Layout and Course
The Vallum comprises 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 distance of the Vallum from the Wall varies. In general there was a preference for the earthwork to run close to the rear of the Wall where topography allowed. In the central sector the Wall runs along the top of the crags of the Whin Sill, while the Vallum, laid out in long straight stretches, lies in the valley below to the south, as much as 700 metres (2,300 ft) away.
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