24 January, 2012

Hadrian’s First Wall [Part 1 of 3]

On Tyneside, between Hadrian’s Wall and the Ditch to the north, archaeologists have found three lines of double postholes, which it is argued, represent an early Timber Wall, which, along with the Ditch, formed part of a temporary frontier while the Roman Wall was being built. Further, it is argued that the Turf Wall represents the continuation of this structure in the western sector of the Wall.  In addition, when the engineering and layout of the Vallum is examined, it appears to be an unfinished road, probably abandoned when warfare interrupted work on the Wall. These insights into the archaeology of Roman military engineering are the key to a new understanding how and why Hadrian’s Wall was built.

An updated  summery of a series of articles from this site on the timber and earth structures predating Hadrian’s stone Wall.

Presented in three parts:

1. The Timber wall

1. The Timber Wall
The evidence for the double postholes, often referred to as ‘cippi pits’, had been picked up in several excavations, and was compiled by Paul Bidwell of Tyne and Wear Museums Archaeology, who were responsible for several of the excavations, [1].   His paper sought to set the evidence in the wider context of other Roman frontiers, and drew on Julius Caesar’s Account of the Gallic war, [De Bello Gallico],[3], particularly the siege of Alésia, in reaching his conclusion that these postholes represented ‘obstacles’ on the berm, probably sharpened wooden entanglements, similar to the ‘cippi’ referred to by Caesar. We shall return to these arguments, and Caesar, later, but it is clear that I consider term 'obstacles’ to be somewhat underselling this remarkable structure, which I understand as a rampart.   

[left] The Wall, ditch, and 'cippi' pit at Shields Road excavation, Byker, Newcastle [2]

The Evidence
The archaeological plan of Buddle Street, Wallsend
The evidence was admirably described by Paul Bidwell, so it will not be covered in great detail, but it comes principally from excavations at Buddle St, Wallsend, [4] Shields Road Byker, [2], Throckley, [5], and also from Melbourne St, Newcastle, [6].
The double post pits occur in 3 rows, the inner and outer rows are aligned parallel to the wall (E-W), and the central row perpendicular to it (N-S).

The pits have been robbed, and appear enlarged by this process, particularly at Buddle St; at Shields Road they were more rectangular and better defined. In most cases the traces of a pair of post pipes are evident at either end of the pit, representing posts of 0.2 – 0.3m in diameter.  [below]
A watching brief on a trench being dug by Northumbrian Water in Throckley yielded further important evidence. The trench ran for 2.2 km along the berm, parallel to wall, giving a long, but very narrow, glimpse of the post pits, and proving they extend as far as Throckley.  In addition, it also produced a most important piece of evidence; as the pits approached the position of Turret 11b on the wall, they changed direction towards it. What is significant here, is that the ditch does the same thing, curving to towards the turret.  This suggests that the post pits, ditch, and turrets are part of the same system, at least at this point. It has puzzled archaeologist why the ditch curved in towards the turrets creating a narrowing of the berm, a phenomenon observed in several places, [1], but this is explained if the rampart represented by the post pits ran between, but in front of, the turrets, along with the ditch, as a temporary expedient, while the stretches of stone walling linking the turrets on a more direct line were under construction.

The deviation of the Postpits and Ditch toward the turret 11b at Throckley.

As a structural archaeologist working with postholes, (my) best practice is consider each site on its own merits, and the theoretical models are based on the sites at Buddle Street and Shields Road, where there is clearly defined block of pits. Luckily, we do some Roman literature discussing timber and earth ramparts.

Historical sources: Engineering the battlefield

Julius Caesar's Account of the Gallic War tells the story of Caesar’s campaigns to subjugate Gaul between 58 – 51 bce, a quite extraordinary account of total war Roman style. One of the big set pieces at the climax of the war, and the book, is the siege of Alésia, a hillfort in France, where the Gaulish leader Vercongeterix, was blockaded with most of his army.

