28 December, 2014

De-turfing Hadrian’s Wall


Everywhere that archaeologists  have looked in the last 20 years, three rows of double postholes have been found on the berm between the ditch and Hadrian’s Wall; - Everywhere that they have looked in the last 20 years - that is the important point in the narrative, there was over a century and a half of investigation before these features were recognised.  
Previously, archaeologists found what they were looking for – the remains of a stone wall, with a ditch; except of course for the Turf Wall  section which was made of turf, so they find turf, apparently; what else could it be?
Let’s not beat about the bush, my preference would be wood, and specifically layers of horizontal timbers stacked between three lines of posts, which is the model derived from study of the postholes on the berm.  Timber was the the material used by Caesar's armies and is shown on Trajan’s Column. 
Rampart building in Caesar's writings
Ramparts are mentioned over 40 times by Caesar in his military writings, [2], frequently in connection with timber.
He orders him to fortify a camp with a rampart twelve feet in height, and a trench eighteen feet in breadth.….those who had proceeded some distance for the purpose of seeking materials for the rampart, ……
Caes. Gal. 1.6

. . . . . and after having suddenly assailed the soldiers engaged in procuring wood, came with a large body to attack the camp.
Caes. Gal. 5.26 
Caes. Gal. 5.39 
Caes. Gal. 3.29
This last reference, as Caesar is perusing the foe into dense woodland, would tend to confirm that ramparts were normally made up of horizontal timbers.  The layout of the postholes on the berm had suggested the key feature of this type of engineering was layers of horizontal timbers stacked between the rows of posts; these can be placed parallel, at ninety degrees, and crucially, at an angle to the line of Rampart; this hides the posts, makes the structure incomprehensible, and very strong, [Below].
This is probably the key difference Roman in the approach; The Gallic rampart described by Caesar has a regular and rectilinear structure, using jointed horizontal timbers with layers of stones, which is easily understood. [Caes. Gal. 7.23].
It is clear in this period the Roman Army was building substantive timber and earth fortifications in the field, discussed in detail elsewhere [see Caes. Gal. 7.72].  It is also clear that the breastwork is distinct from and sits on the Rampart, which is distinct from the ditch and its spoil.
Ditches and ramparts are two separate operations, as is apparent from Caesar’s account of his campaign against Afranius during the civil war in Iberia.
Caesar's approaches the enemy camp to encourage them to fight on level ground of his choosing, he arrayed in three line battle order.  When it looks like there is not going to be a battle, Caesar gets his 3rd line to dig a 15’ wide ditch behind his front two ranks and out of sight of the enemy .  After spending a night under arms behind this ditch, he pulled off the same trick the next day finishing off the ditch; but because materials for a rampart must have been fetched from a great distance, it was day three before he constructed a rampart to fortify his position,[Caes. Civ. 1.42].
Caesar makes three references to turf being used in the building of structures, all in Gallic Wars.   In book III Publius Crassus campaigning in Aquitania is attacking an enemy camp; 
Caes. Gal. 3.25
Presumably, the turf, “caespitibus” in this context is used in the construction of an “aggerem”, a mound / [ramp] higher than the defences. This technique is described in more detail in later siege;
….  in twenty-five days raised a mound three hundred and thirty feet broad and eighty feet high. When it almost touched the enemy's walls, and Caesar, according to his usual custom, kept watch at the work, and encouraged the soldiers not to discontinue the work for a moment: a little before the third watch they discovered that the mound was sinking, since the enemy had set it on fire by a mine; and at the same time a shout was raised along the entire wall, and a sally was made from two gates on each side of the turrets. Some at a distance were casting torches and dry wood from the wall on the mound, others were pouring on it pitch, and other materials, by which the flame might be excited, …. 
Caes. Gal. 7.24
It is clear these structures were primarily of wooden construction, but, even though fresh wood does not burn readily, encasing them in turf or soil would be a sensible precaution. 
In the only reference to the use of turf in forts, Caesar blocks the gates of his camp as part of a ruse to lure the enemy into attacking it
Caes. Gal.  5.51
At which point the Romans launched an all-out counter-attack from all four gates simultaneously, with predictable results. It is a perfect illustration of Julius Caesar’s tactical use of fortifications and subterfuge.  In the only other mention of turf, during Caesars campaign against the Nervii, they surround his winter-quarters;
Caes. Gal. 5.42
This gives an insight into the importance of the Roman soldiers kit such as  the pick-axe [dolabra] and the mattock [ligo].  There can be little doubt that the Roman legions were building timber ramparts and other works with timber; turf may serve as a facing or cover which can both help fireproof and disguise the nature of the structure.  

