#Alongthewallin80days - Day 4
A Vallum is unique on any Roman Frontier. Although there is no definitive historical evidence as to why the Roman army built this unusual barrier along the length of Hadrian's Wall, it is thought that the Vallum established the southern boundary of a military zone bounded on the north by the wall itself, this zone would have been "out-of-bounds" to civilians and those with no valid reason to be there.
Hadrian's Wall Vallum runs from coast to coast and lies south of the wall. It comprises of a ditch, nominally 6 metres (20 ft) wide and 3 metres (10 ft) deep, with a flat bottom, flanked by two mounds about 6 metres wide and 2 metres (7 ft) high, set back some 9 metres (30 ft) from the ditch edges. For a great deal of its length a third lower mound, the so-called marginal mound occupies the south berm (flat area between mound and ditch), right on the southern lip of the ditch.The total width of the fortification (consisting, from north to south, of mound, berm, ditch, marginal mound, berm, mound) was thus about 36 metres (100 ft).
The Vallum is known to have been constructed some time after the wall was completed, as it deviates to the south around several wall-forts which were either completed or under construction when the wall was nearing completion. One of the wall forts that the Vallum would have needed to deviate around would have been milecastle 76, however the location of milecastle 76 has never been confirmed and has only been calculated from the relative distances between other Hadrianic structures. The Vallum is also not visible along where milecastle 76 is believed to be, it is thought that sometime in the 2nd century AD, the Vallum was "slighted" (the ramparts were broken through and the ditch filled in at fairly regular intervals along its length.) Archeologists and historians have deduced that either the Vallum was then deemed unnecessary, or that it was proving to be a hindrance to military and authorised civilian traffic.
(The Vallum near Cawfields)