Near Avebury, Wiltshire, Southern England.
Silbury Hill is one of the largest, unusual and certainly the most enigmatic of all the prehistoric monuments in the British Isles. While it is certainly large enough to be dismissed as just another hill by passing motorists who are unaware of its existence, they must surely do a double-take, as its steep, perfectly symmetrical, upturned pudding bowl sides look so startlingly at odds with anything in the natural world. The grass which covers the Hill today does its best to disguise this immense elephant-in-the-room, but at the time of its construction there would have been no uncertainty as to whether or not it was man-made, as it is built almost entirely of chalk and so would have presented a brilliant white face to the world. The effort taken to raise it, using tools probably no more sophisticated than picks fashioned from antler bone, almost defies understanding. It has been calculated that 500,000 tonnes of material were used in the construction which necessitated approximately four million man-hours. Silbury Hill, with its base occupying an area in excess of five acres and measuring 525 feet across and 120 feet high, is the largest man-made mound in Europe and amongst the biggest in the world.
Why it was built is an utter mystery and may always remain so. It stands at the very heart of the Avebury Landscape, with the great henge itself ¾ mile to the north and West Kennet Long Barrow ½ mile to the south-east, yet it has no obvious function within them. The great antiquarian of the early 18th Century, William Stukeley, imagined the ancient landscape as an enormous snake, with the myriad stones that once formed Beckhampton Avenue comprising the tail, leading on to the henge at Avebury and then twisting along the West Kennet Avenue to what he regarded as the head, The Sanctuary. Silbury Hill stands more or less equidistant from the important points along this great arc, and so Stukeley concluded that it was little more than a viewing platform from which ancient man could marvel at his immense train set. It is, however, an inadequate theory, not least because the construction of the Hill was perhaps a more demanding task than the rest of the ritual landscape put together. It must, therefore, have had some other, far greater significance.
A succession of archeologists have made numerous attempts over the previous three centuries to understand the Hill, almost destroying it in the process. In 1776, Colonel Edward Drax, at the behest of the Duke of Northumberland, engaged a group of Cornish tin miners to plunge a vertical shaft into the heart of the monument in search of a life-size golden statue of "King Sil" (or Zel) riding his horse, which, so local folklore maintained, was buried within the mound; needless to say he emerged empty-handed. Drax, however, having dug down some 95 feet, began to uncover a cavity which measured six inches wide and 40 feet deep; a later excavation discovered tree fragments, believed to be oak, which suggests that Silbury, in the early phases, was constructed around a totem pole. In 1849, John Merewether, the Dean of Hereford, drove a horizontal tunnel towards the centre of the mound, and was followed by further excavations in 1867 and 1886. In 1922, Flinders Petrie, the renowned Egyptologist, set his sights on Silbury and, drawing on his experience of pyramids, hoped to locate a tunnel in the mound leading to a central burial chamber; he uncovered nothing but yet more chalk. The final assault came in 1968 with the BBC's The Silbury Dig programme, following the progress of Professor Richard Atkinson's efforts to get to grips with how and why it was built. Despite media disappointment at the lack of a "Howard Carter" moment, the dig succeeded in uncovering much of what we now know about the construction of the Hill.
Atkinson revealed that Silbury Hill was built in three distinct phases, which he named Silbury I, II and III. Modern radiocarbon dating techniques have established that Silbury I began between 2,450 and 2,350 B.C., and the discovery of winged ants at the base suggest that work started in August. The composition of Silbury I is somewhat mixed and there could well be further phases concealed within it, as evidence has been found of pits, postholes, sarsen stones and another small mound. It appears that in the beginning, a circular wooden fence of stakes was arranged at the base with clay containing flints and various plant life heaped at its centre; a feature common to barrows, suggesting that Silbury was a monument connected with death or the afterlife. Turf was then cut from around the hill and placed over the clay, followed by topsoil containing a mixture of moss and grass. Sarsen boulders were placed roundabouts in no obvious alignment, but on their tops were offerings of bone, ribs of either ox or red deer, sticks, bushes and mistletoe. A considerable amount of gravel and soil, doubling the height of the mound, was heaped on top of this until the fence and all within its confines were buried.
This completed the Silbury I phase, and the construction of Silbury II proceeded without any noticeable delay. Chalk blocks were cut and dressed over the mound, eventually taking the form of six stepped drums, like a wedding cake of six consecutively smaller cakes placed on top of the other. The ends of each layer were tapered at a 60° angle, thereby giving far greater structural rigidity to the monument than a sheer face would have provided. The techniques used in the construction were sophisticated and thorough, with each layer consisting of a honeycomb arrangement of blocks into which chalk rubble was poured and compacted before work began on the next, smaller layer on top of it. It is entirely because of these superior building methods that Silbury Hill has survived to the present day, more or less intact in its original form.
