Cwmystwyth’s landscape tells the story of millions of years of geological processes and the far more recent action of ice, water, and people. During the peak of the last ice age, 25,000-20,000 years ago, Cwmystwyth was covered by ice. Like rivers of ice, glaciers flowed from the uplands, widening and deepening the valleys. Most of the ice had disappeared by about 15,000 years ago, but its influence on the landscape remains. Today, the Afon Ystwyth flows through one of these formerly glaciated valleys. Smaller streams such as Nant y Gwaith and Nant Watcyn flow from the uplands into the deep Ystwyth valley, forming spectacular waterfalls on the valley sides. Upstream of the mines, a large landslide deposit, likely from the end of the last ice age, extends across the valley forcing the Afon Ystwyth against the northern valley side.
As Nant y Gwaith reaches the flatter, wider valley floor, it loses energy and cannot carry sediment any farther. Over time, Nant y Gwaith has laid down large amounts of these sediments and formed a large fan-shaped deposit. Downstream, near the old mines, the river meanders more freely across its floodplain. During floods, the river banks are eroded and gravel and sand are moved, resulting in a constantly changing valley floor.
From the Bronze Age to the early 1900s, unregulated mining for copper, lead, zinc, and silver created the spoil tips on the northern slopes. This mining has left a legacy of hidden and more visible pollution at Cwmystwyth.