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Silbury Hill built over a blowhole

Silbury Hill is one of the most mysterious places of Wiltshire, England. Many have studied this enigmatic hill and hundreds of books have been written about her, but the reason for her existence is still shrouded in mysteries. What if Silbury Hill was built over a blowhole. This would not only answer the question why she was built, but especially the question why she was built at that specific location.

Silbury Hill, Wiltshire, England
Silbury Hill, Wiltshire, England

Silbury Hill is flat-topped conical mound, that sits quietly in the Kennet valley, just south of Avebury and is the tallest prehistoric man-made mound in Europe and one of the largest in the world. The hill as we see her today was not built in a single campaign, but enlarged over several generations, between about 2470 and 2350 BC. According to English Heritage, first people stripped the topsoil and stones from the ground. Then a small mound of gravel 3 feet (about 1 metre) high was built. At some later stage, a ring of stakes was set out to define a larger area, 52.5 feet (16 metres) in diameter, into which basket loads of mud and dark soils were tipped. Smaller heaps were constructed nearby and some pits were dug into the central mound. People continued to add soil and turf and even boulders, creating a mound 107 feet (35 metres) in diameter and about 15 feet (5 metres) high. A massive enclosing ditch with an internal bank was then dug, 305 feet (100 metres) in diameter. It was backfilled and recut a number of times, until the hill had her present day shape and size. It is estimated that 500 men would take 15 years to build Silbury Hill.
The site was first illustrated by the seventeenth-century antiquarian John Aubrey, whose notes, in the form of his Monumenta Britannica, were published between 1680 and 1682.
Later, William Stukeley wrote that a skeleton and bridle had been discovered during tree planting on the summit in 1723. It is probable that this was a later, secondary burial.

John Aubrey William Stukeley
John Aubrey

William Stukeley

There have been several excavations of the mound.
In 1776 Hugh Percy, Duke of Northumberland, financially supported an excavation by Edward Drax, who directed a group of miners to dig a vertical shaft from the summit to the centre of the hill. They failed to find the central burial they had expected.
Later, in 1849, John Merewether, the dean of Hereford, oversaw the excavation of a horizontal tunnel into hill. Again, no central burial was discovered but organic remains including decayed moss and turf were found. John Merewether made an interesting observation though and writes on page 13 of his Diary of a Dean, An Account of the Examination of Silbury Hill:
“I must not omit to state that in many places within this range from the centre on the surface of the original hill were found fragments of a sort of string of two strands each twisted composed of as it seemed grass and about the size of whipcord.”
Professor Richard Atkinson led a third investigation between 1968 and 1970. Sponsored by the BBC, it was the first excavation to be televised as it progressed. Atkinson excavated another tunnel on a similar line to Merewether’s and identified three phases of construction, although Dr Jim Leary believes the mound went through 15 stages of construction and up to 100 different phases within that over four or five generations.

All the research done so far sheds an abundance of light on the question how the hill was built, but no light at all on the all important question: Why? Why was she built? What motivated the builders and what was (or is) her function? And why was she built, where she was built? The location of Silbury Hill (in a valley so that its summit barely attains the level of the surrounding hilltops) raises many eyebrows and questions. Perhaps Edward Drax is holding a clue why this specific spot was chosen. In a letter written by Drax and sent in November 1776 (likely on Monday 18th), Drax writes:
“But when we came to the depth of 95 feet we struck upon a Thing which I am sanguine enough to hope will lead to a great Discovery. It was a perpendicular Cavity That as yet appears Bottomless it is just 6 inches over; we have followed it already about 20 feet[;] we can plumb it about Eleven Feet more but as a great deal of loose chalk has unfortunately fallen in, at that Depth, is a stoppage but as at present a Strong wind comes up the Hole enough almost to blow out a Candle it must have some communication with the Air or some great Cavity somewhere, at first what the miners call a Damp or foul Air come out of it into the Shaft so that they could Hardly breath[e] nor would a Candle Burn, but that is over and now a strong wind comes up[;] as it is in the very centre of this Great Hill and goes perpendicularly down it is matter of Astonishment.”

Can it be that Drax stumbled on a unique feature that was already present in the landscape and that motivated the people at the time to construct the first stage of what later would become Silbury Hill. A so called ‘blowhole’. A rare geologic feature in which air is blown through a small hole at the surface due to pressure differences between a closed underground system and the surface. The blowholes of Wupatki National Monument in northern Arizona are an example of such a phenomenon. Stories among the Hopi refer to these openings as the source of the wind and the home of the wind god, Yaaponcha. Wind is significant to the farming Hopi, for it can create clouds which bring needed rain. Many large pueblo sites in northern Arizona, including Wupatki Pueblo, are located near blowholes.

Wupatki blowhole Bert Janssen at the Wupatki blowhole
Wupatki, Arizona, USA

The blowhole at Wupatki

In case of Silbury Hill, the location of the blowhole would dictate where the hill was to be constructed, resulting in a hill being built at the lowest spot in the valley. It is very well possible that this blowhole was in Neolithic times celebrated as much as the Hopi do. This would result in building a ceremonial site where the blowhole is. Perhaps accentuating the blowhole by constructing a very small mound with a central hole over it. Like a mini volcano. And such a small mound is exactly what English Heritage says was the first stage of what is now Silbury Hill. In later years this small, initial mound was enlarged, whereby the central opening was kept intact. This would explain why Edward Drax stumbled on a shaft of which he wrote:
“. . . a Strong wind comes up the Hole enough almost to blow out a Candle it must have some communication with the Air or some great Cavity somewhere, at first what the miners call a Damp or foul Air come out of it into the Shaft so that they could Hardly breath[e] nor would a Candle Burn, but that is over and now a strong wind comes up. . . ”
The wind coming up did not stop after a short while, but continued. It resembles very much the blowhole I visited at Wupatki in Nothern Arizona. The idea that Silbury Hill is built over a blowhole, feels very natural and logical for me.

More information on the different aspects of Silbury Hill, for instance her connection with ‘squaring the circle’, can be found in my book the Organizing Principle.

Bert Janssen, May 2019
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