It was a series of serendipitous events that got me really interested in and surveying a few stone circles. I'd visited several of them over the years without taking that much notice, but in 1994 we visited the Druid's Circle above Penmaenmawr in North Wales, and as always I took some photographs. When I compared my slides with some of the published sketch plans for this well known site, I noticed that a quite massive stone (~1 ton), now fallen, had "moved" by a significant amount. It was far too large to have been moved by the antics of drunken yobs. Visits to libraries and book stores followed, and I soon discovered that there seemed to be two different plans for this site - one version aligning with my photographs and that from the more often referenced archaeologist having this major stone in a different alignment. It now seemed obvious that many academics simply used the first plans they came across and never bothered to check their accuracy. Unless you're proposing alignment theories it probably didn't matter.
I also had vague memories of a TV programme from a decade earlier when Prof Thom had been demonstrating his theories about the solar and stellar alignments of stone circles in Scotland. I soon found and bought Thom's books, (as well as several by Dr Aubrey Burl), and this really opened my eyes to what could be done by simple surveying techniques.
Over the next few years with family help I surveyed two of the well known Welsh Stone Circles, Kerry (Ceri) Hill in Powys, and Cerrig Pryfaid in Snowdonia. We also made several visits to the huge stone Maen Llia and Cerrig Duon in the Brecon Beacons area.
Kerry Hill Stone Circle. OS map ref: SO 157860 altitude ~455m.
Kerry Hill was not too far from where relatives lived, and we visited it several times, including one visit in 1996 after a light fall of snow. This was useful in that previously un-noticed features on the ground were emphasized, and as for once the sky was clear we could see the summit of Drygarn Fawr about 25 miles away. It was notable that this distant summit was only just visible from the altitude of the Kerry Hill circle, sited on the side of a gentle slope. Lower down the slope and Drygarn Fawr disappeared from view. The direction of Drygarn Fawr seen from the circle aligns closely with that of the setting sun on the shortest day of the year. However, no line joining pairs of the surviving stones aligns with this.
Thom, in "Megalithic Sites in Britain, Oxford 1967", proposes a complex construction of three joining arcs with a complex geometry. The surveyed plan confirms that arcs can be drawn as Thom demonstrates. At p3 in Thom 1967, Thom indicates that he used a theodolite and tape for his surveys, and it is assumed that he took care to apply the correct tension, ensure that the tape was not blown off line by the wind and to set a baseline on a sufficiently horizontal plane. As Thom stresses accuracy, we can assume that his results were then reduced to the horizontal plane. Consequently as the site is on a reasonably steep slope of 1:10 it is necessary to check whether this could have affected significantly any measurements made on the ground surface during the Neolithic period. The point here is that measurements along the slope will be slightly longer than the comparable measurement reduced to the horizontal reference plane, whereas measurements along a horizontal will be identical with those on the reference plane. On a sufficiently steep slope a circle inscribed on the ground using a peg and a fixed length rope will project an ellipse onto the horizontal plane . However errors along the line of maximum slope from this cause on this site are a maximum of about 0.5% and would not appear to alter geometry to a significant degree.
As mentioned above, it is worth considering whether the siting of Kerry Hill circle has any significance in that although offering extensive views to the south and west, more extensive views are obtainable from a short distance higher, on the ridge where a tumulus is sited . Perhaps the tumulus was there first, but there is plenty of space between the tumulus and the site of the circle. A different explanation is offered in "The First Stonehenge" in which Francis Gaynor draws attention to the possible importance which the summit of Drygarn Fawr, 641m, might have had in Neolithic times. The bearing of Drygarn Fawr from Kerry Hill is ~230 deg and this is close to that at which the mid-winter sun sets. Sighting in the direction of Drygarn Fawr from the Kerry Hill circle, Drygarn Fawr is just visible at about 40km range above a small notch in the nearer skyline . The photograph, below, taken in December 1996 from the circle shows Drygarn Fawr clearly on the original slide. Digital enhancement to the skyline has been used in the enlarged copy. From a short distance lower down it is not visible, and whilst Drygarn Fawr is easily visible from higher up on the ridge, the defining notch is much less obvious.
View looking towards Drygarn Fawr (just visible in notch above stone) - see below left for enhanced image.
Above shows Drygarn Fawr just visble at midwinter. RH picture, after light snow, shows suggestions of ditch around part of the circle.
Above, The central stone at Kerry Hill, and Right, a view across the alignment from stone 7 to stone 3, and including the central stone of the circle.
From the measurements I did, I'm reasonably sure that originally the central stone was standing up - may be as some sort of pointer. The alignment from stone 3 to stone 7 goes over what looks like it was the base of the central stone.
Reference: Francis, Gaynor. The First Stonehenge. 1986. Christopher Davies, Swansea. 0 7154 0666 3
Thom, A 1967 : Megalithic Sites in Britain. Oxford 0 19 813148 8
Thom, A 1971 : Megalithic Lunar Observatories. Oxford 0 19 858132 7
Cerrig Pryfaid Stone Circle. OS Map ref: SH 724713 Altitude 443m
This stone circle was suggested to me by the late Dr Dick Spicer of Oxford University whose inspirational course, Ritual Rings of England and Wales, I attended from October 1999 through to Spring 2000. It's sited at about 443m above sea level at OS map ref SH724713 above the Conwy valley in North Wales, and adjacent to a lane with a raised stone wall between the lane and the circle. (It's not possible to see the circle when driving past in a car.) There are extensive views to the east over the Conwy estuary as the land drops away. To the west, from the ring, it is possible to see the col where the track passes between mountains. On the OS map, a Roman road (following an ancient trackway?) is shown close to the lane adjacent to the circle. Two standing stones are sited close to the col.
At the time of our visit, the site was covered in tussocky grass, making searching for stones and holes quite difficult. There were a few outliers, marked on the plot, and what looked more like a natural rock a bit further away.
It had previously been surveyed by Barnatt in 1989 in a rather different style of survey, but several stones listed by him were missing when we did our survey in July 2000. The relative altitudes of the stones are averages of several measurements at the bases of the stones, and are referenced back to the site reference point below the theodolite. These indicate a gentle slope across the circle from stone K to E of ~1.2m in 21m, a slope of 5-6%. There's no central stone in this circle.
Compared to other circles I'd visited in Wales, this one struck me as being rather fragile with several loose stones, and unlikely to survive much longer. (It's rather close to a lane which supports motorised vehicles.)