The Romans built a series of encircling siege works around the hillfort, including diverting rivers, and then, in order to protect himself from any relieving forces and the Gaulish cavalry which had escaped, they built a second set of defences to protect their siege works against attack from outside, [below [7]].

There is quite a detailed account of the building of these works, which is worth reproducing in full:
LXXII. Caesar, on learning these proceedings from the deserters and captives, adopted the following system of fortification; he dug a trench twenty feet deep, with perpendicular sides, in such a manner that the base of this trench should extend so far as the edges were apart at the top. He raised all his other works at a distance of four hundred feet from that ditch; [he did] that with this intention, lest (since he necessarily embraced so extensive an area, and the whole works could not be easily surrounded by a line of soldiers) a large number of the enemy should suddenly, or by night, sally against the fortifications; or lest they should by day cast weapons against our men while occupied with the works. Having left this interval, he drew two trenches fifteen feet broad, and of the same depth; the innermost of them, being in low and level ground, he filled with water conveyed from the river. Behind these he raised a rampart and wall twelve feet high: to this he added a parapet and battlements, with large stakes cut like stags' horns, projecting from the junction of the parapet and battlements, to prevent the enemy from scaling it, and surrounded the entire work with turrets, which were eighty feet distant from one another.  

LXXIII.--It was necessary, at one and the same time, to procure timber [for the rampart], lay in supplies of corn, and raise also extensive fortifications, and the available troops were in consequence of this reduced in number, since they used to advance to some distance from the camp, and sometimes the Gauls endeavoured to attack our works, and to make a sally from the town by several gates and in great force. On which Caesar thought that further additions should be made to these works, in order that the fortifications might be defensible by a small number of soldiers.
Having, therefore, cut down the trunks of trees or very thick branches, and having stripped their tops of the bark, and sharpened them into a point, he drew a continued trench everywhere five feet deep.
These stakes being sunk into this trench, and fastened firmly at the bottom, to prevent the possibility of their being torn up, had their branches only projecting from the ground. There were five rows in connection with, and intersecting each other; and whoever entered within them were likely to impale themselves on very sharp stakes. The soldiers called these cippi.
Before these, which were arranged in oblique rows in the form of a quincunx, pits three feet deep were dug, which gradually diminished in depth to the bottom. In these pits tapering stakes, of the thickness of a man's thigh, sharpened at the top and hardened in the fire, were sunk in such a manner as to project from the ground not more than four inches; at the same time for the purpose of giving them strength and stability, they were each filled with trampled clay to the height of one foot from the bottom: the rest of the pit was covered over with osiers and twigs, to conceal the deceit.
Eight rows of this kind were dug, and were three feet distant from each other They called this a lily from its resemblance to that flower.
Stakes a foot long, with iron hooks attached to them, were entirely sunk in the ground before these, and were planted in every place at small intervals; these they called spurs.

De Bello Gallico LXXII – LXXIII

The term ‘cippi’ seems to apply to a trench filled with branches, and trunks with branches still attached, and then backfilled; presumably, this was wood remaining after the straight lengths of trunk had been used in the main defences. It is placed at a distance from the rampart so that the archers would be able to engage the enemy as they dealt with the entanglement, in addition, once this had been breached, the defenders could concentrate their fire on the breach.

