Afon a'r Graig, Yr - Ceubyllau Afonydd Cymru - Dewi Roberts, Stephen Tooth, Hywel Griffiths - diskuze

Where rivers flow over rock, the combined action of water and sediment can create intricate and beautiful natural sculptures. Some of the most striking forms are potholes: roughly circular depressions carved into the rocky beds of turbulent, upland rivers.
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