth Teller puts campaign ads for and against Scottish independence to the test. (Davin Coburn/The Washington Post)

“I’m English, but I’ve lived in Edinburgh for 40 years. I was married to a Scot. My children were raised in Scotland — one is voting ‘yes,’ the other would vote ‘no,’ but is living in London,” said Bethune, the library assistant. “I understand Scottish pride. I’ve lived with it for a long time. But I think it’s possible to be proud of your country without dividing the union.”

The line of division, if it becomes a true border again, can be hard to find. With no natural geographic features to partition this island, the Romans built Hadrian’s Wall — but much of it is gone. The River Sark, which forms today’s western boundary between England and Scotland, is little more than a stream that can be forded with a couple of hops. Drivers crossing into Gretna on an old stone bridge may not even notice they have entered a new nation.

Nationalists say that won’t change if the union is dissolved and Scotland achieves independence. Much like another island to this one’s west — Ireland — Britain, they say, can be divided without border controls.

But British officials say that they are not so sure, and that differing security and immigration policies may force them to set up checkpoints at the crossings.

That could fundamentally change this area, which always has thrived on the flow of people and goods across the border.

“You don’t solve your problems by cutting yourself adrift and floating off into the North Sea,” said Benet Allen, a 47-year-old helicopter pilot who grew up on the Scottish side of the border and had his first job at a hotel on the English banks of the Sark. “The secessionists want to keep the pound. They want to keep the queen. They want to keep NATO. That’s not independence. There’s no such thing as independence in the modern world. We’re all tied together.”

And yet, his parents and three siblings will all be voting “yes.”

“I’ve come here to campaign against my family,” said Allen, who now lives in southern England.

He and others who gathered this week at the cairn were careful to say that if Scotland does break free, they’ll wish it well.

But there’s no question the debate has brought an unusual degree of discord — especially for an area like this, where the politics tend toward the mundane, and the focus is usually on weddings, not divorce.

“Scotland has never been more divided. And not only is Scotland divided — the whole U.K. is divided,” said Alan Marshall, a barrel-chested 62-year-old who dons full regalia and plays the bagpipes at up to a dozen weddings each day at the Famous Blacksmiths Shop in Gretna Green.

Back in the era of runaway brides, the blacksmith doubled as a priest. Today, the Famous Blacksmiths Shop is Scotland’s answer to Las Vegas’s Chapel of Love — a place to get hitched in a hurry beneath an archway of horseshoes, with Marshall always ready to serenade the happy couple.

After pausing to catch his breath and wipe the sweat from a brow partly obscured by an ostrich-feather headdress, Marshall said he’s a firm supporter of independence. “We can’t stick with the status quo,” he said.

But he won’t get to have his say in the voting booth Thursday.

The bagpipe player — that human symbol of all that is Scottish — lives in England.

Karla Adam in London contributed to this report.