It appears to me, that the term ‘cippi’ in this context is a bit of soldierly gallows humour Caesar is sharing with his readers. A cippi is a marker, a post, often marking a grave, [8], "so that the men call them cippi" – effectively ‘gravestones’, and can be seen in the same light as the expressions ‘lilies’ to describe the  tapering pits, shaped like the flower of a lily, with a sharpened stake at the bottom, and the use of ‘spurs’ for the iron hooks designed to catch peoples feet. As Paul Bidwell noted the expressions only appear in this context and nowhere else.
There is another interesting passage about fort building from earlier in the campaign;IX … … … … He ordered his camp to be fortified with a rampart twelve feet high, with breast-works built on it proportioned to its height; and two trenches, each fifteen feet broad, with perpendicular sides to be sunk: likewise several turrets, three stories high, to be raised, with a communication to each other by galleries laid across and covered over; which should be guarded in front by small parapets of osiers; that the enemy might be repulsed by two rows of soldiers.
The one of whom, being more secure from danger by their height, might throw their darts with more daring and to a greater distance; the other, which was nearer the enemy, being stationed on the rampart, would be protected by their galleries from darts falling on their heads. At the entrance he erected gates and turrets of a considerable height.
De Bello Gallico IX [3]
This passage emphasises the importance of height, and the advantages of having more than one fighting platform, to concentrate firepower at the point of attack, as well as demonstrating the practice of protecting defenders from aerial bombardment. These structures were probably defensible from both sides.
There is an interesting reference to military engineering in Polybius’s description of the Roman general Flamininus's campaign in Thessaly and the Battle of Cynoscephalae in 197 BC.
The context is slightly different: it compares the approach of the Greek and Roman armies to building temporary ramparts for camps. The translation is a bit cumbersome, but quite insightful about Roman thinking on timber ramparts several centuries earlier.

18. Flamininus . . . . ordered all his soldiers to cut stakes for a palisade to carry with them for use when required. This appears to be impossible when the Greek usage is followed, but on the Roman system it is easy to cut them. For the Greeks have difficulty in holding only their pikes when on the march and in supporting the fatigue caused by their weight, but the Romans, hanging their long shields from their shoulders by leather straps and only holding their javelins in their hands, can manage to carry the stakes besides.
Also the stakes are quite different. For the Greeks consider that stake the best which has the most and the stoutest offshoots all round the main stem, while the stakes of the Romans have but two or three, or at the most four strange lateral prongs, and these all on one side and not alternating. The result of this is that they are quite easy to carry — for one man can carry three or four, making a bundle of them, and when put to use they are much more secure.
For the Greek stakes, when planted round the camp, are in the first place easily pulled up; since when the portion of a stake that holds fast closely pressed by the earth is only one, and the offshoots from it are many and large, and when two or three men catch hold of the same stake by its lateral branches, it is easily pulled up.
Upon this an entrance is at once created owing to its size, and the ones next to it are loosened, because in such a palisade the stakes are intertwined and criss-crossed in few places.
With the Romans it is the reverse; for in planting them they so intertwine them that it is not easy to see to which of the branches, the lower ends of which are driven into the ground, the lateral prongs belong, nor to which prongs the branches belong. So, as these prongs are close together and adhere to each other, and as their points are carefully sharpened, it is not easy to pass one's hand through and grasp the stake, nor if one does get hold of it, is it easy to pull it up, as in the first place the power of resistance derived from the earth by all the portions open to attack is almost absolute, and next because a man who pulls at one prong is obliged to lift up numerous other stakes which give simultaneously under the strain owing to the way they are intertwined, and it is not at all probable that two or three men will get hold of the same stake.
But if by main force a man succeeds in pulling up one or two, the gap is scarcely observable. Therefore, as the advantages of this kind of palisade are very great, the stakes being easy to find and easy to carry and the whole being more secure and more durable when constructed, it is evident that if any Roman military contrivance is worthy of our imitation and adoption this one certainly is, in my own humble opinion at least.

