Cwm Idwal, Snowdonia, Wales, UK. Cwm Idwal is a hanging valley, sculpted by glaciation. Charles Darwin visited Cwm Idwal in 1831, and observed marine fossils which he realised were evidence that the rocks must have formed in an ancient ocean, later lifted to the surface by powerful geological forces. He later made further observations, concluding that the landscape had been shaped by glaciers. In 1954 Cwm Idwal was designated a National Nature Reserve, the first in Wales.