The Vallum is an earthwork that runs mostly parallel to, and just south of, Hadrian’s Wall. It consists of a broad flat-bottomed ditch 6m (20 Roman feet) wide and 3m (10 Roman feet) deep flanked by mounds to its north and south formed from the upcast. Both mounds were separated from the ditch by a 9m (30 Roman feet) flat strip or berm. In addition, a so-called marginal mound occurs in some places on the south berm, on the lip of the ditch. This smaller mound does not appear to comprise material from cleaning out the ditch so may represent another obstacle – technically a glacis – for the Vallum ditch. The name Vallum is a modern coinage, reflecting a name given to it by Bede (using the Latin word for rampart). Its function is unknown but it is assumed to have delineated the southern limit of the frontier zone, limiting access to the gates through the Wall.