The naked hard rock surface of the river shores may have round holes of about a hand in diameter and slightly less in depth, from natural origin, which are known as potholes. The potholes can keep water from the rain or from the last flood of the river or stream. Potholes are mainly found in limestone rock, always close to the water flow. The image displays the series of potholes in the nummulitic limestone on the shore of the river Cardener, under the Pont Vell (= old bridge), in Manresa.

The mystery of the potholes, as everything, has one explanation. The potholes result from the abrasion by the sand dragged by the turbulent current of the floods that turns continuously into one cavity. An initial irregularity in the rock can trigger the generation of one pothole.

If the holes in the shore are much larger, the size of a bathtub, they are called giant kettles. Giant kettles appear in the center of a stream flow after the same abrasive action, albeit more constant and on a larger scale adding gravel to the sand.

[photo Jordi Badia]