Hadrian's Wall; understanding The Vallum

The Vallum is one of the largest earthworks in the world, part of Hadrian's Wall World Heritage site, and yet is seldom discussed, perhaps because while its interpretation may work on paper, it makes less sense on the ground.

It is an excellent example of how in archaeology, what we name something conditions the way we perceive it, and how our literary constructs  can develop independently of the underlying physical evidence. 

The Vallum is one of the oldest concepts in the literature of Hadrian’s Wall, originating with the Venerable Bede in the eighth century, and while this structure is not a vallum in any way shape or form, all subsequent literature would appear to have developed from this idea.

In more recent times, it was apparent that the earthwork was not defensive, but it was nonetheless usually regarded as a boundary or barrier between the Wall and something else, with even the language used to describe the earthwork being shaped to accommodate this underlying assumption.

However, to understand the Vallum you have to look at it with the perspective of a structural archaeologist, luckily, I see it every day, so I know with a reasonable degree of certainty that is a construction trench for an unfinished road, an argument I discussed in detail 5 years ago [here]; subsequently and more generally [here].

In terms of engineering the case is straight forward, but Archaeology is also a text based study, so its relevance to the Vallum as a literary construct, secure in its self-reverential and internally consistent space, is less certain.

The Evidence

The Vallum was originally a continuous earthwork running to the south of Hadrian’s Wall, from the bridgehead at Newcastle to the western terminus. It comprised a steep sided, flat bottomed trench, from which the spoil has been moved more than 30 feet and piled in neatly built heaps running parallel on either side. 

It was laid out in straight lengths connected by gentle corners, and where ever possible  it  maintained even or moderate gradients to the extent that it did not always follow the same defensive line as The Wall, particularly in the central section. 

The Vallum had a standard measured design, with the principle variations being the section at White Moss, where, in soft ground, there was only the trench, formed by two mounds, and also the intermittent presence of smaller mounds of spoil along south edge of the trench, known as marginal mounds.

It was one of the earlier features of the frontier, but was built over during later phases of the construction.[1]

The Vallum as mysterious earthwork

“Along the whole line this mysterious earthwork keeps company with the Wall” [2]

The conventional view is that the Vallum formed Boundary between a military zone and something else; with perhaps some wriggle room as to what is to the south of the Vallum that requires such a boundary.This idea has been firmly embedded in the literature for long time and remains more or less unchanged to present times.

“The straight lengths in which the Vallum is laid out are consistent with distances of the uninterrupted view of a surveyor. Changes in direction tend to occur where a new viewpoint is required to obtain another long straight view. This is the system used to lay out roads. It has been argued that as the Vallum was surveyed as a road would be, it must have functioned as a road. This does not follow, and if we accept that a road would require metalling, and metalling on the berms is sporadic, and sometimes doubtful, we have to discount all of the suggested variations of a function related to lateral communication.

Richmond’s (1930) statement of the function of the Vallum remains valid:

‘the Vallum takes its place as a prohibited zone delimiting the south side of the military area, an unmistakable belt in which an obstacle is provided by the great ditch. Neither commerce nor interference with the soldiery could take place across it unchecked.’ ”

Tony Wilmott 2007 [3]

From my student copy of Collins Field Guide to Archaeology in Britain [4]

The continuous nature of the structure, with crossing points, later removed to form the marginal mounds, is perhaps the best argument for it being a boundary, albeit one set out like a road. The layout could conceivably result from the use of a method of surveying normally reserved for roads, but this does not explain its form.

The Vallum corner at Down Hill on Google Earth

In many accounts the idea of a boundary is enhanced by describing the earthwork using the term ditch and bank, which, if this was the case, would support the argument.  However, the key point is that it is not a bank and ditch, as conventionally understood; a flat bottomed hole in the ground is a trench - at least to an engineer and an archaeologist. 

The reason it looks like a bank and ditch now is that steep sided trenches are not stable, which is why the Roman Army dug “V” profile ditches with banks to define a boundary or perimeter, not a trench, and certainly not by moving about 1.5 million cubic meters of spoil 30 feet or more.[5]

One important aspect that should not be overlooked,   is that this form of structure is quite unique in archaeology, and as far as I can ascertain, nobody, anywhere, ever built a boundary like this before or since.

The Vallum as engineering

The resemblance of the Vallum's layout to a road is apparent particularly in its straight lengths, moderate gradients and corners designed to accommodate the poor turning circle of wagons.  The apparent priority to move the spoil keeping the margins of the trench free from obstruction is another aspect of the plan that is only explicable if the object is create a metalled road for carts and wagons with wide margins for ambulatory and equestrian users.

Most archaeologists are familiar with trenches, they are only ever temporary structures, but they are essential for foundations, which, for reasons that should be obvious, require a level, even and stable base to spread the load.

The basic engineering of building a road that works in this type of ground involves filling a uniform trench or cutting with layers of progressively finer compacted material  to create a roadbed, and capping it with a durable surface, such as interlocking cut stones in the case of the Romans.  This approach is not possible where soft or waterlogged ground has to be crossed, so here a floating structure made of wood known as a corded road has to be created.

