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In the days when maps were a rarity the local priest, wardens and a group of boys armed with birch or willow boughs would literally beat the ground marking the boundaries of their Parish. The boys would often be at the receiving end of these boughs. The object being to ensure that the boundaries would continue to be protected.
The city of London boundary extends to the middle of the Thames. This involves the party piling into a boat and beating the water.
NB: No boys were beaten in the marking process in 2010.