Vallum (Hadrian's Wall)

The Vallum is a huge earthwork associated with Hadrian's Wall in England. Unique on any Roman frontier, it runs from coast to coast to the south of the wall.

The earliest surviving mention of the earthwork is by Bede (Historia, I.12), who refers to a vallum, or earthen rampart, as distinct from the wall, or murus; the term is still used despite the fact that the essential element is a ditch, or fossa. It was for long thought that the Vallum predated the stone wall, the most elaborate phasing being presented in 1801 by William Hutton, who thought that the south vallum mound and the marginal mound, with a ditch between, were the work of Agricola, the vallum ditch and north mound were added by Hadrian, and the stone wall was the work of Severus.

Layout and course
The Vallum comprises a ditch, nominally 6 m wide and 3 m deep, with a flat bottom, flanked by two mounds about 6 metres wide and 2 m high, set back some 9 m 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 m.

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 m away.

Archaeology and purpose
Before the middle of the 19th century, the Vallum was most commonly known as Agricola's Ditch, since antiquarians thought that it had been constructed during the period when Agricola was governor of Brittania, the Roman province spanning what is now England, Wales and southern Scotland. After John Hodgson published the final portion of his History of Northumberland in 1840, it became generally accepted that the Wall and Vallum had been built during the reign of Hadrian. Hodgson based his view on evidence which included a stone tablet in the Museum of Antiquities, Newcastle upon Tyne, which had been found in Milecastle 38 on the Wall in the previous century, its significance having been overlooked. The inscription on the tablet, probably made and erected to mark the completion of the milecastle, includes the names of Hadrian and Aulus Platorius Nepos (governor of Brittania during Hadrian's reign), as well as "Legio II Augusta" (Second Augustan Legion). It is now accepted that units of that Roman legion built the section of Hadrian's Wall which includes the milecastle, and they would have automatically included the names of the current emperor and governor on the tablet.

The first excavation was undertaken in 1893 at Great Hill (Heddon-on-the-Wall, near Newcastle upon Tyne), where it was observed that the Vallum ditch was cut through a seam of fire-clay which was deployed in both mounds. This excavation demonstrated that the main north and south mounds were contemporary and built using material dug from the ditch. In the late 20th century several excavations established that the marginal mound was also contemporary.

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. There would have been a crossing-point like a causeway or bridge to the south of each wall-fort—several such causeways are known, such as the one at Condercum in Benwell, a suburb of Newcastle. Causeways have also been detected to the south of several milecastles.

Although there is no definitive historical evidence as to why the Roman army built this unusual barrier, modern archaeological opinion is that the Vallum established the southern boundary of a military zone bounded on the north by the wall itself. The zone would have been "out-of-bounds" to civilians and those with no valid reason to be there.

Sometime in the 2nd century AD, the Vallum was "slighted"—that is, the ramparts were broken through and the ditch filled in at fairly regular intervals along its length. Archaeologists 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. Some have suggested that this coincided with the building of the Antonine Wall in Scotland and the partial abandonment of Hadrian's Wall.