![Top curves](../../../../jsp/jahia/templates/GSL/gsl_templates/img/keystage3/main_top_grad.gif)
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.
The shape of the Cwm Idwall valley today is a classic glacial feature, eroded by ice that only melted about 10,000 years ago.
Snowdon, North Wales
![Snowden, North Wales](../../../../webdav/site/GSL/shared/images/education_and_careers/RockCycle/RocksAroundBritain/SnowdonSyncline%20xtrawide.jpg)
![Bottom curves](../../../../jsp/jahia/templates/GSL/gsl_templates/img/keystage3/bottom_curve.gif)