Heritage conservation is concerned for the long-term future of our ancient monuments, but this is usually considered in terms of the immediate coming centuries. However, what the survival of monuments over millennia? How can we, and should we, consider the future-proofing of our valued ancient monuments for thousands of years to come?

I’d like to suggest that Britain’s longest ancient monument – Offa’s Dyke – will likely survive longest – for thousands, if not tens of thousands, of years – not where it is situated in woodland or farmland, or where it is protected as a scheduled ancient monument, but where it has already been sealed by 19th-century railway embankments.


The conservation of Britain’s largest ancient monument – Offa’s Dyke – is a pressing topic for archaeological and heritage debate. Regarded as built as an early medieval (Mercian) frontier work of the late 8th century to control movement and protect territory against Welsh rivals, it is a stupendous military, economic and ideological edifice. Yet despite its enormous scale and length (both of which remain issues that are hotly debated), this monument’s long-term future is uncertain.

Offa’s Dyke runs for large sections of over 170 miles intermittently from Gloucestershire to Flintshire. In some places it may have never been present, marked instead by rivers and other features. In further places it may have been constructed by was already denuded by medieval agricultural activities. Yet so much of the damage to the monument has been the result of 19th-early 21st-century agricultural activities, extractive industries, road and rail building, development for housing and other construction activities. On a far more modest level, it has been affected by erosion caused by walkers enjoying it as a public right of way and long-distant route: the Offa’s Dyke Path. Beyond all of these factors, Offa’s Dyke is affected by passive neglect as it is damaged by vegetation, burrowing animals and animal husbandry and farming practices, as well as by wind and water action by the elements (especially where it runs along steep slopes and in river valleys).

Conserving Offa’s Dyke for future generations is therefore not as strightforward as it might seem. Looking forward through the 21st century, the monument faces many potential risks to its long-term future including not only the processes outlined above, but also our disinterest and inability to conceptualise its monumental scale and historical significance. This is why the Offa’s Dyke Association, Historic England and Cadw are working together and funding an eagerly awaited Conservation Management Plan of the monument.

But let’s take a really long-term view for a moment. Which parts of Offa’s Dyke will survive longest?

Offa’s Dyke has been a dominant feature of the British landscape since at least the late 8th century: it survives in patches of staggering monumental scale, with a ditch up to 2-3m deep and bank up to 4m high, and a horizontal breadth of up to 20m. It has endured in varying scales for over 1220-1260 years! Surely, even if subject to ongoing damage, it will remain a feature through the 21st and 22nd centuries, and arguably the 31st and 32rd centuries AD, perhaps even until the 41st and 42nd centuries AD, perhaps long after humans have vanished from the planet. For the long term, and while humans are still here, surely scheduling the ancient monument will best ensure its survival for millennia to come?

Certainly I support the legal protection afforded to Offa’s Dye, but I’d also like to suggest that for all our conservation strategies, the sections of Offa’s Dyke likely to survive longest aren’t those subject to scheduling. Instead, I propose those that have already been ‘destroyed’ will survive longest. Buried sections of the ditch at many  locations along its length are well-preserved out of site, but in terms of the bank’s survival, I’d propose railways have already ‘saved’ some sections of Offa’s Dyke for future millennia by late 19th-century railway embankments.

Offa’s Dyke looking S to the railway embankment of the line from Acrefair via Rhosllanerchrugog to Rhostyllen and Wrexham

Railways and the Dyke

Offa’s Dyke is crossed by railway lines in multiple locations in its northern stretches associated with the Denbighshire coalfield. Many of these railway lines are parts of wider industrial operations that have destroyed the Dyke. For instance, at Ruabon the GWR crosses the Dyke in a cutting, as well as being damaged by road and rail at Ruabon Brick and Terra Cotta Works, cut by the Wrexham and Minera line at Cae-llo Brick Works, destroyed by sidings at Vron Colliery, and damaged by being cut by the Wrexham and Minera joint line at both Ffrith Hall and also at Pontystain Crossing.

Yet there are at least two locations where it seems that Offa’s Dyke has been buried and not necessarily destroyed by railway construction. In these locations, Offa’s Dyke’s bank and ditch might be reasonably expected to survive as well-preserved buried features since they were sealed beneath metres and metres of overburden during the construction of railway embankments. I make this argument presuming that railway embankments were built by a human workforce: not by large-scale earth-moving devices that might have damaged and destroyed the bank during the embankment’s creation (therefore contrasting with modern road-building). I’m happy to be corrected on this point.

First of these is at Brymbo Colliery where sidings are laid out over Offa’s Dyke with coal waste and now landscaped.

Brymbo Colliery
Offa’s Dyke runs into the sidings at the Wonder Pit, Brymbo

The second is a site I visited for the first time yesterday: north of Pentrebychan Crematorium in Wrexham borough. Here, the line of Offa’s Dyke was crossed by the Wrexham-Rhosllanerchrugog railway line. Constructed in the 1880s and ’90s, it was opened to passenger traffic from 1902.

Offa’s Dyke crossed by the line of the Wrexham-Rhos railway line

My only reservation is that, from the Ordnance Survey map, it might be taken to suggest that the line of the Dyke is denuded by the building of the bridge over a lane joining farm fields. However, on close inspection, it seems that the bank should be largely preserved, at least at the northern end, beneath the railway embankment just east of the crossing. Sir Cyril Fox (1955: 49) came to the same conclusion, stating that ‘It is certain that the Dyke is present under the railway embankment’.

Looking S from the edge of Offa’s Dyke – one can see the line of the Dyke approaching the embankment at an angle such that it will be preserved beneath the embankment on the east (left) of view.
A denuded bank of Offa’s Dyke runs N towards the Wrexham-Rhosllanerchrugog-Acrefair railway embankment

So will Offa’s Dyke be saved by the railways? Thinking of the deep-time persistence of this important monument might require we think of places where the Dyke has already disappeared, not where it is scheduled and preserved.DSC00156