The structure is named after Offa, the 8th century king of Mercia. He is believed to have ordered its construction. Its original purpose is not known for sure. It followed the border between Anglian Mercia and the Welsh kingdom of Powys. The dyke was up to 65 feet (20 m) wide (including its flanking ditch) and 8 feet (2.4 m) high. It ran over low ground, hills and rivers. Today the earthwork is protected as a scheduled monument. 