Silbury Hill is an artificial chalk mound near Avebury in Wiltshire and at 40 metres high, is the tallest prehistoric man-made mound in Europe, it is also one of the largest in the world.
The hill is composed mainly of chalk and clay excavated from the surrounding area and covers an area of about 5 acres. Archaeologists calculate that Silbury Hill was built about 4750 years ago and that it took maybe 500 men working for 15 years to deposit and shape nearly a quarter of a million cubic metres of earth and stones on top of a small natural hill.
The base of the hill is circular and 167 metres in diameter. The summit is flat-topped and 30 metres in diameter. There are indications that the top of the hill originally had a rounded profile, but this was flattened in the medieval period, possibly to provide a base for a building.
The first phase, carbon-dated to around 2400 BC, consisted of a gravel core with a revetting kerb of stakes and sarsen boulders. Alternate layers of chalk rubble and earth were placed on top of this: the second phase involved heaping further chalk on top of the core, using material excavated from an encircling ditch. At some stage during this process, the ditch was backfilled and work was concentrated on increasing the size of the mound to its final height, using material from elsewhere. The step surrounding the summit dates from this phase of construction, either as a precaution against slippage, or as the remnants of a spiral path ascending from the base which was used during construction to raise materials and later as a processional route.
There have been several major excavations of the hill, from both the top and from the base in to the centre. All that was found was construction material along with clay, flints, turf, moss, topsoil, gravel, freshwater shells, mistletoe, oak, hazel, sarsen stones, ox bones, and antler tines.
The purpose for which Silbury hill was constructed is unknown. If you visit the site the first thing you will notice is that the hill is set within a valley, with high ground all around. If creating a structure to dominate the local landscape was the intention of the builders, then building in a valley was a strange choice of location.