Offa's Dyke

Between the Romans leaving and the Normans arriving, the largest single construction project in Britain was a bank and ditch running roughly eighty miles down the Welsh border. It took an enormous amount of organised labour, it is still there in long stretches, and there is no contemporary account of why it was built.

What Asser says

The attribution to Offa, king of Mercia from 757 to 796, comes from Asser, a Welsh monk writing a life of Alfred the Great around 893 — a century after Offa died.

Asser says that Offa had a great dyke built from sea to sea between Wales and Mercia. That single sentence is the entire documentary basis, and it is written by a Welshman in the service of a West Saxon king with no particular reason to be accurate about Mercian public works.

The conventional account — one king, one plan, one campaign of building — is now seriously doubted.

It also does not run from sea to sea. There are long gaps, particularly in the south, and whether these were never built, have been ploughed out, or were closed by dense woodland is argued about.

What it looks like

A bank with a ditch on its western side, so the ditch faces Wales. That is the single most quoted fact about it and it is what makes most people read it as defensive or at least as facing outward.

Where it survives well the bank can still stand two or three metres above the bottom of the ditch, and the original profile was considerably larger. It uses the ground intelligently: running along the eastern slope of valleys so that anyone approaching from the west is exposed and below.

What it was for

Several readings, none conclusive.

A defended frontier. The traditional view. The problem is that eighty miles of earthwork cannot be manned continuously, and there is little evidence of forts, gates or garrison posts along it.

An agreed boundary. A line settled between Mercia and the Welsh kingdoms, marked monumentally. This fits the absence of military infrastructure.

A control line. Managing movement, cattle and trade rather than armies, with crossing points where tolls or inspection could happen.

A statement. The labour required is the point. A king who can compel that much digging across that much territory is demonstrating something, and the audience may have been as much Mercian as Welsh.

The most persuasive recent work treats it as several of these at once, and as an assertion of hegemony rather than a defensive line in the modern sense.

Wat's Dyke

There is a second, shorter earthwork running roughly parallel to the north, Wat's Dyke, about forty miles from the Dee to the Severn.

Its relationship to Offa's Dyke is unresolved. It has been dated variously to the fifth, eighth and ninth centuries, and the sequence between the two — which came first, and whether one superseded the other — remains open.

Where to see it

Knighton is the obvious place. The town takes its Welsh name, Tref-y-clawdd, from the dyke, the earthwork runs through the town, and the Offa's Dyke Centre is there. The stretch running north from Knighton towards Llanfair Hill is the best-preserved anywhere on its length.

The Vale of Montgomery has excellent sections, and it is instructive country because the ground is flat and open — the Mercians had to build a barrier because the landscape offered none.

Discoed and Evenjobb in Radnorshire have substantial surviving lengths.

The Offa's Dyke Path, opened in 1971, runs 177 miles from Sedbury on the Severn estuary to Prestatyn. It follows the earthwork where it can and the border where it cannot, and it is on the dyke itself for rather less of its length than most walkers expect.

Looking after it

The dyke is a scheduled monument along most of its course. Damage to it — ploughing, gate-cutting, vehicle tracks, unauthorised works — is a criminal offence, and sections have been lost to exactly those things within the last few decades.

If you see damage, report it to Cadw or to the Offa's Dyke Association.