The last passage is also one of the many that demonstrates that in Gaul the natives were not simply helpless foils for roman military superiority, but were tactically aware and adaptable.  It is important to understand that structure designed to hold off a full scale enemy assault has to be fit for purpose.  This passage describes the typical tactics used by the Gauls attacking roman defencive positions.
Caes. Gal. 7.86   
This chapter describes Gallic countermeasures against Roman siege works.
To the extraordinary valour of our soldiers, devices of every sort were opposed by the Gauls; since they are a nation of consummate ingenuity, and most skilful in imitating and making those things which are imparted by any one; for they turned aside the hooks with nooses, and when they had caught hold of them firmly, drew them on by means of engines, and undermined the mound the more skilfully on this account, because there are in their territories extensive iron mines,.......
Caes. Gal. 7.17
Ditch, Bank and Palisade
While Trajan’s column shows structures built from layers of wood, [left] and Caesar’s writings are full of references to timber fortifications, one later writer, Vegetius, [3], gives a different impression, leading to a perfectly understandable convention that Roman soldiers built marching camps by digging a ditch creating a bank with the turf and spoil, which was then fortified with a palisade, perhaps of stakes carried by the infantry for that purpose. 
It is also certainly what is described Polybius [4]  years earlier, when in 197 BCE, the Roman infantry were ordered to carry palisades stakes to build their camp; although his point is that were able to, because, unlike their enemy the Macedonians, they not already encumbered crying pikes, [sarissa].
Polybius Histories 18 [4] 
This passage comes before the battle at Cynoscephalae, where Roman infantry triumphed over the Macedonians, which was not only important in the development of the empire, but also a significant tactical achievement over the previously dominant Greek Phalanx based on the use of the sarissaMuch of the narrative has this theme, but what it is clear from the rest of the passage that the Romans interlocked their stakes more cunningly which helped strengthen and disguise the structure.  The stakes are cut because he wants to be able to encamp quickly, but he has to order the troops to carry stakes, so presumably it is not routine.
However, the Military Institutions of the Romans’ [Epitoma rei militaris], by Flavius Vegetius Renatus, often known as Vegetius,[3], and probably written in early in the fifth century, is another  important source for ideas about the Roman Army.  He discusses the construction of camps with reference to basic training.
Epitoma rei militaris 1.20 [3]

There are two things about this account, firstly the qualification; "But this valuable art is now entirely lost", implying that building proper  timber ramparts was no longer practiced, and that defences now consisted of a ditch and bank with a palisade.
And secondly, I am suspicious of the translation which implies the soldiers routinely carried the materials for a palisade.  That they routinely carried the tools necessary makes sense and is well known, but only in very particular circumstances would you want to be carrying the additional weight of several pieces of wood.  As we have seen, in earlier times wood was sourced locally, and quite frankly, it is normally envisaged that Roman infantry had enough to carry.
The Turf Wall at Appletree
Hadrian’s Wall West of the fort at Birdoswald was known to be different from the characteristic sections that survived to the East in having a non-stone phase, which was particularly obvious at Appletree in Cumbria, where the stone replacement was on a different line further to the North.  This was first sectioned in 1895, and a very good account of the history of the archaeology of this section of the wall can be had here [5].  In essence, a paper by F G Simpson and Ian Richmond in 1935 proposed that this Eastern section of the Wall was built from turf with the help of pioneering palynology by Dr A Rainstock, [6]
More recent work on this section by English Heritage in 1999, [7], also contains an excellent summary of other recent results, along with the latest reports on the pollen [8]  and plant macrofossils, which are well worth a closer look.[9]  
The archaeology that remains at Appletree [right] is a about 1m tall at best, and what interests us here is the “Peaty” layer which is the signature archaeology, and looks like a complex piece of Russian cake with dark chocolate layers interleaved with lighter greys yellows and greys.
In the 1999 section [below] some of these dark layers are continuous for over a meter, the lowest one is continuous and is thought to represent the original ground surface, [Layer 53], sealed by 45 -50cm of Turf wall material, [Layer 54].  In essence current thinking is that this section was built of turves and with a timber breastwork to a height of 3.6m / 12” with a near vertical northern face and sloping south side.[10] 
So let’s be clear about this, turf is sods of earth held together by the roots of plants such as grass, a substantial part of which is a block of top soil, so, in theory, the archaeology of a turf structure be layers of top soil interleaved with thin bands of organic humus, depending on the nature of the groundcover and the nature of subsequent soil formation processes.
It was this complex peaty layer [L52], the layer below it thought to be the old ground surface [L53], and similar material from the ditch [L45] that was sampled with aim if establishing the origin of the turves.