Polybius Histories 18 [9]
In short, the Greeks preferred stakes with lots of branches all round, but they had long pikes, so could not carry them anyway, while the Romans, with a javelin, could carry several standardised stakes, with only a few short branches, that interlocked together so that they were difficult to pull apart.
This is the same basic principle evident in the Timber Wall, emphasising that even the trimmed branch stubs contributed to the interlocking nature of the rampart. Visually, it was probably not very pretty, but it was a very efficient form of rampart, being difficult to break through or pull apart - this being the object of the exercise.
Another important source of information on Roman military engineering is Trajan’s column in Rome, which depicts his two victorious campaigns against Dacians, (101/2 &105/6 ce), [above]. One scene shows a ballista set on a structure made from layers of logs laid at right angles to each other, while another shows legionaries cutting and extracting timber baulks.
We have another general source that discusses the organisation of the Roman army and its approach to warfare; Publius Flavius Vegetius Renatus, often known as Vegetius, wrote ‘The Military Institutions of the Romans’ [Epitoma rei militaris], [10], probably early in the fifth century.  While little is known of Vegetius, or the veracity of his sources, it does provide a glimpse of the Roman army’s general philosophy and practice, albeit written quite late in the period.
He discusses the importance of earth and timber ramparts in the construction of camps with reference to the training of recruits.
Entrenched Camps
Recruits are to be instructed in the manner of entrenching camps, there being no part of discipline so necessary and useful as this. For in a camp, well chosen and entrenched, the troops both day and night lie secure within their works, even though in view of the enemy. It seems to resemble a fortified city which they can build for their safety wherever they please. But this valuable art is now entirely lost, for it is long since any of our camps have been fortified either with trenches or palisades. By this neglect our forces have been often surprised by day and night by the enemy's cavalry and suffered very severe losses. The importance of this custom appears not only from the danger to which troops are perpetually exposed who encamp without such precautions, but from the distressful situation of an army that, after receiving a check in the field, finds itself without retreat and consequently at the mercy of the enemy. A camp, especially in the neighbourhood of an enemy, must be chosen with great care. Its situation should be strong by nature, and there should be plenty of wood, forage and water. If the army is to continue in it any considerable time, attention must be had to the security of the place. ……………………………………… ………………………………………There are two methods of entrenching a camp. When the danger is not imminent, they carry a slight ditch round the whole circuit, only nine feet broad and seven deep. With the turf taken from this they make a kind of wall or breastwork three feet high on the inner side of the ditch. But where there is reason to be apprehensive of attempts of the enemy, the camp must be surrounded with a regular ditch twelve feet broad and nine feet deep perpendicular from the surface of the ground. A parapet is then raised on the side next the camp, of the height of four feet, with hurdles and fascines properly covered and secured by the earth taken out of the ditch. From these dimensions the interior height of the entrenchment will be found to be thirteen feet, and the breadth of the ditch twelve. On the top of the whole are planted strong palisades which the soldiers carry constantly with them for this purpose. A sufficient number of spades, pickaxes, wicker baskets and tools of all kinds are to be provided for these works.

There is no difficulty in carrying on the fortifications of a camp when no enemy is in sight. But if the enemy is near, all the cavalry and half the infantry are to be drawn up in order of battle to cover the rest of the troops at work on the entrenchments and be ready to receive the enemy if they offer to attack. The centuries are employed by turns on the work and are regularly called to the relief by a crier till the whole is completed. It is then inspected and measured by the centurions, who punish such as have been indolent or negligent. This is a very important point in the discipline of young soldiers, who when properly trained to it will be able in an emergency to fortify their camp with skill and expedition.

