When was Offa's Dyke built?

It's one of the great monuments of early medieval Britain, and one of the most mysterious.

Image from www.cpat.org.uk

Offa's Dyke is a linear earthwork (bank and ditch) that runs roughly along the English-Welsh border. It isn't continuous, but the surviving stretches (shown on the map to the left) add up to 129 kilometres.

These stretches are part of a 283 kilometre-long National Trail between Sedbury and Prestatyn that I walked in 2010 (and can recommend!).

It's named for King Offa of Mercia (757-796) basically because a hundred years after he lived a Welsh monk called Asser attributed the building of the dyke to him. No source from Offa's lifetime actually mentions the dyke.

Archaeologists love a good argument. There are debates not only about when Offa's Dyke was built and why, but about whether it even ran 'from sea to sea', as Asser claims. Probably it didn't. If it did, a lot of it must have been destroyed.

But very recently the Clwyd-Powys Archaeological Trust has uncovered evidence that the dyke was built much earlier than Offa - perhaps 200 years earlier. This is based on radiocarbon results from a northern section of the dyke near Chirk in Wrexham (Wales), which give a date range of 430-642. A best guess would therefore place the building of dyke at Chirk in the later sixth century - a period about as dark as the Dark Ages get.

A hiker on a section of surviving bank. The kingdom of Mercia lay on the left side, Powys on the right.

Isolated C-14 dates should always be taken with a fistful of salt, but if they turn out to be reliable this really upsets the apple cart. Could the dyke really be this much earlier? If so, who built it, and why did Asser give Offa the credit? Maybe parts of the dyke were built long before other parts, and no single king was responsible. Could it be that Offa only refurbished and extended an older earthwork?

Most likely, this is just one new piece in a highly complicated and mysterious puzzle...

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