Silbury Hill is a man-made chalk mound near Avebury in the English county of Wiltshire. It is 40 metres (130 ft) high, and lies at grid reference SU100685. Silbury Hill, part of the complex of Neolithic monuments around Avebury (which includes the Avebury ring and West Kennet Long Barrow), is the tallest prehistoric man-made mound in Europe and one of the world's largest. Its purpose however, is still highly debated. There are several other Neolithic monuments in the area, including Stonehenge. Few prehistoric artefacts have ever been found on Silbury Hill: at its core there is only clay, flints, turf, moss, topsoil, gravel, freshwater shells, mistletoe, oak, hazel, sarsen stones, ox bones, and antler tines. Roman and medieval items have been found on and around the site since the nineteenth century and it seems that the hill was reoccupied by later peoples. Composed mainly of chalk and clay excavated from the surrounding area, the mound stands 40 metres (130 ft) high and covers about 5 acres (0.020 km2). It is a display of immense technical skill and prolonged control over labour and resources. Archaeologists calculate that Silbury Hill was built about 4750 years ago and that it took 18 million man-hours, or 500 men working 15 years (Atkinson 1974:128) to deposit and shape 248,000 cubic metres (8,800,000 cu ft) of earth and fill on top of a natural hill. Euan W. Mackie asserts, that no simple late Neolithic tribal structure as usually imagined could have sustained this and similar projects, and envisages an authoritarian theocratic power elite with broad ranging control across southern Britain. The base of the hill is circular and 167 metres (550 ft) in diameter. The summit is flat-topped and 30 metres (98 ft) in diameter. A smaller mound was first constructed, and in a later phase much enlarged. The initial structures at the base of the hill were perfectly circular and surveying reveals that the centre of the flat top and the centre of the cone that describes the hill, lie within a metre of one another (Atkinson 1974:128). There are indications that top originally had a rounded profile, but this was flattened in the medieval period to provide a base for a building, perhaps with a defensive purpose.