Epitoma rei militaris 1.20 [10]
Clearly, earth and timber fortification are fundamental to Roman field tactics, and engineering is as important as fighting to the training of a legionary.   One important point he makes, is that priority should be given to building the fortifications, and that, if the enemy is close by, half the troops and all the cavalry should remain at battle stations to cover the engineering works.  In a sense, this has implications for the building of the wall, since in hostile circumstances, and without a completed defensible line or perimeter, large numbers of troops would be required to guard the workforce.
In book two of Epitoma rei militaris, Vegetius discusses the different ranks and responsibilities of officers in the legions.  Two ranks in particular illustrate the importance of engineering to the army.
The Praefect of the camp
"The Praefect of the camp, though inferior in rank...,  had a post of no small importance. The position of the camp, the direction of the entrenchments, the inspection of the tents or huts of the soldiers and the baggage were comprehended in his province. His authority extended over the sick, and the physicians who had the care of them; and he regulated the expenses relative thereto. He had the charge of providing carriages, bathorses and the proper tools for sawing and cutting wood, digging trenches, raising parapets, sinking wells and bringing water into the camp. He likewise had the care of furnishing the troops with wood and straw, as well as the rams, onagri, balistae and all the other engines of war under his direction. This post was always conferred on an officer of great skill, experience and long service, and who consequently was capable of instructing others in those branches of the profession in which he had distinguished himself."
Epitoma rei militaris  2.5  [10]
The Praefect of the workmen
The legion had a train of joiners, masons, carpenters, smiths, painters, and workmen of every kind for the construction of barracks in the winter-camps and for making or repairing the wooden towers, arms, carriages and the various sorts of machines and engines for the attack or defence of places. They had also travelling workshops in which they made shields, cuirasses, helmets, bows, arrows, javelins and offensive and defensive arms of all kinds. The ancients made it their chief care to have every thing for the service of the army within the camp. They even had a body of miners who, by working under ground and piercing the foundations of walls, according to the practice of the Beffi, penetrated into the body of a place. All these were under the direction of the officer called the prefect of the workmen.
Epitoma rei militaris 2.6 [10]
These extracts give an insight into engineering and logistics in the Roman army and the range of skills that underpinned it, which more often involved the use of timber rather than stone.
Marcus Vitruvius Pollio, better known as Vitruvius, served as a soldier with Julius Caesar, probably in the artillery, and he wrote the De Architectura [x], the only significant surviving book about Roman architecture.  While this book provides no particular insights into rampart building, it covers most things from building a temple to making a ballista, demonstrating the wide range of skills and knowledge expected of an engineer or architect in the Roman world.
Application of these skills was one of the Roman armies’ principle advantages in the field. The speed of construction and effectiveness of their siege works and forts, even the range of their artillery, upset many of their enemies’ most basic strategic assumptions.  They out-engineered their opponents.

The Obstacle theory
Paul Bidwell’s theory that these post pits once held pointed pieces of wood, similar to those described by Caesar, is much easier to draw than to realise, and has several disadvantages as an explanation. [1]
The postholes at the Wall suggest trees were 0.20-0.40m in diameter, suggesting, if oak was used, trees were harvested at c. 50- 80 years, this being typical of the prehistoric timber.  The closely spaced groups of branches envisaged in the Obstacle Theory may occur at the crown, and for a tree to be 0.20 –0.40m in diameter at the top, it would have to be quite substantial at its base.  Branches and crowns are by their nature irregular, and are usually thrown away; Caesar could use them only by burying them in a ditch.  
In short, the trees required would be of a very unusual form, conveniently, and somewhat uniformly branched just above the 0.20 – 0.40m diameter point of the trunk, and available in large quantities.  
Another objection is practicality; once this entanglement was in place, how was it maintained? It would rapidly become overgrown with weeds, and soon start to develop the characteristics of a hedge, and would end up providing cover for attackers. Over time, as the wood dried out, it would become flammable.
My third objection is on grounds of strategy; entanglements are designed to slow down attackers, and concentrate them into a field of fire in a battlefield context. As Caesar described, obstacles can be overcome by counter measures, (except by cavalry, which is unlikely to be attacking a wall). Placing two timbers in each hole would make them easier to pull out.  The most practical method of attacking a wall is with scaling ladders, which is one of the principle reasons for having a ditch, which is why the position of the Hadrian’s Wall Ditch is so odd, too far from the wall to be effective, with its normal position, apparently usurped by a wooden entanglement.  

An artists impression of the 'cippi' style wooden entanglements curving towards the turret at at Throckley , [11].
The evidence, everywhere it has been found, is of standard double postholes in three even spaced lines, to create such a pattern, it requires thousands of regular posts, rather than multiple irregular shaped timbers cut from a previously unknown form of tree.

