The ancient mountains were more recently – between 2.5 million years ago and just 10,000 years ago – by glaciers during the ‘Ice Age’, creating the great valleys and ‘cwms’ which were the seat of glaciers and form a distinctive semi-circular ‘scoop’ out of the mountain side.
You can see glacial cwms on Moel Eilio & Moel Cynghorion shortly after strating the ascent from the Snowdon Café and more dramatically in Cwm du’r Arddu seen as you ascend steep section of the path known as ‘Allt Moses’ (with the second railway bridge at the top). More cwms are carved into the sides of Snowdon higher up, especially after the Pig Track joins the Llanberis path near the summit.
To discover more about the geology read Paul Gannon’s book especially written for those with no previous knowledge of the subject: Rock Trails Snowdonia
The Geology of Snowdon
A walk up Snowdon on the Llanberis Path is a fantastic lesson in ancient geology as local geologist and writer Paul Gannon explains:
The rocks around you as you walk up the Llanberis Path from the Snowdon Café are truly ancient. Close to the café they are around 500 million years old and were formed by material eroded from even older mountains and dumped into the sea, eventually hardening to form solid ‘sedimentary’ rock.
Shortly after you pass the Halfway house, however, the rocks are ‘volcanic’ and date from violent undersea eruptions that occurred about 450 million years ago. About 400 million years ago all these rocks – sedimentary & volcanic – were pushed up into mountains by the collision of ancient Europe and ancient America.
If you look hard enough you can find the fossil remains of sea creatures – ‘brachiopods’ – just below the trig point on the very summit of Snowdon, now at 1085 metres above sea level.