Of the several crossing points between England and Scotland, the most celebrated is also the ugliest. Ever since a toll road stretched across the border from Carlisle in the late 18th century, Gretna has been making money from the traveller. At first it was weddings; Scotland’s more lax marriage laws made the country an obvious destination for any young English couple whose marriage faced parental veto and who could afford the journey. Anywhere in Scotland would have served, and almost any respectable figure could have officiated; the job wasn’t restricted to the local clergyman or registrar. But early publicity established Gretna and its little spouse, Gretna Green, as the places to get married in – whoever heard of runaways running away to, say, Falkirk? Gretna promised speedy consummation, with lodgings only a few hundred yards from the border, and the fairytale-like presence of a village blacksmith who would conduct the ceremony over his anvil. By the time legislation in England and Scotland removed the need for runaway weddings, the Gretna ceremony had become a marketable tradition, though today it must count for far less to the local economy than the Gretna Gateway Outlet Village.

Drivers and their passengers can see the back of this shopping centre from the M6 – a long, low huddle of buildings decorated with brand names. Coming and going by car every summer, I’ve sped by it for nearly 20 years and never once been tempted to slow down and turn off to buy an end-of-the-line perfume or a slightly flawed anorak, despite the promise of reduced prices implied by the phrase “factory outlet” and by the border location, which to a foreign tourist confused by British devolution might suggest goods that are duty-free. But this week, searching for a monument, I drove by mistake into the car park and discovered a discrete world of hyperactive commerce that must have closed shops in town and village main streets from Hexham to Dumfries. Traffic queued to find parking spots; men and women laid long parcels carefully into open tailgates; the air of satisfaction was general – among those who had spent their money and those who were about to spend it.

It seemed impossible that the Auld Acquaintance cairn should be located here, but that’s what I’d been told: “It’s one field south of the outlet village.” I peered through the bushes where the shops end and could see nothing. An online consultation yielded a different approach. I followed instructions and drove south down the Glasgow Road as far as the old toll house that advertises itself as the first and last house in Scotland, and there beside the bridge in the field next to the river lay the unprepossessing heap that the Tory MP, Rory Stewart, hoped would be “a lasting marker of our union, something that future generations will look back at and remember, with deep gratitude, the moment we chose to stay together”.

That was only a year ago. The referendum was a few weeks away. It was Stewart’s idea that the people of the United Kingdom demonstrate their historical friendship in an act of monumental participation – by bringing stones from all over the British Isles to this site beside the River Sark, which here marks the national boundary. Slate came from Wales, quartz from Skye, chalk from Sussex: one of the cairn’s enthusiasts called it “a geological love poem to the union”, echoing the then current analogy that the UK was like a failing marriage heading for divorce. In Stewart’s words, you couldn’t say to a partner who was threatening to leave that they were too poor to survive on their own, or that you didn’t care if they left or not … “You have to say, I love you,” Stewart said. He hoped this would be the message of the stones.

How many people came with a stone isn’t clear – certainly only a tiny fraction of the people who travel up and down the M6, which is reported to carry more than four fifths of Anglo-Scottish road traffic. But the stones are believed to number 100,000, to which I added two. Late last summer, when opinion polls were tilting towards a Yes vote and for the first time causing genuine anxiety south of the border, many distinguished supporters of the Union came forward to give Stewart’s cairn their support. Most were English: the explorer Ranulph Fiennes; the historians Simon Schama, David Starkey, Antony Beevor and Max Hastings; the philosopher AC Grayling; the ubiquitous signature-appender Joanna Lumley. The stones rather let down the glorious names and the noble ambition. They have been beautifully penned in by drystone walls of red sandstone, which cut a passage to the cairn’s centre as though it were a neolithic dwelling or burial chamber, but the frame here is better than the picture. Many of the stones left by visitors have been painted with bright flags and messages: “UK OK” and “NICOLA STURGEON LOVES BRITS.” A monument intended to celebrate beautiful generalisations – political unity, friendship between peoples – has been altered (an old-fashioned aesthete would say damaged) by the intrusion of the specific and the everyday.

Unlike the carpark at the Gretna Gateway Outlet Village, the space around the Auld Acquaintance cairn was empty. The river flowed quietly towards the Solway. Not much more than a 100 yards away, lorries and cars buzzed along the M6. After I laid my stones – selected from a nearby pile – I thought how long ago it all seemed. Gordon Brown, Alistair Darling, Danny Alexander, Douglas Alexander; every sitting Labour and Lib Dem MP in Scotland, except two. Other than Brown and Darling, who were already going, how many could have foreseen that their Westminster careers would be over before a year had passed? That pan-British political parties in Scotland might be finished? I looked at the cairn and understood that it came from that different time.

On our way north, we stayed the night at a B&B near Lanercost Priory, most of which has been a beautiful ruin since the Reformation. It lies in the wooded valley of the Irthing close to Hadrian’s Wall, which is why many people come – in fact, why I first came as a boy in 1956 on the back of a tandem with my dad. He was a terrific Roman wall enthusiast – this may have been his fourth trip – and I think he hoped to share his sense of wonder with his son: to introduce me to the world of hypocausts and centurions there on the northern fells. Unfortunately, I remember little of this beyond the names of forts. (Birdoswald was a strange one.) My clearest memory is of sheltering from the rain below an ancient bridge in Lanercost, while Dad fiddled with the Primus and a tin opener and eventually served me the warm contents of a small tin. Heinz baked beans with infant sausages! It was my first taste of this novel concoction, of which so many forkfuls were to follow.

On my return this week I identified the bridge, which is now closed to traffic, and tried to imagine Dad and I sheltering beneath it and about to taste ambrosia. Impossible – it was easier to imagine Romans roasting larks and pouring wine from clay jugs, or whatever they got up to in their Northumbrian villas. The past may well be a foreign country, but the recent past is a different continent. From the priory tearoom’s extensive menu, we chose flat whites and fruit scones. Sausages and beans would have been … an enormity.