Onnum (Halton Chesters) Fort

The Hadrianic Fort at Onnum – A dedicatory slab from the west gate of the fort tells us that the Sixth Legion were responsible for the initial building work, but unfortunately, does not give us the name of the original garrison. It is likely, but not proven, that the Hadrianic unit was a cohors quingenaria equitata, an auxiliary force containing a nominal five-hundred men, approximately half of which were mounted. Units of this type have been identified at many Wall forts, and would have been ideally placed here, the infantry contingent to guard the Fort and Wall, and the cavalry to patrol along Dere Street and the Devil’s Causeway to the north.

Post-Hadrianic Alterations

At some time during the campaigns in Britain of emperor Septimius Severus between 208 and 211AD, an extension was added to the south west corner of the fort south of the wall, which housed a new bath-house and gave the fort an unusual, L-shaped plan.

A large cross-hall was also added to the front of the Principia during the same period. The bath-house was later demolished and replaced with barrack-blocks and stables, probably during the reign of Constantius Chlorus at the end of the third century, when a new bath-house was built in the north-west corner of the fort. Bath-houses for the use of the soldiers were a usual feature of a permanent Roman fort, and they were built mainly outside the defences. The presence of a sequence of internal bath-houses here at Halton Chesters, though not unique, is nontheless quite remarkable.

Post-Hadrianic Building Inscriptions

The Severan Cavalry Fort

A monumental slab roughly dateable to the third century was found at Halton Chesters bearing an inscription which links Ala Sabiniana with Onnum. The inscribed slab is now held at Trinity College, Cambridge. The alterations made to the fort in the Severan period were probably due to the replacing of the original garrison by Ala I Pannoniorum Sabiniana, the first cavalry regiment on the Wall, a five-hundred strong unit raised in Pannonia (modern Hungary). This unit would require more barracks and stables than the original part-mounted garrison, and the south-western extension of the defences more than likely served this purpose.

The Onnum Garrison

The presence of Ala Primae Pannoniorum Sabiniana would also explain the new cross-hall fronting the Principia; this could have been used to assemble the officers when they were mounted on horseback, which would have been impossible in the Principia itself, where the days orders were traditionally issued. The Notitia Dignitatum confirms that the same regiment was retained at Halton Chesters through to the beginning of the fifth century.

The Notitia Dignitatum Entry

The Gods of Roman Halton Chesters

Altarstones from Onnum

There are only three altarstones from Halton Chesters, all of them damaged, and all are shown above. There are in addition three other stones which may be votive in nature (see below).

Other Possible Votive Stones from Onnum

Onnum Today

References for Onnvm/hunnvm

• Britannia xxxii (2001) p.328 & fig.8 p.329;
• Hadrian’s Wall Map and Guide by the Ordnance Survey (Southampton, 1989);
• Hadrian’s Wall in the Days of the Romans by Ronald Embleton and Frank Graham (Newcastle, 1984) pp.78-90;
• The Roman Inscriptions of Britain by R.G. Collingwood and R.P. Wright (Oxford 1965);
• Air Reconnaissance of North Britain by J.K. St. Joseph in J.R.S. xli (1951) pp.52-65; right (Oxford 1965); Air Reconnaissance of North Britain by J.K. St. Joseph in J.R.S. xli (1951) pp.52-65;

Roman Roads near Onnvm/hunnvm

Wall: W (6) to Cilvrnvm (Chesters, Northumberland) Dere Street: S (2.5) to Corstopitvm (Corbridge, Northumberland) Devil’s Causeway: NNE (29) to Learchild (Learchild, Northumberland) Dere Street: NNW (13) to Risingham (Risingham, Northumberland) Wall: E (7) to Vindobala (Rudchester, Northumberland) Wall: W (1) to Portgate