The pollen analysis discuses previous work on a less specific turf wall sample [probably 52+53],
The pollen results revealed that a relatively open, probably grazed moorland existed immediately prior to construction of the Wall with the main; taxa represented being a mix of alder, birch, hazel, oak, heather and wild grasses. Other species present in low but consistent numbers were sedges, ribwort plantain, ferns and bracken”. [8]
The 1999 samples also show a similar pattern with material from the “Turf” layer being dominated by tree pollen, particularly hazel, with samples of the ditch fill show a similar pattern of Hazel, Alder and later birch pollen dominating.
The report sheds little light on the source of the turf;
 “The source area of the hazel-dominated turfs is probably still from very close to the site and the higher values of hazel may suggest that the turf was removed from closer to a field boundary or woodland edge.”[8]
The study of plant macrofossils involves breaking these peaty deposits down, sieving the content in graded sieves, and examining the residues.
While the sample from the lowest layer [53], the probable old ground surface, contained some sand and a little gravel along with charcoal, the layer above [52] produced sand, with charcoal from both trunk and branch up to 10mm.[9]  The report suggests that perhaps the turves came into contact with brushwood burnt during the land clearance for the Turf Wall.  The remains of Ceratodon purpureus, Fire Moss, was also present in quantity, which as the name suggests is an early coloniser after forest fires, and among other things it commonly grows on is dead wood.  
Having prejudged the nature of the material nothing quite makes sense, the “turf" is the dominated by  pollen from trees species, and appears to lack lacks the range of inclusions that might derive from a top soil component, being dominated by charcoal fragments.  Even given that the organic materials decompose, and that soil is often composed of finer particles which can leach away, a greater range and concentration of inclusions from a boulder clay top soil component might be expected.

A 3.6m high wall made of turf, 6m wide at the base, would require eighteen 20 cm layers of turf, which would require up a 100m wide strip turf, depending on how it sloped.  A 3.6m pile of turves would decay into a significant large grassy mound, which presumably would have little requirement for the aid of mosses to facilitate regeneration.
The dominance of tree species in the pollen record, the abundance of charcoal, and the presence of fire moss are all consistent with the structure being the remains of a pile of wood.

