Rock-cut basin

Such plucked-bedrock pits are created by kolks; powerful vortices within the water currents which spin small boulders around, eroding out these rock basins by their abrasive action.

[2][3] The conditions on the river bed must be just right, so that the 'abrasion stones' remain in approximately one area as they circle, allowing the processes which create the rock-cut basins to occur.

[6] Below the confluence of the North Teign River and the Walla Brook on Dartmoor there is a large boulder covered with rock-cut basins, one of which the kolks have completely worn through and therefore the stone has an almost perfectly circular hole.

To add weight to this idea it has been recorded that sick children were also passed through double-rooted bramble hoops, split ash trees and even holes in the ground.

The hole in the stone might also represent the female birth canal in the Druid or 'pagan' mind and by passing through it a person was symbolising the act of rebirth and therefore regaining innocence or being cleansed of post-parturition illness, etc.

[18] The Killoch Burn and glen near Neilston in East Renfrewshire, Scotland has become associated with a witch because at low water the numerous 'pot-holes' have worn into one another, giving fantastic shapes.

The prehistoric village at Maes Howe had similar sized and shaped basins made from flat stones and possibly sealed with clay, used perhaps for storing bait.

The water conditions must be precisely right to create a fully formed kolk as shown by their absence or partial formation in sub-optimal parts of the same river system.