dyke

In earth science, a sheet of igneous rock created by the intrusion of magma (molten rock) across layers of pre-existing rock. (By contrast, a sill is intruded between layers of rock.) It may form a ridge when exposed on the surface if it is more resistant than the rock into which it intruded. A dyke is also a human-made embankment built along a coastline (for example, in the Netherlands) to prevent the flooding of lowland coastal regions.

dyke

In archaeology, a linear earthwork consisting of a line of bank and ditch. Most dykes in Britain are products of the post-Roman period. They may be double or occasionally treble, and were designed to serve as a boundary or frontier. Offa's Dyke along the Welsh border is an outstanding example in Britain.

Certain dykes in England may have been constructed at the end of the early Iron Age, but most date from about the 5th–9th centuries, and may have been inspired by Roman frontier works such as Hadrian's Wall and the Antonine Wall.