Five cavalry units are known to have been based along Hadrian’s Wall. That does not, of course, mean there may not have been others we do not yet know about.
Auxiliary cavalry were organised in units known as alae (literally ‘wings’, from the fact that they used to be placed on the flanks of a Roman battle line). They were either quingenaria (regular strength; literally 500-strong) or milliaria (double strength; literally 1,000-strong), although their exact strengths are a matter of dispute (and even the occasional mud fight). Alae were composed of turmae which are thought to have been around 32 men, including a decurio and standard bearer (other interpretations of the limited evidence are available); probably 16 turmae for an ala quingenaria (giving a theoretical total of 512 men) and 24 for an ala milliaria (and thus 768 men).
The units along the Wall came from three distinct regions: Gaul, Spain, and Pannonia. The two Asturian alae originated in one particular region of the Iberian peninsula (Asturia, naturally), and may have enjoyed a fondness for cider. The Wall garrison included the largest (and most prestigious) of the cavalry units in the Exercitus Britannicus, the ala Petriana, and we shall start with them.
Ala Gallorum Petriana milliaria civium Romanorum bis torquata
Named after its first commander, T. Pomponius Petra, and ultimately the only milliary cavalry unit in Britain, the ala Petriana was still apparently only quingenary at the time that Flavinus (RIB 1172) died and was buried, most likely in the Flavian period and probably at Corbridge. By AD 98, it had received a block grant of Roman citizenship and then by AD 122 it had been increased to double-strength, both recorded on diplomas. An inscription from Carlisle notes the unit as milliary and torquata (meaning it had received a block grant of torques as a distinction), whilst a career inscription of a former commander, C. Camurius Clemens from Matelica, in Umbria, records the unit as bis torquata. Ala Petriana is placed in the Notitia Dignitatum at an eponymous base, generally supposed to have been Stanwix. Support for the identification of Stanwix as its base comes from a tile stamp and tombstone missing its inscription from the site, which − at 3.27ha − is the largest fort on the Wall (Housesteads, by comparison, is only 2ha).
Ala Augusta ob virtutem appellata [Gallorum]
This is now thought to be the same as the ala Augusta Gallorum Proculeiana civium Romanorum known from diplomas. It has been suggested that the unit was originally called the ala Flavia by Domitian, but that it was subsequently renamed to Augusta following his damnatio memoriae. The unit’s presence at Lancaster is attested by the tombstone of Insus (RIB 3185), before next being recorded under Hadrian at Chesters. It had settled at Old Carlisle by the middle of the 2nd century, with dedications by prefects from AD 185, 188, and 191, and is last recorded there in AD 242. An inscription from Carlisle (RIB 947) may also belong to this phase, possibly between AD180 and 184, rather than actual occupancy at Carlisle. Whether it is to be identified with the ala I Herculeia of the Notitia Dignitatum at that same station is unknown. The unit’s presence (under its formal title ala Gallorum Proculeiana) in the Exercitus Britannicus is recorded on diplomas from AD 122 through to 145/6.
Ala I Hispanorum Asturum
This unit may have arrived in Britain with the initial invasion of AD 43, although there is no clear evidence for this. By the 3rd century AD, it was at Benwell, where it is also attested in the Notitia Dignitatum. The presence of a tombstone belonging to a trooper’s freedman from South Shields is probably not relevant to the location of the unit.
Ala II Asturum
This unit is recorded in Pannonia in the Claudio-Neronian period (CIL III,14349). It appears to have transferred to Britain with Petilius Cerealis at the beginning of his command of the Exercitus Britannicus in AD 71 and to have been based at Ribchester at the end of the 1st/beginning of the 2nd centuries AD. A member of the unit (one Caravus) may have been the owner of the Ribchester cavalry sports helmet. Ala II Asturum subsequently moved to Chesters, presumably replacing the ala Augusta, where it was to remain.
Ala I Pannoniorum Sabiniana
This ala is first listed in the famed Hadrianic diploma of AD 122 (CIL XVI, 69). Its early postings are unclear, but by the 3rd century it was based at Haltonchesters, where it may have been producing brick and tile (a stamp comes from South Shields). Lead seals bearing its stamp are also known from South Shields, Corbridge, and Pittington Farm and it is assumed that these were destinations receiving communications or goods from the unit when it was at Haltonchesters. The Notitia Dignitatum records it as still being based at Haltonchesters. Although normally just referred to as the ala Sabiniana, a former prefect is more formal by calling it, like the diplomas, ala I Pannoniorum Sabiniana in an inscription.
Those, then, are the alae. They are not, however, the only mounted troops on the Wall. The mixed cohortes equitatae contained an element of cavalry in them and it is those we shall examine next.
NEXT: The garrison: Part II