Depth of a marching army.

[ValentinianVictrix]

I have been trying to work out how long a column of 20,000 infantry and 6000 cavalry would have been.

If the column was marching with the infantry four abreast, and the cavalry were divided with 500 to the front, 500 to the rear, 2000 to the right flank and the rest of the cavalry in the column with the infantry, how long would the column actually be?

I've tried finding sources for this but the only measurements I can find are within Vegetius and that to do with deploying and army into lines.

[Nathan Ross]

Have you seen Gary Brueggeman's site? His section on marching armies provides some helpful sources (secondary) and calculations:

Roman Army - The March

Remember that a marching force would be accompanied by a sizeable baggage train, which would be incorporated within the column. The total length of the column in march formation could easily be ten miles or more.

[Conal]

Gary Brueggeman's site is very informative, he's put a lot of work in there.

I have read, on here some where, that cavalry may not have marched on the road but beside it. Gary suggests that they rode on the road.

Does anyone have any literary mention either way please?

[Mark Hygate]

The main detailed source for marching is, I believe, Josephus. I would note that the cavalry numbers/proportion seems quite high.

He would have, it seems, the infantry marching in columns of centuries, each 6 wide x 10 long with the, most likely I'm thinking, antesignanii detached; half to the advance guard/camp; and half to guard the general + officers.

If the infantry marched as troops tend to today then each century would occupy about 50ft of length; but this could be halved as I have postulated previously.

We know of the tactic that was used when marching in hostile territory of marching in 3 columns which can then directly deploy into the standard triplex acies of the manipular period by wheeling; where the baggage train occupied the gaps between. If your 20,000 men represent a Consular Army of the same period then the minimum length of the baggage train could be as little as 2 x 2,000ft (the distance of the deployed frontage at a minimum).

Transpose that linearly and you're looking at a minimum of 10,000ft (2 Roman miles) for the infantry and baggage train and, given that in the same period we would expect the extraordinarii to be detached rather than the antesignanii (later periods), you could possibly get a Consular Army into those 2 miles. That could be your minimum and it could easily be up to 5 miles if the spacing doubled as I suggested.

I am sure the cavalry would be out to the flanks and rear, let alone with the advance guard (of whichever type) and, even if some were closer, they would not be on any road/track but enjoying the fresher air and lack of dust and crap. They can also forage (essential for cavalry) and even find supplementary food sources, apart from their general scouting and guarding duties.

[Frank]

Gary Brueggeman's site is very informative, he's put a lot of work in there.

I have read, on here some where, that cavalry may not have marched on the road but beside it. Gary suggests that they rode on the road.

Does anyone have any literary mention either way please?

I remember that Junkelmann (Die Reiter Roms I-III) wrote, that riding beside the road made a lot of sense. He and his team rode some hundred miles across Europe with roman equipment as part of an experimental archaeology project. Roman horses had no horseshoes. On the road the hoofs of the horse wear off way too fast.

Furthermore, at least in hostile territory the role of the cavalry was protection of the flanks. So it is a good guess, that in this case they did not march with the infantry on the road.

[Renatus]

My late father was an officer in the Royal Artillery during World War II and I am clearing out his house at the moment. I have found a device called 'The M. T. Slide Rule' by which, if you know the number of vehicles in a convoy and the number of vehicles per mile, you can work out the length of the convoy. I reckon that, if you take an average length for a vehicle and an average distance between vehicles ( I will call that 'the vehicle/separation unit') , you can get a notional figure for the number of vehicles per mile. Then, if you calculate how many ranks of marching soldiers would occupy one vehicle/separation unit (I will call that 'the vehicle equivalent'), you can work out how many vehicle equivalents make up the marching column. Setting the one against the other, the slide rule will give you the length of the column.

If you also settle upon a marching speed in mph, the slide rule will tell you how long it would take the column to pass a particular point. I reckon that, if you take that point to be the end of the march and add the time it would take the front of the column to reach it, you can probably work out how long the column would be on the march before reaching its destination.

[Frank]

According to this detailed analysis, an army bigger than 3 legions had to be splitted.

http://www.bandaarcgeophysics.co.uk/arch...rching.pdf