THE GEOLOGY OF SNOWDON

Cwm du’r Arddu (the Cwm of the Black Heights) – in the Ice Age this area was the seat of a glacier that carved out the massive rock wall on the left. Photo © Paul Gannon

Cwm du’r Arddu (the Cwm of the Black Heights) – in the Ice Age this area was the seat of a glacier that carved out the massive rock wall on the left. Photo © Paul Gannon

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 very tough ‘sedimentary’ rock.

Volcanic rocks, known as a ‘welded tuff’, formed some 450 million years ago in the hottest and most violent type of volcanic eruption. Photo © Paul Gannon

Volcanic rocks, known as a ‘welded tuff’, formed some 450 million years ago in the hottest and most violent type of volcanic eruption. Photo © Paul Gannon

Shortly after you pass the Halfway house, however, the rocks are ‘volcanic’, created by extremely violent undersea eruptions that occurred about 450 million years ago.

Then, about 400 million years, ago all these rocks – sedimentary & volcanic – were pushed up into mountains by the collision of two ‘tectonic plates’ 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.

‘Brachiopod’ fossils that lived below sea level some 450 million years ago and were later pushed up over 1,000 metres above sea level when theSnowdonia mountains were built. Photo © Paul Gannon

‘Brachiopod’ fossils that lived below sea level some 450 million years ago and were later pushed up over 1,000 metres above sea level when theSnowdonia mountains were built. Photo © Paul Gannon

More recently – between 2.5 million years ago and just 10,000 years ago – the mountains were home to glaciers during the ‘Ice Age’. These glaciers carved out the great valleys such as the Llanberis Pass and the many glacial ‘cwms’ on Snowdon – these cwms were the seats of glaciers and now form distinctive semi-circular ‘scoops’ out of the mountain sides.

You can see some small glacial cwms on Moel Eilio & Moel Cynghorion shortly after starting the ascent from the Snowdon Café and even 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.

A walk up Snowdon on the Llanberis Path is a fantastic lesson in ancient geology.

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 - available from local outdoor shops or on the internet.

Paul Gannon Books

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