Dr Aubrey Burl has spent 20 years travelling more than 20,000 miles throughout Britain, Ireland and Brittany, in search of ancient stone circles and similar monuments, and has succeeded in tracking down some 2,200.
His detailed analysis of their design and geographical distribution is now shedding important new light on a 2,300-year-long period of British prehistory. Differing widely in size, the sites fall into four main categories - stone circles, stone avenues, stone rows and multiple stone rows.
The site with the most stones - a multiple row complex at Kerzerho in Brittany - consists of 1,129 standing stones, while the largest site in terms of area is the 28-acre great stone circle at Avebury, in Wiltshire.
The longest row, running across Stall Moor on Dartmoor, is two- and-a-half miles long and has 2,000 stones; while the largest single stone (now broken into five pieces) - near Locmariaquer in Brittany - was originally 75ft high and weighed more than 300 tons. In Ireland the most complicated complex, at Beaghmore, Co Tyrone, has seven circles and eight rows.
The 2,200 sites studied by Dr Burl consist of a grand total of around 30,000 stones. His comparative analysis of all these British, Irish and Breton prehistoric monuments has revealed the way in which architectural and religious traditions developed and spread between approximately 3300BC and 1000BC (the late Neolithic and Bronze Age). In particular, his in- depth study of stone avenues and rows - just published in book form by Yale University Press - shows, for the first time, how those monuments first developed in very rudimentary form in the Lake District, and then spread south to Wessex, Devon and Cornwall, and finally to south-west Ireland.
Northern Ireland was directly influenced by Lake District standing stone 'architecture', while western Scotland seems to have derived its inspiration from Brittany. This suggests not only that ideas travelled long distances, but that the sea was probably even more important as a cultural highway than scholars have hitherto believed. Dr Burl's study also shows how prehistoric architectural fashion developed over time.
Initially, the stone avenues start off as two- or four-stone extensions to stone circle entrances. Then, after many generations, they were gradually extended to become avenues, slowly increasing in length. After around 2000BC, some avenues and their simpler derivatives, stone rows, were being built on their own, no longer in association with pre-existing stone circles.
In some cases, stone rows were constantly added to, with new, roughly parallel, rows being erected. The most complicated in Britain, in Mid-Clyth, Caithness, has 23 lines of stones (some 300 in total), while at Kerlescan in Brittany there are 13, with more than 500 stones in total.
Finally, as the standing-stone monument tradition began to falter, both rows and avenues became less complicated and less impressive, ending up in the late second millenium BC as simple pairs of stones.
Dr Burl has also carried out detailed calculations to determine whether any of the avenues and rows are aligned astronomically. His research now shows that, whereas stone circles show a mixture of solar and lunar alignments, detached stone avenues and rows built after around 2000BC are almost exclusively lunar in their orientation.
Together with the architectural changes, the spread of exclusively lunar avenue and row alignments will lead archaeologists to speculate as to the causes of these trends. Perhaps wider demographic, military or social developments lay behind these religious changes. Stone circles, avenues and rows can normally only be built in areas where suitable stone is easily available. Certainly the distribution of almost all the 2,200 standing-stone monuments coincides with the geological presence of hard stone. However, evidence assembled by Dr Alex Gibson has revealed that in geologically non-stone areas, prehistoric circle and avenue builders resorted to timber.
Dr Gibson, a prehistorian with the Clwyd-Powys Archaeological Trust, trawled through thousands of archaeological reports from lowland England, Scotland and Ireland and found around 45 timber circles - a dozen of which had timber avenues attached to them.
They seem to follow the same 'rise and fall' chronology shared by most stone circles and avenues. For example, small and simple from 3000 to 2400BC, large and complex from 2400 to 1800BC, and finally small and simple again from 1800BC to 1000BC.
Most of the 45 timber circles were discovered by accident, usually when archaeologists were excavating other earlier or later monuments on the same sites. Up to 70 probable timber circle sites have been discovered through aerial photography.
It is therefore probable that there were literally hundreds of timber circles and other similar monuments scattered all over lowland Britain. One particularly large example, near Godmanchester in Cambridgeshire, was a great horseshoe-shaped complex covering 10 times the area of Stonehenge.
All the prehistoric circle and avenue evidence (both stone and timber) suggests that some areas of Britain were more populous - or simply more sacred - than others.
For instance, Devon has 159 stone circles, avenues and rows, while Cumberland has 40 and Somerset 34. Perthshire and Argyll have 74 and 31 respectively, while Cork, Kerry and Tyrone have 214, 67 and 52 respectively. The Breton department of Finistere has 55. Most areas have standing-stone monument densities of less than 1 per 10 square miles - although some small areas of, say, Dartmoor boast as many as 3 monuments per square mile.
Despite Dr Burl's years of survey work, the precise function of the circles and avenues remains a mystery. It is thought they were almost certainly used for religious activities, and perhaps also non-religious tribal gatherings. Monument density and the size of the circles may give clues as to population density and the size of tribal, clan or even family units.
Certainly dead humans and animals - probably sacrificial victims - were buried when some of the larger monuments were first constructed. At Stonehenge, for example, archaeologists discovered deposits of probable sacrificial victims - mothers and children - at both entrances.
The astronomical alignment evidence for many circles, avenues and rows, shows that the prehistoric tribesmen were interested in the rising and setting of the sun and moon, and in the cardinal points - especially the north- south axis.
And surviving ancient folklore contains clues which suggest that the monuments had some ritual mathematical function - perhaps the counting of days, seasons or even years. It hints that taking part in these counting rituals may have been strictly forbidden for ordinary mortals, and were perhaps activities which were the exclusive preserve of some now long-forgotten shamanic priesthood.
Although rituals at most monuments ceased some 3,000 years ago, local reverence for standing stones continues in some places, and in Brittany new stones were being added to some stone rows as late as the last century.
From Carnac to Callanish - the prehistoric stone rows and avenues of Britain, Ireland and Brittany by Aubrey Burl, published by Yale University Press, pounds 25; The Stone Circles of the British Isles, by the same author, also published by Yale, pounds 19.95.
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