The Megalithic Yard: A Consequence of Circumferential Distribution?

Design Considerations

The Bi-metric Hypothesis has possible implications with respect to ring designs that are not truly circular. Thom identified four design classifications: Flattened, Egg-shaped, Ellipse, and Compound, as shown at Figure 17. He presents four types of flattened circle, based upon segments of either 180 degrees or 240 degrees with specific radial divisions.
Examples of Thom's Designs
Figure 17: Examples of Thom's Designs.
The flattened design at left in Figure 17 is one of the four, this having a segment of 180 degrees (a semicircle) at its base with the radii divided into three parts to determine the source of the side arcs. The source of the bridging arc bisects the perimeter of the base semicircle.

Thom identifies three egg-shaped rings, all based upon a semicircle. The design at Figure 17b. has its radii divided in two parts to identify the source of the side arcs. The source of the bridging arc is at the point of intersection of the radii of these Secondary Arcs at a derived angle. The ellipse (17c.) is of standard form. Compound designs (as 17d.) require specific instructions for each case.

Thus, Thom’s designs are all differently constituted - the flattened circle is formed in one way, the oval in another and the ellipse in yet another - but Arc-Construct designs have common roots (see Figure 18). Essentially, they are all drawn in much the same manner and subject to the same design criteria.
The Arc-Construct Family
Figure 8: The Arc-Construct Family.

Ellipse v. Ellipson

There are few distinctly elliptical rings (8% of all those analysed) and evidence supporting the case of the ellipse or the ellipson is difficult to find. For example, there could be traces of markers at the foci. However, in the case of arc-construct ellipson, the key points are not foci, as such, but the origins and limits of the component arcs. Three cases may serve to demonstrate.
Amesbury 61 Ellipson

Figure 19: Amesbury 61, after Ashbee
A stake-ring discovered beneath a barrow at Amesbury 61, Wiltshire (Ashbee, 1984), provides an example. Here, there is a break in the perimeter at the southwest, and there appears to be a separate arc which almost connects at the east but not at the west.

To achieve this as two partial ellipses would require the completion of three-quarters of the perimeter followed by the removal and incorrect repositioning of the foci, as at left in Figure 19. However, were the design an arc-construct ellipson this might simply result from sourcing or, more likely, re-sourcing the southwest arc at the wrong point of intersection marked on the ground (where the line crosses the defining circle), as shown at right in the figure.

Consider that the ellipson may have been set out correctly and, some time later, two teams begin to insert the stakes: team A at the north, and team B at the south. Team B finds that the southern arc is indistinct so sets about redrawing it (beginning the arc at the east), but places the centre of the arc at the wrong point upon the southeast arm of the original design. The stakes are then inserted (starting at the west) and team B continues to complete the east face while team A finishes at the west.
Bedd Arthur Ellipson

Figure 20: Bedd Arthur
The ring at Bedd Arthur (Figure 20) could also be a case for the ellipson, and a good example arguing against the ellipse. The ring has two stones upon the major axis, which might look to lie at the foci (F1 and F2 in the figure), but no ellipse based on these as foci fits the stones on the perimeter. However, if these ‘focal’ stones are taken to be defining the source of the contact arcs of an arc-construct ellipse then the rest follows naturally, and with ease.

With a format specification of E,45,[4/5],I, it becomes apparent that the source of the northwest arc matches the defining stones, but the source of the southeast arc is slightly offset (it appears not to be diametrically opposite.) This results in the stones marking the defining circle appearing to be shifted about 50cm (1.6 feet) to the southwest.
Horncliffe Ellipson

Figure 21: Horncliffe as an Arc-Construct Ellipse
The Horncliffe elliptical ring on Ilkley Moor, Yorkshire (see Figure 21), may be interpreted as a further instance of an arc-construct ellipse. In 1929, a plan was produced by Arthur Raistrick, who found the ellipse to measure 25ft. x 32 ft. (7.6m x 9.8m), confirmed more or less by recent survey, though there appears to have been some disturbance since that time. It has a stone-lined central pit quite unlike that on Raistrick’s plan.

Whether this site is truly a stone circle, the remains of a cairn, or simply a later enclosure, is open to question (Burl classes it as a stone circle). However, the stones do seem to conform to a design consistent with other stone circle geometries.

This can be appreciated when an arc-construct ellipse is superimposed on Raistrick’s plan. The southern arc of eight stones, in grey, is prominent still, though the northern arc is less so.

The western arc contains the largest stones - the two marking the extents being shown here in black. The northern arc commences at west with two stones of a shape and size much as those in the southern arc, and the eastern arc then begins with a tall(ish) stone, appreciably different from the rest.

The perimeter may have contained 42 stones, distributed as at Figure 21. Potentially, the ring has an originating circle of 144 diametric units, drawn as format E,90,[1/2], the inner face of the central pit being contained within the defining square, though slightly offset.

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