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It is a lone sarsen in a field - as far as I know nothing is known about it. But obviously it has been recognised as having some significance as it hasn't been dragged to the field edge.
Reminds me a bit of the Cuckoo stone, Torstone and Heel stone...
More details at:
The Hanging Stone (Wiltshire) Standing Stone (Menhir) : The Megalithic Portal and Megalith Map:: "Site Name: The Hanging Stone (Wiltshire)
Country: England County: Wiltshire Type: Standing Stone (Menhir)
Nearest Town: Devizes Nearest Village: Woodborough
Map Ref: SU099605 Landranger Map Number: 173
Latitude: 51.343500N Longitude: 1.859252W"