A frontier work is a man-made boundary feature usually comprising linear earthwork formed of a single bank and ditch, which appear to have stretched more or less continuously across the landscape for between about 300m and 240km, in some cases cross-cutting the natural grain of the landscape.
Frontier works are generally identified in the field as earthworks, but where these are slight or non-existent aerial photography, documentary sources, placename evidence, and field boundary patterns may also contribute to their recognition. Unless unequivocal documentary evidence is available, excavation is usually necessary to establish the date of these monuments since similar linear earthworks were constructed in later prehistory and during the Roman period. Frontier works may also be confused with hedge banks, hollow ways and lynchets; these, however, are not usually as long as frontier works, nor do they usually follow alignments for which a defensible or other strategic purpose can be argued.
Specifically excluded from this description are linear earthworks less than 200m in length and other defensive works, for example those around towns.
Medieval frontier works have been interpreted as military and/or territorial boundaries and, although few have firm dates attached, most are thought to date to the Early/Middle Saxon period.