Wat's Dyke is a linear earthwork running from Basingwerk (on the coast of the Dee Estuary) in the north to Maesbury in the South, a course of some 40 miles. Consisting of a bank of earth and a ditch, which faces Wales, the embankment averages 33 feet (10 meters) wide and the ditch averages 17 feet wide (5 meters) and 6.5 feet deep (2 meters). Wat's Dyke runs generally parallel to Offa's Dyke, sometimes as close as a few yards, sometimes as far as 3 miles away.
The origins of Wat's Dyke are obscure; at one time, scholars believed it dated not much further back than Offa's Dyke (a late-eighth-century construction). Archaeological investigations in the 1990s indicated it may have been built as far back as the fifth century; this dating rests on the discovery of a single fire site in which Romano-British pottery shards were discovered. The fire-site is not on top of the dyke, so the dyke may have been constructed at a later time. Most recent excavations suggest a construction even later than Offa's Dyke, possibly in the early 9th century and possibly sponsored by Cenwulf of Mercia, who became king a few months after Offa's death.
The origin of the name "Wat's Dyke" is unclear, as well. Wat was a personal name in use during the Anglo-Saxon period, but if indeed the dyke is named for an individual, who Wat may have been is unknown.