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The main henge at Avebury is surrounded by the remains of an enormous ditch and bank system. The bank is outside the ditch which is the opposite way round to what we would expect if their purpose was to keep human enemies away, plus the entrances to the henge would not have provided much impedance to external attackers. It seems likely then that the bank and ditch were intended to be spiritual in nature, designed to keep spirits inside the henge.
According to English Heritage "The shape formed by the ditch is subcircular and is divided by causewayed entrances into four unequal arcs. The bank is now some 14–18 feet (4.2–5.4 metres) high but was once nearly 55 feet (17 metres) above what was originally a 30-foot (9-metre) deep ditch". The bank and ditch would have displayed blindingly white surfaces of white chalk, what we see today is merely the decayed, collapsed remnants of an astonishing wonder of the ancient world.
Image copyright David Baldwin Night Photography