Enclosures, henges, circles - PURPOSE, USE & design

Terminology

A note to clarify our terminology:

·        A ‘survey station’ is a location from which surveyor’s set out new survey lines.

·        A ‘fixed’ location is one whose direction from at least two other fixed locations is known

·        The ‘baseline’ for a survey station is a surveyed line passing through the station’s centre. The directions of new survey lines can be measured from it.

·        Timber posts settings can form Timber Circles and have been found associated with henges too. It seems highly likely that timber posts sometimes assisted the alignment work at survey stations. As we haven’t yet included timber circles in our scope, we do not explicitly describe their use below, but the reader will readily appreciate that one or more posts on such sites might be of assistance.

Monument purpose

We propose that causewayed enclosures, tor enclosures, stone circles, henges, and circle-henges are all the remnants of ‘survey stations’. Furthermore, these five types all operated in a similar way in that:

·        Their location was fixed with reference to other survey stations.

·        The line back to one of those survey stations was considered the baseline.

·        Their purpose was to set out new survey lines at known angles to the baseline.

·        Banks and stones (or timber posts) assisted a surveyor at some distance from the survey station to position themselves precisely on new survey lines.

Survey Method

The location of a new survey station was planned so that its location could be fixed using lines from at least two existing survey stations.

Once the new location was fixed, lines were set out in new directions. A new line might be designed to fix a further planned survey station, or to fix a landmark.

The new lines could only be set out at angles which could be measured against the baseline. In the early surveys, they were only comfortable using angles which could be derived by simple divisions of a circle.

The setting out of angles may have been done in a similar way at each type of monument. We suggest that ropes, probably made from plant material, were used. A possible method is illustrated below:

Fig 1.      A rope was used to draw out a circle from the point that had been fixed.

Fig 2.      The circle was divided into 60-degree segments by using the same rope to set out six equilateral triangles around the circle centre.

Fig 3.      The length of the 60-degree arc was measured and divided into quarters or sixths to get divisions equivalent to 10 or 15 degrees.

Fig 4.    A distant surveyor would then align with one of the divisions. The mechanism to do that varied with monument type. 

Design of Enclosures and Henges

At enclosures and henges, a bank was built for a surveyor to stand on. The bank’s height was set so that the sight line from the circle centre to a surveyor at some distance from the survey station would pass through the poles of a surveyor standing on the bank. If a surveyor on the bank stood on a line at a known angle from the survey station baseline, then the distant surveyor could align himself on the bank surveyor and the fixed point, which is now at the centre of the enclosure.

When causewayed enclosures are sited on hills, they are often not on the summit but a bit down slope.  As others have noted, that is very strange if the prime purpose of the enclosure was defence. However, the positioning makes sense if the aim was to have sight lines which went to surrounding countryside, not out into open air.

In the case of henges, once the circle was drawn out, the easiest way to build the bank was for the ditch to be dug following the outside of the circumference of the drawn circle, and the spoil heaped on the outside of the ditch.

In contrast, at the earliest henges (e.g. Stonehenge) they piled the spoil inside the ditch line. One disadvantage of this approach was that they then had to draw out a second circle within the bank before being able to set out the triangles. More significant is that to deliver a bank of a given radius, the earlier design requires either a longer ditch be dug, or for spoil to be moved along the bank line where no ditch was cut. In either case the earlier method requires more labour. This is why henge design quickly moved to the ditch being on the inside of the bank.

At causewayed enclosures, the ditch does not always match the bank. In other words, they sometimes dug out spoil and moved it along to create the bank. Furthermore, the irregular shape of causewayed enclosures will generally require a longer bank than that required for a circle of similar diameter. Hence, we can discern a path of improved building efficiency from causewayed enclosures through henges with an outer ditch, onto henges with an inner ditch.

Design of Stone Circles

At stone circle sites, a circle was drawn out in a similar way and marker stones set at the desired angles on its circumference. The stones were erected on each side of the circle so that a surveyor at distance could align himself on the two stones at each end of a diameter. Where the stones were short and/or roughly hewn, he may have required colleagues to stand at the two marker stones he wanted to align on. In later circles, where the stones are often regularly shaped and relatively tall, he may have managed the alignment alone.

For a stone circle to be usable, the distant surveyor must be able to move into the same plane as the two stones he wants to align on. This is why stone circles aren’t found on hill summits – generally there is not enough space for a surveyor to stand in the same plane as the circle and sight through it to another hill side.

Burl (2000, page 106) alerts us to the connection between the topography and the flattening of circles. We hope to explain his observation more fully because a change in slope at the circle site can require stones to be brought closer to the centre for them to come onto the sight lines from the distant surveyor.

The immediate topography around a site may also explain why some stone circles have stones grading in height from one side to the other. The larger stones were needed to be visible to the distant surveyor who was beyond a slight change in slope.

Choice of Henge versus Stone Circle

Burl (2000, page 114) discusses the amount of work required to build a henge versus a stone circle of the same size. He estimates that a henge took five times more effort - assuming there was suitable stone close to the site. Obviously, stone circles can only be built where there is stone, but if there was no stone, a timber circle would often be an option. So why did they choose henges so often?

The reason is that circles are useless in topography where the plane of sight cannot be made to work. The henge design is inherently more flexible because you can raise or lower the bank to enable the surveyor on it to align on landscape of varying heights. Indeed, building banks was the only option where they wanted to set lines from flat lower ground to higher ground.

Our suspicion is that the smaller circle-henges were needed where a single method did not enable all the required lines to be set. If so, the henge would have to be built after the stone circle had finished being used, because the stones could no longer be viewed from distance once the henge bank was built. However, this does not apply to Avebury and other mega-henges which may have had circles within them. We will consider these separately.

There may have also been hybrid situations where a circle was built but a bank / henge method used for short arcs. This may explain sites, such as in Cumbria, (Burl 2000, fig 6, page 107), where there are just short stretches of henge bank at a circle.

Future work

Demonstrate surveyor line of sights at multiple sites

We have so far tested these ideas at just a few sites. They seemed plausible at those sites, but a more extensive and rigorous validation is now required.

Cumbria - regional case study

Cumbria is a centre for some of the oldest large circles. They are large in diameter, with a high number of stones, often close set and the stones are unworked. Burl (2000, page 126) explains there are also, later, circles in the region which are still in the ‘large’ group but tend to have just 12 stones, widely spaced, and more shaped. He also points out how similar in design two of the older type are. There are also henges. The region would benefit from a case study to understand the geometric relationships between the circles / henges and see if the logic and sequence of the survey, or surveys, can be interpreted.

Flattened Circles

Field work to prove, or disprove, that circle flattening matches what is required for stones to align across their diameters, given the local topography. The work might start at the Cumbrian circles mentioned by Burl 2000 page 106. What % of flattened circles does it work for? What % of perfect circles can be shown to support such alignment?

Graded Circles

Field work to prove, or disprove, that the height grading matches what is required for stones to align across their diameters, given the local topography. Example sites: Brisworthy, Fernworthy, Gors Fawr.