Posts Tagged ‘underwater cropmark’

A medieval weir. Part 2

November 19, 2011 Leave a comment

Part 1 of this article described an ‘underwater cropmark’ – a band of riverbed vegetation 3m wide and 40m long crossing the River Great Ouse in Bedford in a diagonal direction relative to the current, visible on aerial photos. This was identified as indicating the former position of a medieval weir. But could the construction of the weir actually date to the late Anglo-Saxon period?

Weirs as defensive features
The word “weir” has its roots in the Old English werian, meaning to defend (as in “beware” or “to be wary”). As well as providing the head of water to drive mills, weirs could be used to obstruct and help control traffic along rivers. For this reason towns on rivers may once have incorporated weirs into their systems of defence: after all, it makes little sense to surround a town with earthen ramparts and deep ditches if river approaches were left open.

An associated role of weirs may have been to create the head of water to direct flow through defensive earthworks. This is an important but neglected area of study. Urban defences are usually discussed in terms of their solid materials – the walls or earthen ramparts which presented physical barriers to attack. Water flowing through ditches outside of those ramparts receives little attention. In the rest of this article, I will argue that a large linear earthwork in southern Bedford, commonly supposed to be of late Anglo-Saxon construction, could only have functioned if there was an associated weir across the river.

The King’s Ditch
When Edward the Elder fortified the southern part of Bedford in AD914, effectively turning the town into a “double-burh”, the terrain was so flat on that side that the river was the obvious source of water to use. The Anglo-Saxon Chronicle records that King Edward’s army stayed in the town for four weeks in order to build the defensive earthworks. A kilometre-long semicircular earthwork known as King’s Ditch was constructed. This survives today in places as a broad ditch alongside a sizeable inner bank, covered in dense undergrowth. Other stretches have been levelled and built over, so that it runs through culverts beneath roads, offices and schools.

A former system of flow, with Reynolds map of 1849 used as base plan (Bedfordshire and Luton Archives and Records Service). Click on picture to enlarge

The most interesting feature about the King’s Ditch is that it flows. This is no stagnant ditch. Nor does it have a languid current like the Great Ouse from which its water is derived. Its flow is fairly fast and strong. And this is not just a modern phenomenon, brought about through recent re-engineering of the watercourse. Evidence that there was always a good flow of water in the King’s Ditch throughout medieval and post-medieval times is provided by the existence of fishponds just inside the ditch on its south-eastern side. These fishponds originally belonged to St John’s Hospital, a medieval religious institution. Maps show that there were channels bringing water in and out of the fishponds, which presupposes a strong flow in the ditch itself. Although later in date than the King’s Ditch itself, the fishponds tapped into and made use of an already existing flow of water.

Detail showing flow through medieval fishponds connected to the King’s Ditch

By rights the water should be stagnant, or should back up into the river the way it came, given the flat terrain of southern Bedford. The fact that it flows suggests that this was an important element of the initial design. The monument was built with a constant gradient and a contrived fall of water of about 2m between inlet and outlet – much greater than the natural fall of the river between those two points, with gravity enlisted as the principal force impelling water to flow through the ditch.

The significance of flow
Why would the builders of the earthwork go to the trouble of making it flow? Consider for a moment why a town boundary ditch full of stagnant water would not work. Imagine how quickly such a ditch would start to silt up, fill with rubbish, get polluted and clog with vegetation. Carrying out periodic maintenance would be difficult. Thick slimy mud thrown up from the ditch would slip down into the water again, and the earthwork bank alongside would soon fall into disrepair. Above all, the water – since it has no momentum or energy – cannot be exploited for industrial purposes. Without flow, it cannot be moved around or managed in any way at all.

The King’s Ditch and rampart as it was 100 years ago, and a similar stretch today. Despite recent silting up and vegetation growth, the ditch still has a flow of water running through it

Now consider the many benefits of a flowing ditch. The water itself does some of the work of maintenance, carrying away smaller particles of silt and leaving the gravel behind, making the ditch easier to clean out. It is self-scouring. The ditch takes away rubbish and pollutants, acts as a sewer and a drain, while replenishing itself with fresh water. It also serves as a flood defence. Unclogged by vegetation and mud, the stream can be used for bathing and washing of clothes, or for transport of goods by boat, with direct access to and from the river. Flowing water is a vibrant material which can be controlled and moved around as required. It can be diverted through sluices into side channels – to fill further water features or flush them out, to cool forges, to serve potteries and perform innumerable other industrial functions. And that is not to mention the symbolic significance of encircling towns with flowing water.

Liquid flow – a force driven by gravity yet intensively modified, shaped and utilised. This picture was taken at the existing 19th century weir on the river between inlet and outlet of the King’s Ditch. Although the position of the weir has been changed, the King’s Ditch still relies on a river weir to provide the necessary fall of water to ensure flow.

How, then, was the artificially contrived fall and the flow of water through the ditch accomplished? There was one obvious way to do it, and that was to construct a substantial weir across the river somewhere between inlet and outlet of the King’s Ditch. Its purpose was to create the necessary artificial step in water levels. Such a weir on the river is actually a logical necessity for the King’s Ditch to function as a flowing earthwork, and its former existence might be deduced even if there were no material trace surviving of the structure itself. The technology of stepping the river through the building of weirs, in order to create a head of water to drive mills, was already well established. It made sense to apply the same principals to ditches of defensive earthworks too, especially if a large workforce was available to carry out the work.

Thus the weir across the river revealed by the ‘underwater cropmark’ could have been built as an integral and functioning part of the King’s Ditch, even though it appears to be physically separate from it. Flow as a material force is normally largely absent from archaeological explanations, leading to a somewhat static and fragmented view of landscapes and townscapes. Only when we take relationships of flow into account can the dynamic interconnections between otherwise disparate sites and monuments be perceived.

A modified version of this article was published as ‘The weir and flowing earthworks of Bedford’ in British Archaeology, Nov-Dec 2011, 22-7, and featured on Radio 4’s ‘Making History’ programme on 25/10/2011.