Offa's Dyke, ancient entrenchment of W England and E Wales, from the Dee estuary to near the estuary of the Wye River. It was built in the 8th cent. by Offa, king of Mercia, as a barrier against the Welsh and lies mainly along the England-Wales boundary. Watt's Dyke, a similar work, roughly parallels a section of Offa's at a distance of c.2 mi (3.2 km). Parts of the dikes are well preserved.
"Offa's Dyke." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (July 23, 2018). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/offas-dyke
"Offa's Dyke." The Columbia Encyclopedia, 6th ed.. . Retrieved July 23, 2018 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/offas-dyke