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Silbury Hill a prehistoric artificial chalk mound, south of Avebury, in Wiltshire. Silbury Hill is the tallest prehistoric man-made mound in Europe and one of the largest in the world; similar in size to some of the smaller Egyptian pyramids of the Giza Necropolis. Silbury Hill is part of the complex of Neolithic monuments around Avebury, which includes the Avebury Ring and West Kennet Long Barrow. Its original purpose is still debated. Several other important Neolithic monuments in Wiltshire in the care of English Heritage, including the large henges at Marden and Stonehenge, may be culturally or functionally related to Avebury and Silbury. Composed mainly of chalk and clay excavated from the surrounding area, the mound stands 40 metres (131 ft) high and covers about 5 acres (2 ha). The hill was constructed in several stages between c.2400–2300 BC and displays immense technical skill and prolonged control over labour and resources. Archaeologists calculate that it took 18 million man-hours, equivalent to 500 men working for 15 years (Atkinson 1974:128) to deposit and shape 248,000 cubic metres (324,000 cu yd) of earth and fill. 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 (548 ft) in diameter. The summit is flat-topped and 30 metres (98 ft) in diameter. A smaller mound was constructed first, and in a later phase much enlarged. The initial structures at the base of the hill were perfectly circular: 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. There are indications that the 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. The first clear evidence of construction, 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 a series of surrounding ditches which were progressively refilled then recut several metres further out. 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, used during construction to raise materials and later as a processional route.