Syncline, N Wales

The rocks of Snowdon were erupted as volcanic lavas and ash flows around 450 million years ago. They were later folded in the Caledonian mountain-building, as northern and southern Britain were joined in an enormous plate collision about 50 million years later. Evidence of this downfolding (syncline) is clearly seen in the back-wall of Cwm Idwall.

The shape of the Cwm Idwall valley today is a classic glacial feature, eroded by ice that only melted about 10,000 years ago.