Offa's Dyke

Offa's Dyke is a massive earthwork between England and Wales, running from the estuary of the River Dee[?] in the north to the River Wye in the south.

The earthwork is widely attributed to Offa, King of Mercia in the 8th century. At the time it was used it as a defensive measure against the depredations of the Welsh and as a boundary between England and Wales. It is not known to what extent Offa was indeed responsible for building the dyke - important parts of it may date from earlier periods.