CHAPTER 9

SILBURY HILL

In the Avebury area a few miles north of Stonehenge there is another very substantial megalithic construction we should look at. No one has any idea what the purpose was in building it. Here it is:

This is the largest artificial ancient mound in Europe. It's 130 feet high and covers 5 1/2 acres at its base. The top is flat. The surrounding ditch was originally about 125 ft. wide and 30 ft. deep with a large extension to the west. It is now almost completely silted up as a result of long continued flooding. Here's what we're told about the construction:

The construction of the mound shows an almost obsessive concern for stability. The piled material has been laid in horizontal layers, each divided into numerous sectors by radial and circumferential walls of chalk blocks; and the inner side of the ditch has been protected from frost-weathering, which would have undermined the edge of the mound, by piling against it horizontal layers of chalk rubble, held in place by timber revetment. These are features unknown elsewhere in earthworks of this early date, and they show that the builders of Silbury had a remarkable empirical knowledge of civil engineering, which they applied so effectively that the shape of the mound has hardly changed at all since it was built.

Several points come to mind:

-- It's about 50 feet in diameter at the flat top;

-- The terrace wall is about the same height as the Stonehenge uprights -- about 15 feet.

-- This is high enough to stop a man standing on another's shoulders reaching the top.

In modern times Silbury Hill has been dug into from the top down and from the side into the centre. Nothing has been found in it. But as a result we now know how it was constructed. First there was a primary mound or core of clay, with flints, then stacked turf and soil, then four coverings of chalk, gravel and topsoil, like this:

But that's just the core. On top of the core we have this, like a six tiered wedding cake, made of chalk blocks, with rubble infill:

To all intents and purposes it's the same shape today as on the day it was completed. Apparently no trees or shrubs have grown on the slope of the hill. Since its purpose is unknown, let's consider it more closely. First, why build an artificial hill in a valley when there are natural hills near by. The answer seems to be that the builders wanted a moat around it. It is close to Swallowhead natural spring, so close that although the adjoining river dries up in summer and goes underground, the Silbury moat always has water in it. Here's the plan of the hill and moat:

The moat is deep -- 30 feet -- and Roman wells dug 2,000 years later between Silbury Hill and the spring are only 20 feet deep. This means that the moat drew underground water from below the top of the water table. The extension of the moat in its long axis is 1109 feet. Was the moat a water reservoir for use of the people -- I would say probably the half-Immortals -- using Avebury, West Kennet Long Barrow and the other megalithic constructions in the area? If it was intended as a reservoir only, then it was not necessary to build the hill.

I conclude that the hill was built in a valley beside a spring because it was necessary to have water surrounding it all year long, including during a hot dry summer. This probably relates to a risk of fire on the hill which was turfed and bare of trees or other combustibles.

Why flatten the Silbury hill top and make it difficult of access by humans?