Rock-cut basin

A rock-cut basin, in this usage of the term, is a natural phenomenon. They are cylindrical depressions cut into stream or river beds, often filled with water. 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. These basins are frequently found in streams and rivers with a relatively soft rock substrates such as limestones and sandstones. The rather unusual and man-made appearance of such depressions has led to various folk-tales becoming associated with them, such as their identification as petrosomatoglyphs, including knee prints, elbow prints, etc. of saints, heroes, kings or supernatural beings.

Rock-cut basins are formed by the action of fast running water currents that cause small boulders to move in a circular motion or Vortex. The friction created by these kolks propelling small boulders in a circular motion erodes the natural rock substrate to create concavities called rock-cut basins, that increase in depth and circumference over the years. One or more rounded stones may be found within them, often of varying sizes as these stones also wear away with the physical abrasion effect.

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. During times of heavy flood, river currents provide considerable energy to stones lying on the bottom, as can be witnessed beside many rivers where audible sounds are made by boulders as they are tumbled downstream over the bedrock or as they clash against other boulders. The term Bed load is used to describe the material carried by a river by being bounced or rolled along its bed.

If a rock-cut basin forms on an overhanging ledge at a waterfall or drop in the river level, then a circular holed stone may eventually form. These may have been used in the construction of megalithic monuments, especially tombs. Such holed stones are often associated with folklore as healing stones through which sick children are passed etc., as with the Mên-an-Tol in Cornwall, where the legend is that passage through the stone will cure a child of rickets (osteomalacia), also scrofula as well as spinal conditions in men and women. For centuries, children with rickets were passed naked through the hole in the middle stone nine times, or three times against the sun.

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