Between Hadrian’s Wall and the ditch to north, archaeologists have found three lines of double postholes running parallel to the Wall, which may represent an early timber 'Wall', albeit temporary, comprising a box rampart and the ditch. This was almost certainly the largest structure timber ever built in this country, its full extent is not known for certain, but it was quite probably 117 km long, and would have required an estimated 2.5 million trees.
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,  Shields Road Byker, , Throckley, , and also from Melbourne St, Newcastle,. As a structural archaeologist working with postholes, (my) best practice is consider each site on its own merits, and my analysis is based primarily on the site at Buddle St, where there is clearly defined block of pits, [below].
The model does not specify the distribution of spoil, but it must have gone somewhere, and filling the base of the rampart and creating a glacis is the most likely scenario, most excavations have found trace of a bank on this southern edge of the ditch, but its origin is unclear.
One obvious variation is to have double thickness facing by placing baulks on both sides of the outer posts, [right]. The structure is very flexible and adaptable, the upper part can be lightly constructed, while the lower portion can be of much more massive, and part filled with spoil from the ditch, which can also be piled on the berm at the foot of the rampart to form a continuous glacis slope with the face of the ditch.
"The line between disorder and order lies in logistics…” Sun Tzu The Art of War (c6th bce)
“Forget logistics, you lose"Lieutenant General Franks, 7th Corps Commander, Desert Storm 
The construction of the Hadrianic frontier, like any military operation, was foremost an exercise in logistics; in terms of the ‘Wall’, one obvious problem being the lack of existing sources of stone, aggregate, and lime, and the infrastructure necessary to exploit them, which would have contributed to the 6 years that it took to build it. The retreat from Southern Scotland, and the creation of the frontier in this form, suggests that the overriding concern in Northern England was ‘security’, without it, the job of constructing the wall would be much more difficult, thus creating a temporary defensible frontier could be seen as entirely logical, and almost a prerequisite. 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.
2. The density of trees in woodland
3. The proportion of woodland in the countryside
As we have discussed before , the growth pattern of trees, in terms of size and density of woodland, can be modelled from standard forestry tables . The 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 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 new money, each 100m stretch requires between 867 - 3953 trees, very roughly 3 ha worth.
What the model shows is that the larger more mature trees, [60 –80 year old oaks left], were the most efficient source of timber, with small younger trees being the least, despite growing more densely. As noted in earlier articles on this site, this is exactly the type of tree would have been grown for building in prehistory, .
Julius Caesar's Account of the Gallic War, is a must read far all archaeologists of the Iron Age, granted it's partial, biased, and in places, little short of Julian propaganda, but we don’t exactly have a plethora of first hand accounts of warfare in ‘Barbarian’ Europe to choose from. It tells the story of Caesar’s campaigns to subjugate Gaul between 58 – 51 bce, a quite extraordinary tale of total warfare Roman style.
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.
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.
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.
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 left]. 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.
The Obstacle theory
Generally, trees are grown to be straight, in conditions which discourage branching; in prehistory most postholes are 0.20-0.40m diameter, suggesting, if oak was used, trees were harvested at c. 50- 80 years. Close spaced groups of branches, may occur at the crown, , which is generally thrown away, however for the crown of a tree to be 0.20 –0.40m in diameter, the basal diameter would have to be quite significant greater, certainly larger than those that are represented in postholes of the period.
The Buddle Street road mystery.
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, Which is what we do here!
Postholes, although often little regarded, are the key to understanding much of the archaeological built environment, and it’s best not to ignore them, or explain them away – they may be filled voids now, but they are called postholes because they once held posts, and therein lies their utility – as Lao Tzu might have said.
It is important to stress that as a researcher specializing in postholes, I am not necessarily that concerned with the broader implications of this, or any other, of my theoretical models. Evidence for posts can occur in a variety of periods and contexts, and I am in reality no more a Roman specialist than someone studying the carbonised remained of wood would be. I study the commonest of all features, postholes, and the structures they represent, which is not even regarded as a specialist subject, and I am probably the only archaeologist in the country studying these features from a structural as opposed to a structuralist perspective.
