On the banks and beds of the Sousa and Ferreira rivers there are several “giant pans”, with varying diameters and depths.
Albergaria da Serra - Aguiar de Sousa
Paredes
These circular or elliptical depressions excavated in the rock result from the erosion caused by rotating movements of sands and pebbles, triggered by the action of water, which will deepen, widen and round the concavities, at the same time that it will make these sands and pebbles spherical.
Source: https://www.cm-paredes.pt/