Timber Wall theoretical models
The theoretical model describes the general form of the rampart, and is based on the precise position of its posts, and on the principle that military engineering has to be credible and fit for purpose. In addition, we have some invaluable descriptions and insights from classical sources which explain the general form and thinking behind this type of military engineering.
Box ramparts were widely used, particularly in the bronze age, in essence it is a massive log built cavity wall, filled with material from a ditch which was usually dug in front.
Log built walls are fairly easy for an attacker to scale, [above A], so some form of parapet would be added to prevent this, [Above B]. The ditch is an obstacle to direct assault on the rampart face, and making it more difficult to scale with ladders, [Above c]. The spoil, in addition to filling the structure, can be built up on the berm to form a continuous slope with the ditch known as a glacis, giving the attacker no firm or level position for the attack the rampart from, [Above d].
The glacis material also strengthens and conceals the base of the rampart. Spoil may be added to the outer edge of the ditch, where it is known as a counterscarp.  Keeping the attacker off balance on sloping, preferably loose, ground makes it more difficult for them to use their weapons.
Putting some form roofing over the structure protects the defender and the structure from missiles and the elements. Similar features, built on medieval fortifications, often overhanging, (although they seldom survive), are known as hoardings, or even hoards, [12], (hiding and protecting the defenders).  
An important thing to understand about the three lines of double postholes on the berm is that their utility comes from the spaces between them; the interlocking layers of horizontal timbers form the rampart.  Each set of posts is the foundation of a stack of wooden baulks arranged so that they cannot be readily pulled apart or pushed down.
Structurally, having two posts in each hole makes more sense if they are different heights.  Each set, joined at the top, allows two levels of superstructure to be supported.  The upper level could have served as both a hoarding and a second fighting platform.

The Model is based on the use of a 10’ timber baulk as a standard component, although the posts are in some cases twice this length. Clearly, a variety of different lengths could have been used, and lengths in the range 10’ –12’ can readily be accommodated in the structure, and of coarse shorter lengths can also be used. As a 10’ baulk appears to be inherent in the design, and would greatly simplify the logistics and foraging, the model is illustrated on this basis.
Despite the denuded state of the individual post pits the structure at Buddle St is laid out with a significant degree of accuracy and regularity. Each pit is centred at 4’, in Roman feet, (Pes - 0.296m), with the entire structure being 10’ wide edge to edge.    While this this structure could be simply viewed as a traditional box rampart, the overall geometry is heavily triangulated, which would facilitate 60° bracing, and begins with an offset, suggesting that this was also an important aspect of the design to help avoid ‘racking’.
Polybius’s description gives rise to a different emphasis in the model, suggesting a far greater use of horizontal timbers running through the structure at 60° in the construction of rampart body.  Importantly this 60 °geometry is one of the most characteristic features of these posthole alignments, and at Shields Road Byker seems more dominant that the 90° relationships.

The Timber Wall Model is formed by pairs of posts set holes, which:
  • Form the foundations of the wall;
  • Hold the horizontal components in position;
  • Support the fighting platforms.
  • Two sets of posts allows for two fighting platforms at different heights.
The body of the rampart is formed by horizontal timber baulks [valli]
  • Thick end at the back, thin end pointed to the front;
  • Made from standard lengths of timber 10 –12’;
  • Stubs of branches help interlock the baulk.
Baulks may be placed:
  • At an angle 60° to axis, [Bracing];
  • Running across the structure [90° to axis];
  • Parallel to the axis behind or in font the lines of posts.

Design Features:
  • Simple design;
  • Standard components;
  • Simple to fabricate;
  • Flexible, offering multiple configurations;
  • Very strong interlocked and braced structure;
  • Difficult to penetrate;
  • Individual components difficult to remove;
  • Can be dismantled and reused.
The possible road crossing at Buddle Street.