The dividing line between the end of Silbury II and the start of Silbury III was discovered in the form of a thin layer of grass separating the two. Analysis of this grass has discovered that it is of a type which tends to grow on derelict wasteland, and, as it takes up to 15 years to fully colonise an area, this suggests that there was a delay of some considerable time between the construction of the two phases. From the radiocarbon dates, it is possible that the entire monument could have been constructed within about 100 years, and so could, in theory, have been witnessed from conception to completion by one individual. It seems more likely, however, that it took considerably longer to build, possibly as much as 400 years. In any case the final stage of construction continued in the same vein until it reached the size of the monument that we see today. When it was finished, the steps between all of the layers, excluding the top one which can still be seen today as being clearly stepped, were filled in with chalk and smoothed flat. This smoothing of the steps and the general method of construction would appear to scream parallels with the pyramids of Ancient Egypt; indeed Silbury Hill was being constructed at about the same time as the Great Pyramid at Giza.
We know a great deal about how Silbury was constructed, therefore, but for all of these investigations, no one has yet produced any remotely convincing theories concerning its purpose. It is interesting to note, however, that its construction began around a time of great social upheaval brought on by the arrival of Beaker Culture and the Bronze Age. The old form of ancestor worship, of interning a social elite within burial chambers, came to an end about this time and coincided with new ideas of burial, and also the construction of henge monuments such as Avebury. The neighbouring West Kennet Long Barrow was constructed in 3,600 B.C. and, over a period of many hundreds of years, was slowly occupied and filled up with earth, until, at around the time when construction began at Silbury and Avebury, it was finally filled to the roof and its entrance sealed. This seems to point to a revolution in religious attitudes, although there may not be as much difference between them as we might think, after all both henges and burial chambers are commonly associated with ancestor worship and the tracking of the heavens, particularly the winter solstice. It has been suggested that Silbury was the effort of a tribe neighbouring Avebury, who had their own ideas about what a new religious monument should be. It seems unlikely, however, as the Avebury landscape covers a significant area of monuments which are definitely, indeed physically linked, and so it is hard to imagine that Silbury Hill, so close to the centre of all of this, was not a part of that landscape, or indeed at the very heart of it.
Whatever meaning it may have had for those who built it, and the generations which followed, is now completely lost on us. We do have evidence though that it was important to later peoples. The road which now passes at its feet is the A4, which was once a Roman Road which linked London with Bath; the latter being a world-renowned site of religious significance at the time. As pilgrims would have certainly passed Silbury Hill on their way to Bath, it is quite plausible if not certain that the Romano-British held it in high esteem. There is very persuasive physical evidence to support this, in the form a rather large, well-ordered settlement directly opposite the Hill on the other side of the road. In a later era, it would appear that the Anglo-Saxons admired Silbury Hill too, but for much more pragmatic reasons. A posthole dating to the 11th Century has been discovered on the summit of the hill, and a find of an armour-piercing arrow head lends weight to the theory that this was a military outpost of some description. It is not hard to see why as the Roman Road was still an important route during those troubled times, indeed the Anglo-Saxon Chronicle tells us of a battle at East Kennet in 1006 between the Saxons and the Danes. It would seem most desirable, therefore, to overlook a part of this route with a small fort, very well protected by the steep sides of the hill, which would be exhausting for an enemy to attack.
The determination of three hundred years worth of archeologists to solve the mystery of Silbury Hill has, ironically, almost resulted in its destruction. In May 2000, the vertical shaft which Colonel Drax cut into the mound 224 years earlier, finally gave way, exposing a large crater at the top of the monument 14 metres wide and nearly 20 metres deep. Subsequent investigations revealed that the tunnels dug in 1849 and 1968, neither of which had been properly backfilled, were also slowly collapsing. Were it not for the skilful construction techniques employed by those who created Silbury Hill, it is entirely possible that the damage could have been irreversible. The Hill, nevertheless, was in serious danger of collapse, and so English Heritage commissioned a project to repair the damage that its predecessors had wrought, and, as a temporary measure, plugged the gaping hole at the top of the mound with polystyrene blocks (see the aerial photograph). By a further irony, it was the 1968 tunnel which gave the greatest hope of restoring Silbury Hill to its original condition, and in 2007 engineers entered the tunnel to begin their work. Bad weather brought about further serious collapses within the mound, but the rotting tunnel supports were shored up, and the process began of properly backfilling and packing the interior using, most appropriately for a restoration project, some 1,465 tonnes of locally sourced chalk. While this work was underway, archeologists took the last opportunity that they may ever have to gather what evidence they could from the exposed tunnels, and their efforts yielded further information about how it was built and enabled the various construction phases to be more precisely dated. The restoration project has since been successfully completed, and so Silbury Hill, this most magnificent of ancient monuments, has at last been restored to as much of its original condition as human effort can contrive.