The profiles recorded in the 1890’s contain much of the important information required to support this argument and understand this structure.[6] 

The section at limestone corner shows the desired profile cut into the hard bedrock at the crest of the hill, while a few yards away the Roman Army left the Wall Ditch unfinished; this is engineering, if it is not done this way, the combination of the crest of the hill and the transition in the underlying ground could have been problematic.  Similarly, the reversed profile at White Moss is only intelligible if the intention was to build a corded road between the two banks; there was probably no need for additional lanes, as all traffic would have had to use the road. If we accept that the marginal mounds result from the removal of crossings [across the trench], then this was not an engineering decision, although as far as possible it does preserve the integrity of the lanes, it can be only seen in the context of a “Dislocation”, a gap in the construction program presumably caused by warfare.[7]   Dislocations are key idea in the understanding the confusing structural sequence and changes of plan apparent in the archaeology of Hadrian’s Wall; they also explain why a road project requiring as much material as the Wall itself should be abandoned.

Understanding the argument.

I have detailed sixteen general features of the archaeological evidence, and scored the two theories in terms of how well they explain it. The theory scored 1 if it provides a reasonable explanation, and 0.5 for a less obvious connection; if the theory provides no account for a specific feature it scores 0.

	Feature	Road Foundation		Boundary
	Runs from the Newcastle bridgehead	1	Normal	0	?
	Runs south of the Wall	1	Frontier Road	1	Military Zone
	Does not follow defensive line	1	Normal	0	?
	Continuous	1	Required	1	Reasonable
	Laid out in straight lengths	1	Normal	1	Reasonable
	Gentle corners	1	Required for Wagons	0	?
	Moderate gradients	1	Required	0	?
	Consistent measured design plan	1	Normal	1	Why not
	General form	1	Normal	0	Unprecedented
	Marginal mound as removed crossings	0.5	Spread along edge	0.5	Why not dumped?
	Flat bottomed	1	Normal	0	Not maintainable
	Steep sided	1	Normal	0	Not maintainable
	Displaced spoil heaps	1	Required	0	Inexplicable
	Archaeological precedent	1	Yes	0	No
	Profile at White Moss marsh	1	Corded road over marsh	0	?
	Abandoned following dislocation	1	Unfinished/labour shortage	0.5	Administrative change?
	SCORE:	15.5/16 or 97%		5/16 or 31%

Even if we argue that there is no reason that a boundary should not have gentle corners, moderate gradients, vertical sides and a flat bottom, allocating an extra couple of marks, this explanation still only offers 43%.

Conclusions; Soil in Garbage out
In terms of the assessment above, the conventional explanation, that the Vallum was constructed as a boundary, is evidently erroneous; if this was an exam, it would have failed, as it cannot account for evidence, and is clearly the wrong answer, irrespective of whether a better answer was available. However, for a variety of reasons and interests, mostly academic, the existence of the Vallum as a textural construct is not wholly dependent on evidence, being more akin to the product of a literary tradition.
By far the most significant development in Wall Scholarship is the idea of Dislocations, periods of disruption, probably caused by continuing warfare along the Northern frontier, which resulted in significant changes in the speed, specification and quality of the work.  Old ideas about “Decisions”, prompting the various changes, can now be seen as a response to unfavourable circumstances, rather than as a result of imperial whim or some administrative ineptitude.
Previously, our own imperial history has created a cultural bias that tends to perceive the Roman Empire in terms of success and superiority, which has often made it difficult to understand their archaeology in terms of failure. The Vallum is one aspect of the Wall literature that has not kept up with recent ideas about events.
Archaeologists find soil / dirt and not text, but the process that converts the former into the latter, the very essence of archaeology, is not always understood in the teaching and study of the subject. It is also apparent that peer review encourages faith based thinking, a suspension of disbelief, where ideas are accepted on the basis of the perceived status of individuals or publications, rather than on an understanding of the argument's veracity.

Sources and further reading.
[1] The basic archaeology of The Vallum and other aspects of the Wall has been recently reviewed in: Wilmott, T., [ed]. 2009. Hadrian's Wall: Archaeological Research by English Heritage 1976-2000.

[Most of the general information about the Vallum, used in this article, is drawn from the summaries of P72 –75 & 131 –136 cover, along with that from individual excavation reports] at

http://www.english-heritageshop.org.uk/mall/productpage.cfm/EnglishHeritage/_51324/288647/Hadrian's%20Wall [Accessed 29/11/10]

[2] Conybeare, Edward,  1903 Early Britain--Roman Britain –[https://books.google.com/books?isbn=146553377X]

[3]Wood, Eric S., 1973, Collins Field Guide To Archaeology In Britain, Book Club Associates, (first published 1967)

[4] Wilmott, T. 2007. The Vallum. 

http://www.dur.ac.uk/resources/archaeological.services/research_training/hadrianswall_research_framework/project_documents/Vallum.pdf

[5] Hill, P. R.. 2006. The construction of Hadrian's Wall. Tempus. [Vallum logistics p 126-7]

[6]Profiles after: Hodgson, E. 1897. "Notes on the Excavations on the line of the Roman Wall in Cumberland in 1894 and 1895," Trans Cumberland Westmorland Antiq Archaeol Soc, o ser, 14, 390-407. Plate I.

http://archaeologydataservice.ac.uk/archiveDS/archiveDownload?t=arch-2055-1/dissemination/pdf/Article_Level_Pdf/tcwaas/001/1897/vol14/tcwaas_001_1897_vol14_0026.pdf

[7] Breeze, D.J. 2003. "Warfare in Britain and the Building of Hadrian's Wall," in Archaeologia Aeliana 32, 13 –16

This article uses illustrations based on images from Google Earth: http://www.google.com/earth/index.html