Formation of Giant’s kettles

During heavy rain, the torrent carries stones and sand which, when whirled around by ferocious eddies, dig rounded, polished cavities into the riverbed. These holes, called potholes (or “Giant’s Kettles”) take on three forms which attest to the depression of the river:

Permanently active potholes, at the bottom of the river.
Potholes that are still active during flooding, higher up.
Fossil potholes, at the waterway’s former level and which are no longer active.