Offa's Dyke is a large linear earthwork that roughly follows the border between England and Wales. The structure is named after Offa, the Anglo-Saxon king of Mercia from AD 757 until 796, who is traditionally believed to have ordered its construction. Although its precise original purpose is debated, it delineated the border between Anglian Mercia and the Welsh kingdom of Powys.

In archaeology, earthworks are artificial changes in land level, typically made from piles of artificially placed or sculpted rocks and soil. Earthworks can themselves be archaeological features, or they can show features beneath the surface.