One of the most puzzling aspects of the Buddle St plan is that rampart starts on the east of the site, but is missing from the centre and west of the site, which is covered by a motley collection of postholes and other features. While I do not intend to pursue this idea very far, an obvious explanation occurs that explains most of the postholes to some degree.

An interpretive plan of the archaeology at Buddle St, Wallsend

There are a series of ditches and gullies, crossing the site roughly N-S, but about 85° to the line of the wall, the spacing of these gullies about 90-100’ [pes] apart with a central pair about 40’ apart, together with some areas of possible surfacing, is suggestive of a Roman road crossing the site. This being the case, then the road would have lead to the facilities on the riverbank prior to the construction of the wall, running north along the east side of Wallsend Golf Course, where its line may have been preserved in the landscape.

Sketch plan of the possible tower at Buddle Street
When the timber phase was constructed, the road was still in place, and the rampart starts from edge of the central carriageway. To the west is a series of postholes, mostly aligned on the road, but while respecting the line of back of the Timber Rampart. My initial interpretation of these features is that they represent a gateway structure constructed across the central carriageway with a tower and further barrier extending to the ditch at the west edge of the road. This ditch may have been re-cut, 4’ to the west at this stage, the ditch to the east appears to have been diverted through the timber wall, also moving it to the west by about 4’. The structures built across the road are quite complex, and very probably ad hoc, and require more detailed study in their own rite, and are on the long list for further study
If the road was still in use, and required a gateway, then the ditch must also had a gap in it at this point, and this may be reflected in the differing profile of the ditch at this point. By the time the wall was built here, the road would have been realigned on the Segudunum Fort just to the east.
The approximate position of the possible road crossing the Buddle St site

Timber logistics
The construction of a temporary wall, like any military operation, was foremost an exercise in logistic.  To be feasible, the project has to have realistic logistics, but since timber was the principle material used for the construction in the Iron Age, it should have been readily available, along with the roads and tracks, necessary to exploit it.
It is possible to estimate the quantity of timber required for the model, but what this represents, in terms of the area land needed to supply it, is dependent on 3 main factors;
  1. The size range of the trees.
  2. The density of trees in the woodland.
  3. The proportion of woodland in the countryside.
The growth pattern of trees, in terms of size and density of woodland, can be modelled from standard forestry tables [13]. The actual amount woodland is not easily discerned for this period; however, Northern England was historically quite well forested, when a figure of 15% coverage was a rough average for the medieval countryside. Using this figure as a minimum, the model visualises from 15-50% woodland coverage in the area around the wall, in terms of the timber required to build it.

The model examines building the rampart from 10’ baulks taken from small, medium, and large trees; - you need fewer larger trees, but they grow in less dense woodland. The model assumes the use of timber in the round, rather than squared baulks split from larger timber. Each 4’ section of the basic box rampart model requires between 300’ and 470’ of timber, depending on thickness, which corresponds to about 20 – 47 trees, depending on their height; scaling this up into metric, each 100m stretch requires between 867 - 3953 trees, very roughly 3 ha worth.
Above is a graphical representation of the timber required for the rampart, in terms of area of woodland shows is that the larger more mature trees, were the most efficient source of timber with small younger trees being the least, despite growing more densely. This is exactly the type of tree would have been grown for building in prehistory, and correspond to those represented by postholes of the north of the Wall.
The model, though crude, and quite conservative, produces a reassuring answer; even with only 15% woodland cover, sufficient timber could probably be found within 1 km of the construction site. So even if we double the amount of timbers required, and then significantly increase its scarcity, the timber wall would appear logistically feasible.
However, this assumes there was woodland available to be exploited, but an apparent change in construction technique further west may reflect of a scarcity timber resources.

