[split] Distances between files and ranks

[Bryan]

When Metellus at the Muthul River became aware that he was walking into a trap, Metellus halted his army, and changed its march order formation. The right flank, which was nearest the Numidian army, Metellus strengthen with three lines of reserves (subsidia). Sallust (The Jugurthine War 49 4-6) Here we find Metellus had to halt his army in order to change his order of march, and it is a vital clue as it is the opposite to Polybius and Tacitus. What Metellus is doing has been written about by other ancient authors and totally ignored by modern scholars.

Based on the context of Sallust's description of the battle it seems that Metellus was marching into a plain inside a sort-of boxed canyon surrounded on two sides by ridges, left and right, the far side by the river Muthul, and with the army exiting other hill/ridge from their direction of travel. Scouts saw Jugurtha's forces on top the ridge on the right, which would have been at around 2 o'clock to their direction of travel at the exit into the plain. Metellus correctly judged that it was an ambush that had been compromises and knew it was a trap to march into the plain in standard march order, so Metellus maneuvered his army to triplex acies battle array, with skirmishes between maniples, to counter the threat on the right flank. Which meant the army after reassembling would march forward toward the river (with Rutilius Rufus commanding the left wing of cavalry and light forces to secure a position near the river to assemble a camp), with the rest of the column able to face right quickly if necessary to form into immediate battle formation of triplex acies to withstand an attack by Jugurtha's forces who were atop a ridgeline parallel to Metellus' direction of travel to the river. 

Other accounts of marching order and deduction by scholars suggests that in terms of infantry it was typical for in the Mid to Late Republic for consular armies for the Extraordinarii to lead the way as the vanguard, then the rest of the Socii Dextra Ala, then the two Roman legions, then the Sinistra Ala. With that formation they could, if necessary deal with flanking threats, but more importantly it was a formation that allowing them to hit a pivot point at the fore, column right, and then form into battle array to deal with a frontal threat. But it wasn't optimal for a guaranteed flanking battle. So at the Muthul River, had they stayed in that marching order it would have been basically backwards. So Metellus' move to shift the marching order to right facing triplex acies made sense. 

However, there is no way of knowing what the actual battle array was in terms of rank and file width and depth, its absolutely not mentioned in the source, so its pure speculation what it was in the march and battle, or if they shifted again, likely, once once the force exited the narrow exit from the hills into much wider open plain.

I'm pretty sure I figured out where the battle happened based on descriptions: Muthul Battle Location. Metellus' army was likely following the Muthul River and was coming south on the road identified now as P-17 from what would later be called Bulla Regia. Jugurtha's forces were likely positioned along the east-west ridge identified as Foret Nord de Nebeur. There was no reservoir or dam in 109 BC , just the river to the west, that was Rutilius Rufus' destination to establish the Roman marching camp. The battle occurred in the plains between Nebeur and the river, with Metellus holding the center, Marius the right. Upon the Roman army entering the plain Jugurtha sent a detachment that closed off the exit into the plain that Metellus' army had marched from, sealing the Roman army inside the sort-of boxed canyon. Then another Numidian force of infantry and cavalry, to include elephants, attacked Rutilius Rufus' wing near the river (which had marched ahead of the main body), with the bulk of Jugurtha's army of infantry and cavalry attacking the Roman center and right.