Mons Graupius Identified
What space would Agricola’s army require on the field?
This is another aspect which has considerable bearing on a contending sites suitability.
We remind ourselves that Agricola drew up only his 8,000 auxiliary infantry with a further 3,000 cavalry deployed on the wings.
He left his legions in reserve and placed his reserve cavalry in a position that must have been hidden from Caledonian view given the cataclysmic impact on the course of events it made when suddenly unleashed upon the unsuspecting tribesmen.
Agricola, fearing envelopment of his flanks “extended” his line till deemed “dangerously thin” by some of his officers.
What does this mean?
The Roman line of battle usually took a chessboard formation (quincunx) though as a convenience we will call it by its more readily recognised name “manipular order” (though strictly speaking that actual term relates to Republican deployments).
As mentioned above manipular order saw cohorts drawn up chess board fashion, one line up, one back, with the individual cohorts in each battleline maintaining a large space between them to allow manoeuvring while the cohort behind, offset in echelon was positioned to guard this gap. The number of battlelines (acies) in this chessboard arrangement could be greater than two cohorts arranged thus, all depending on the wishes and concerns of the general as well as the manpower available.
If in action the front line cohorts required reinforcing or relieved the rear line cohorts would move forward between these gaps in the front battle line. This classic tried and tested technique critically provided great strength to the formation through defence in depth.
If each auxiliary regiment at Mons Graupius was some 500 strong (we believe Agricola recruited Britons to get existing auxiliary units up to full strength as opposed to creating new distinct units) then this gave Agricola sixteen cohort sized auxiliary regiments available for deployment in manipular order.
Traditionally Roman close order infantry formed up eight men deep (if not deeper) while a six man deep line could in extreme circumstances be adopted. Anything less was impractical, carried insufficient weight and could be easily broken through.
At eight men deep therefore each cohort sized auxiliary regiment would have around a sixty man frontage – assuming the auxiliary unit was not further broken up chess board fashion by individual centuries. However we keep matters simple and allow a notional metre per file for auxiliaries formed up in close order giving each auxiliary regiment in manipular order a 60 metres wide frontage with possibly a gap no greater than 40 metres between it and its neighbouring auxiliary regiment in each battle line.
A minimum of two battle lines (duplex acies) is required to create manipular order. This gives eight cohort sized auxiliary regiments per line. Each line, including the gaps, based on an eight man depth would have averaged around 760 m in length. As one line overlapped the other the entire auxiliary infantry formation would have extended to a width of around 810 m
Allowing each rank in close order slightly more than 1m depth, and a space of around 40m between the two main battlelines would give an overall formation size in manipular order of 810m by some 60 metres depth.
It is a little more difficult to speculate on the space the cavalry divisions on the wings took up.
Cavalry were generally based on multiples of around 30 troopers. Two distinct blocks of 750 troopers on each flank, one behind the other would appear a sensible arrangement that allowed a degree of flexibility. If these were 150 troopers wide by five deep and if we allow each trooper 2m width each, before closing up on the final stages of a charge, then an overall width of around 300m added onto each flank can be expected.
This gives Agricola’s possible initial deployment on the fateful morning a frontage of around 1,500 m. This deployment - with infantry in correct manipular order and cavalry wings doubled into two powerful squadrons (alae) one behind the other on each flank- provides reasonable defensive depth.
Clearly this width was exceeded by the nature of the hill, the benefits of which the more loosely formed open-order Caledonians could take advantage of to swirl around the flanks of the Roman lines.
We should not envision the Caledonian deployment on the hill regimented in neat extended lines marshalled to match the Romans. A battleline at the foot of the slope was evidently formed to counter the Roman advance, this will have consisted of the tribal elite and their retainers. It was the lie of the land above however that would enable the Caledonians to move to outflank the Romans, not their numbers. 30,000 men soon become very thin on the ground in a continuous deployment over only 3 Km - equates to 10 men per lineal metre. This would hardly warrant the description of "rising in tiers" and is yet another nail in the coffin of overly large contending sites.
The poorer equipped and less frontline worthy members of the tribes will have coalesced into clumps of tribal bands on the heights behind this front line and it is these that Tacitus attempts to describe, and their ability to manoeuvre on the topography of the hill that concerned Agricola sufficient to force him to massage his deployment in response to this potential threat.
How far did Agricola have to extend his line – Tacitus’s “open out his ranks” - to satisfy himself the worst of the threat to his wings was countered?
We shall never know with certainty, however, on the basis above it would appear the maximum he could shuffle this force about would be to:
- Extend each auxiliary regiments frontage to eighty men by reducing the depth of the files to six men.
- Abandon manipular order and move the second line up into the front line, maintaining only minimal gaps between units and presenting to all intents a single extended battleline (known as simplex acies), six men or so deep and around 1,400m long.
- Extend his cavalry wings by bringing up his rear squadron to line abreast with the first. Placing cavalry on a static battleline like this seems an incredible waste. The prime role of cavalry was one of shock action and pursuit but circumstances must have dictated otherwise to Agricola. This would extend each flank by a further 300m.
If Agricola used all these options to maximise his frontage to a worryingly “thin” degree then its frontage could have extended anywhere up to around some 2,700m plus some room to manoeuvre.
We should therefore search for a site that would allow a Roman battleline some 1,500m (1.5km / 0.9 mile) long to be threatened with envelopment and one anywhere up to 2,700m (2.7km / 1.7 mile) long where such a threat may be considered to a degree countered.
The Roman Marching Camp
26,000 men require a marching camp (at the most practical rate discussed above of 4.5 acres per thousand) of around 117 acres and one of Flavian proportions (i.e. a squareness / sub-rectangularity in plan) and not exhibiting the classic morphology of later Roman camps.
The camp must be located near to the hill; sufficiently close to enable the Romans to be aware of the unfolding events as recorded by Tacitus and for the legions deployed nearby to be able to intervene and come to the auxiliaries aid if required.
Burnswark near Ecclefechan in southern Scotland is the finest example anywhere of how “up close and personal” Roman camps in action could be placed next to a hill.
A camp therefore located several kilometres away is impractically remote.
Camps placed next to battlefields and siege works were almost always orientated orthogonally to its target -i.e. they addressed it by sitting square on.
©2009 Roman Scotland. All Rights Reserved
First Published February 2009