The Turf Wall

The Turf Wall is the remains of a temporary Wall and Ditch found in the western sector, after the Wall crosses the River Irthing. It is generally thought that it once was 15-20’ tall, built from turfs, although the precise details of its construction have yet to be fully explained.[14] Certainly, a timber superstructure and layers of brushwood or other material to spread the load would have been advantageous.  Importantly, just like the timber wall in the east, the Turf Wall was a temporary fortification, with a ditch immediately in front with its spoil mostly thrown to the north.
The Vallum and Turf Wall west of Birdoswald Fort
It is believed that this was the last section of Wall to be finished in stone. In places, such as to the west of Fort Birdoswald, the masonry Wall was built further north. As elsewhere on the Wall, the Vallum, earthwork comprising a trench and two parallel spoil mounds, runs close behind the Turf Wall  
One possible explanation for the change of construction is that, unlike the eastern half, this section of wall was close to the old Stanegate frontier between Corbridge and Carlisle, and it is likely local timber resources had already been significantly denuded. Forty years of Roman military activity on this part of the frontier may have had a negative impact on local forestry. Removing tree cover has strategic advantages, and the longer-term impact on the local supply of timber may not have been seen as important.
The Turf wall survives only as a mound of earth, which appears to be built with turf stripped from the surrounding area. I would suggest that the structure might have been reinforced and stabilised with layers of brushwood or even timber.

No evidence of the nature of the upper part of the rampart has been found. The model speculates that the breastwork sat on top of the turf wall and would have been structured in a similar manner to the Timber Wall. There is still the requirement for credible fighting platforms and some form of parapet to prevent climbers. We have to assume it, like all sections of this temporary wall, was built in the expectation that it would be attacked, and it has the same basic layout with turrets and milecastles as the stone Wall that superseded it. Unlike its Timber equivalent, the Turf Wall could not be easily dismantled, so its replacement in stone was not as straightforward

The Ditch and the Temporary Wall
The postholes, which have been found as a result of area excavations and watching briefs under the urban sprawl of Tyneside are the only physical evidence for the Timber Wall. Its presence elsewhere can only be inferred from the width of ‘the berm’, the space between the Wall and Ditch. [15]
It is probably worth pointing out that the spoil from a ditch was often used to form a bank or rampart on the defenders' side. For stability, a small berm may be left, but this gap is often covered by a glacis slope. The idea behind this continuous slope is that it offers the attacker no level ground from which to fight. This is why the wide flat berm between ditch and Wall required some explanation.
The presence of the timber wall explains not only why the berm is so wide, but also why most of the ditch spoil was thrown to the north, with just a small amount to the south. Placing the spoil in a wide low bank to the north has little strategic advantage. In this context would appear that it is being ‘disposed of’ rather than being ‘used’. However, in the context of a temporary Wall, soon to be replaced with a stone one, the distribution of spoil and the width of the berm make sense. We might also expect that the long-term plan would be to widen the Ditch, once the Wall was completed.

Dislocation and The temporary Wall
There is a very important idea in the understanding of Hadrian’s Wall. It’s called “dislocation,” and refers to evidence that the construction of the wall was disrupted, perhaps more than once. It is a tacit assumption that if the army was not working on the Wall, it was fighting and engaged in warfare.
Peter Hill’s detailed analysis of the stonework noted that when the work resumes, a decline in its standard is evident, as well as the more obvious narrowing of the specification.[16] The wall was built to two different standards. Before the dislocation, completed parts of the Wall and milecastles were about 10’ wide; when work resumes the Wall is narrowed to about 8’, even where wider foundations and footings existed.
At Birdoswald Fort, a break in its construction, marked by the formation of a soil, suggests that this interruption may have lasted several years.[17] In addition, David Breeze has gathered together other archaeological and literary evidence that also indicate that there was warfare in the North during the early part of the Second Century.[18]   The evidence for a temporary timber rampart, with the Ditch, constructed in the first season fits well with the other evidence for warfare in this period.


Gabriela said...

Interresting read

Geoff Carter said...

Thank you - I have developed my ideas since I wrote this.
Interestingly, What was found preserved of Vindolanda's timber rampart was, much as I predicted, and made of horizontal timbers perpendicular to the ditch.