I am now fairly sure that this is the remains of a timber structure, I think the "turf" is a red herring, and illustrates both the problem of naming things and the tendency of archaeologists to find what they are looking for.  It was a long time before the postholes for the timber Wall were found, and I strongly suspect that there should be postholes in the west.  To understand a structure like this would require a different type of excavation.
I have advanced three lines of argument which suggest a Wall made of Turf is probably erroneous; firstly, Caesar makes no use of it, fortifications were made of timber, secondly, it is not good military engineering; finally, the environmental evidence and the nature of the deposits themselves are not really consistent with them being simply the remains of turves.
Turf is ideal as a skin, disguising and fireproofing a structure; it is useful for stabilising banks, especially in a makeshift or temporary barrier, as in a marching camp of the sort alluded to in Vegetius.  If the objective had been a permanent “earth” wall, the efficient thing to have done from an engineering point of view, would have been to incorporate the spoil from the ditch.
Turf is a useful building material, and is used in Northern Europe particularly when no is better material is available, [i.e. timber] [11].  The work at the Lunt [12] and Vindolanda [13] have demonstrated that large structures can be built with earth and turves using the minimum of wood; for experimental archaeology turf is cheap and easily sourced. However, these structures are not regularly subjected to violent assault, and a structure that is, in effect, a pile of soil, would be simple to undermine, and palisade type defences are easily pulled over, especially if planted in loose earth.  
The Romans Army, whose lives could depend on the efficacy of such structures, were probably less constrained by cost or environmental impact; deforestation was a feature of the Roman occupation in the North [14], it was was good for security, and timber was still the principle building material.
The crucial point about the postholes of the eastern sector is that this structure was removed without any mess; it was clearly only ever conceived of as a temporary measure, yet another reason timber was used rather than turf or earth.   I have previously suggested, as I am sure others may have also inferred, that the Turf Wall should have required layers of timber for structural stability alone.
In the final analysis, the precise method of construction does not matter, the recognition that this is the earliest phase in the west is more important, although without the Timber Wall in the East it makes no real strategic sense.  The idea dislocation/s, break/s in wall building, presumably due to war, is now widely accepted, [15]; it provides a context for this Western temporary wall to become rotten or damaged, and what we see is now at Appletree is probably the remains of a pile of the wood from the rampart that could not be salvaged.

Sources and further reading
[2] C. Julius Caesar, Gallic War 
[4]  Polybius, Histories 
[7] Hadrian’s Wall Archaeological Research by English Heritage 1976–2000
edited by Tony Wilmott
[8]op cit p. 114 the pollen  James Wells
[9] op. cit.116 The plant macrofossils,  Allan Hall, 
[10] op. cit. p.118, fig. 225
[11] Building with Turf by by S Sigurðardóttir - ‎2008 English version by Nancy Marie Brown  ISBN 978-9979-9757-4-8
[12]  http://www.luntromanfort.org/ [Accessed 25/12/2014],
[13] http://www.vindolanda.com/ [Accessed 25/12/2014],
[14] Dumayne L. and Barber K.E. 1994, The impact of the Romans on the environment of northern England: pollen data from three sites close to Hadrian's Wall, The Holocene, June 1994 vol. 4 no. 2 165-173


Odin's Raven said...

Where would they have got all that wood? Was there really a lot more woodland there in Roman times? Surely most of it would not have been so nice and straight and even? Were they cutting down the great Caledonian forest, and perhaps annoying the natives thereby? Wouldn't these wooden fortifications have been very vulnerable to fire? Maybe they were rain sodden, but perhaps some wily Greeks might have been available to sell the natives an early version of Greek Fire!

Compliments of the Season!

Geoff Carter said...

Hi OR,
The pollen record records a landscape of "a mix of alder, birch, hazel, oak,"
Plenty of wood, smaller trees are most useful, easily felled, processed, and moved.
In a society whose material culture is substantially based on the use of wood clear felling trees is a form of economic and cultural warfare.
Fresh wood does not burn easily, and an accelerant is required; Using turf facing might have made them more difficult to set alight.
These fortifications along the Wall were not long term structures, I think the plan was always to replace them in stone.
If the object is to make a serviceable breach in the wall, setting it alight may be counter-productive, certainly in the short term.

Odin's Raven said...

Here's a blog I've just come across which might relate to your interest in old buildings, farming techniques and trees.

The author suggests that some stone circles were threshing floors, doubling as agricultural calendar shadow clocks.He also writes about 'Christmas trees' and that acorns were harvested, ground and cooked long before corn, and has a link to a book about oak trees and how humans brought them to Britain.

Old European Culture

Geoff Carter said...

Thanks OR,
Interesting ideas but;
stone circles are not threshing floors - due to the climate we have indoor threshing [Barns], these ideas might work in the Middle East.
Acorns are not particularly good food and take a lot of processing to be palatable, they tend to be used in times of famine and in some subsistence cultures [as in N American].
As far as I am aware Oak was not an introduced species in UK, it is a key component in the natural climax vegetation.