The model suggested for Buddle street works reasonable well for the other sites, but given the flexibility inherent in the design, and the need to adapt the structure to changes in slope, features such as streams, and differing ground conditions, combined with likelihood of towers, stairs, sally ports, and other ancillary features, we cannot be too prescriptive about the model, or its application. The detail of its construction, its height, number of galleries, disposition of towers was probably quite variable, and cannot be known with any degree of certainty.
I consider the general feasibility of a timber faced box ramparts built predominantly in alternating, and probably overlapping, layers of standard baulks to be proven as a good match for the evidence. The timber requirements are not excessive, materials for flooring, roofing, parapets, etc, would have come from the smaller trees of the woodland under story like hazel. In the context of establishing a secure military corridor across northern Britain, one would envisage the clearance of a wide strip of land, with a road, and defendable boundaries of both sides; several hundred feet to south of the Timber rampart, was the ditch and earth rampart system known as the Vallum. This defendable military corridor would help protect the workforce, materials, supplies, grazing animals, and stock, from the potentially hostile native population, whose former territory was being effectively split into two zones.
When, after the probable six years or so, the Wall was complete, the timber wall, probably one of the largest wooden structures ever created, was dismantled, probably section by section, as the wall was completed; its timbers almost certainly recycled into the dozens of new forts, and, into the new stone wall, - it should not be forgotten, that without addition timber works, a simple wall and parapet, while impressive, was not a particularly defendable structure, (especially when your ditch is so far away).
So this enormous structure disappeared leaving only postholes, which, as so often happens, could have been easily overlooked, which would have been a shame, and rather a waste of about 2.5 million trees.
Latest article on Hadrian's Wall; Reverse Engineering the Vallum
Sources and further reading
 Bidwell, P T, 2005 'The system of obstacles on Hadrian's Wall; their extent, date and purpose', Arbeia J, 8, 53-76.
 Caius Julius Caesar "De Bello Gallico" and Other Commentaries
English translation by W. A. MACDEVITT, introduction by THOMAS DE QUINCEY (1915) http://www.gutenberg.org/etext/10657
 Bidwell, Paul T.; Watson, Moira. 1989 'A Trial Excavation on Hadrian's Wall at Buddle Street, Wallsend'. Archaeologia Aeliana, 5th ser., 17 (1989), 21-28.,
Grey literature: Shields Road, Newcastle, Phase 2b, archaeological excavation. TWM archaeology 10/2006
 T. Frain, J. McKelvey & P. Bidwell 2005 Excavations and watching brief along the berm of Hadrian’s Wall at Throckley, Newcastle upon Tyne, in 2001-2002. Arbeia J, 8
 Grey literature; Throckley, Newcastle upon Tyne, archaeological excavation and watching brief. TWM archaeology 12/2003
Grey literature: Newcastle, Melbourne Street, Archaeological excavations. Archaeological services, University of Durham.
 See; http://structuralarchaeology.blogspot.com/2009/01/17-not-seeing-wood-or-trees.html
 James. 1989, Forester's Companion -Cambridge University Press, ISBN 0631108114
 After; http://en.wikipedia.org/wiki/File:SiegeAlesia.png
 Cip´pus n. 1. A small, low pillar, square or round, commonly having an inscription, used by the ancients for various purposes, as for indicating the distances of places, for a landmark, for sepulchral inscriptions, etc.
Webster's Revised Unabridged Dictionary, published 1913 by C. & G. Merriam Co.
 After Fig 15 p73 Bidwell, P T, 2005 'The system of obstacles on Hadrian's Wall; their extent, date and purpose', Arbeia J, 8, 53-76.
 See Also: Carter, G. A. 1998: Excavations at the Orsett ‘Cock’ enclosure, Essex, 1976. East Anglian Archaeology Report No 86.
 Burley R 2009, Vindolanda A Roman fort on Hadrians Wall. Amberley ISBN978-1-84868-210-8