NGRef: NY 996 685
OSMap: Hadrian's Wall, OL43, LR87.
Type: Wall Fort, Fort, Minor Settlement, Bath House
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

Onnum - 'The Rock'

The name of the Halton Chesters fort first appears in the Notitia Dignitatum of the late-4th/early-5th centuries as Hunnum, between the entries for Uindobala (Rudchester, Northumberland) and Cilurno (Chesters, Northumberland). The wall fort is also mentioned in the Ravenna Cosmography of the seventh century, appearing as Onno (R&C#146), again between the entries for Vindovala (Rudchester) and Celunno (Chesters).

The Hadrianic Fort at Onnum

Plan of Halton Chesters in the 3rd C.
(Adapted from Embleton & Graham, p.83).
"The Sixth Victorious Legion, Loyal and Faithful, made this."
(RIB 1429)

The fort at Halton Chesters was built across the line of the wall facing north, half way between milecastles 21 and 22 about ½ mile east of Dere Steet. The original Hadrianic fort was rather squat in outline, almost square, measuring some 440 feet north-south by 400 feet east-west, with an area just over 4 acres (c. 134 x 122 m; 1.6 ha).

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.

Dedicatory Inscription from Onnum West Gate

IMP CAES T[ra hadr]
AVG LEG VI V[i p f]
LEG AVG PR [pr f]
"Imperator Caesar Traianus Hadriani
Augustus Legio Sextae Victrix Pia Fidelis
Aulus Platorio Nepote
Legatus Augusti Pro Praetore fecit"
To the emperor Caesar Trajan Hadrian
the August, the Sixth Victorius Legion, Loyal and Faithful
[made this, under] Aulus Platorius Nepos,¹
the emperor's Legate with propraetorian power."
(RIB 1427)
  1. Aulus Platorius Nepos, an old and trusted friend of the emperor Hadrian, was the Roman Governor of Britain between A.D.122 and c.125, and thus entrusted with the initial construction of the Rampart Wall.

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 Second Augustan Legion made this." "The Sixth Victorious Legion and the Twentieth Valiant and Victorious Legion made this." "The Twentieth Legion Valiant and Victorious, century of Hortensius Proculus [made this]."
(RIB 1428) (RIB 1430) (RIB 1431)

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

"[...] of the Norican nation, thirty years old, Messorius Magnus his brother, a duplicarius of the Sabinian Wing, ordered the making [of this memorial]."
(RIB 1433; tombstone)
  1. A duplicarius was a long-serving or meritorious soldier on double the normal salary.

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.

Praefectus alae Sabinianae, Hunno
"The Prefect of the Sabinian wing at Hunnum"
(Notitia Dignitatum xl.37; 4th/5th C.)

The Gods of Roman Halton Chesters


Altarstones from Onnum

Inscription Togo-Translation RIB
DEAE FORTVNAE ... CVR ... HD... "To the goddess Fortuna [...] under the direction [...]" 1423>
DEABVS MATRIBVS...E... "To the Mother Goddesses [...]" 1424
NVMINIBVS AVGVSTORVM ... "For the Living Spirits of the Emperors [...]" 1425

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

"Lightning from an open sky." "For Saturn." "Anyone venerable."
(RIB 1426) (RIB 1432) (RIB 1437)

The Portgate and Civil Settlement

Hadrian's Wall is pierced by only three Roman roads, at Banna (Birdoswald) and Uxelodunum (Stanwix) on the western half of the Wall in Cumbria, and also here at Onnum. Dere Street was laid-down by governor Gnaeus Julius Agricola c.80AD to serve his vigorous campaigns in Scotland, and this important highway was in constant use by the Roman military in Hadrian's time. Whereas the Cumbrian roads both led through the centres of forts positioned on the Wall, Dere Street was here conducted via a purpose-built gateway through the Wall itself, the Portgate; situated just over ½ mile west of the Halton Chesters fort, near milecastle 22.


Tombstones from the Onnum Settlement

"To the shades of the departed Aurelia Victorina, Aurelius Victor, father to daughter."
(RIB 1435; tombstone)
"To the Spirits of the Departed [...] Hardalionis, the College of Conservatores, well deserving, placed this."
(RIB 1436; tombstone)

The vicus or civil settlement at Halton Chesters extended past the vallum for three or four hundred yards to either side of the road leading from the south gate of the fort. It is possible that this road was continued south-westwards to join up with Dere Street about a mile north of Corstopitum (Corbridge), near Chantry Farm (NY986667).

Recent Archaeological Activity at Halton Chesters

NZ 9975 6830 - Geophysical survey to the south of the B6318 minor road in 2000 revealed a bath-house and southward-leading Roman road flanked by civilian strip-buildings. These features were partially obscured by overlying Medieval ridge-and-furrow.

Onnum Today

Halton Chesters
Admission Free Car Parking
Although the outline of the fort and annexe can be made out, there is nothing much else to be seen. The remains of milecastle 22 lie about ½-mile west of the Onnum fort near the A68, and the site of the Portgate lies nearby, just to the south-west of the Errington Arms roundabout on the modern road.

Onnum Bibliography

See: 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;
All English translations, including any inherent mistakes, are my own. Togodumnus

Onnum Related Lynx

Link to maps of the area from: StreetMap Old-Maps MultiMap

Roman Name

Ravenna Cosmography: Onno

Page Citation: Kevan White (2018) "Roman Britain: ONNVM - HALTON CHESTERS"