The deep, narrow gorge of the Afon Mynach at Devil’s Bridge is one of the best examples in Wales. Gorges, waterfalls, and other connected features such as potholes, typically form very slowly through the continued action of water and sediment. The slow rates of change to the waterfalls here suggest that they are very old, at least hundreds of thousands and quite possibly millions of years. This means that the waterfalls survived through many of the large number of ice ages that have occurred in Wales, and that they are significantly older than any major human structures on Earth, including the earliest known Egyptian pyramids that are no more than 5,000 years old.
The Afon Mynach flows into the larger Afon Rheidol, joining the Rheidol where the river turns abruptly westwards from its previous southerly course. This change of direction is a result of a process known as river capture. Over time, rivers can gradually cut into the surrounding land, lowering the river bed and extending channels upstream. In this case, Afon Rheidol cut back eastwards towards a higher elevation river known as the Teifi, which originally flowed towards the south. Once it reached the Teifi river, the Rheidol opened up an easier route for water to flow west downhill through its own channel, allowing Afon Rheidol to ‘capture’ and redirect the Teifi’s flow. The steep drop in elevation between the Teifi and Rheidol increased stream power, causing the river to start carving the deep, narrow gorge we see today.