Geology of Snowdonia

For logistical reasons (Sherpa buses) our Paul Gannon geology walk starting at Pen y Pass was switched to the foothills of Snowdon, just above Llanberis. I’d done this walk with Paul a couple of years ago and had only half understood what was said so was hoping to get the other half of the story.

Snowdonia Society geology walk

The weather was fine but hardly tropical as I tried to come to terms with the explanation of tectonic plates and that the spot we were standing on had been beneath the equator about 600 million years ago.

If I’ve got it right our rocks were created by underwater volcanoes, when our continental plate collided with an oceanic plate, but the mountains followed much later when our continent collided with another continent. An oceanic plate is denser than a continental, so when they collide the denser one goes beneath and this is called subduction. But when two continents collide they buckle, fold and squeeze the rocks into the mountains.

Our mountains emerged about 400 million years ago and were about the height of the Alps, but have been shrinking ever since.  The Alps are much younger, a mere 30 million years, and are still growing, but eventually erosion and weathering will bring them down to size. For the next few thousand years Snowdonia will stand still; rising spring-like since the melting of the glaciers but eroding by a similar amount. 

Gneiss!

Our rocks are mainly from the Cambrian era, about 450 million years ago, but lying in the footpath was a gneiss rock from a much earlier era - how did that get there? Either by glacier from Anglesey or imported rock for footpath maintenance. 

Paul explained the three types of rock, volcanic (or igneous), sedimentary and metamorphic, but we would be concentrating on sedimentary. The three types of sedimentary rock are called mudstone, siltstone and sandstone with the names referring not to the chemical composition but the particle size with sand being the largest and mud the smallest.

From here we were shown an example of graded bedding, layers of sediment with the larger particles sinking towards the bottom of each layer, and could see the angle at which the rocks had been buckled and bent.

Paul explains tension gashes

Looking down steeply onto Nant Peris we were next to a fantastic example of tension gashes. These sandstone rocks had been horizontal in the sedimentary phase but during the mountain building phase had been put under huge amounts of tension and bent upwards. Cold rocks would be ‘brittle’ and shatter but hot rocks, 10 to 15 kilometres beneath the surface, would be more malleable or ‘plastic’.  We were presumably looking at sandstone coming from the crossroads of that zone with brittle deformations at one end and plastic deformations at the other.

Along the way I began to feel more comfortable with the geology beneath my feet, it started to make sense and I thought yes, you’ve almost cracked this subject. From ignorance to a little bit of knowledge with tentative understanding and then despair as I realised the enormity of how much more there is to learn. Maybe it will all become clearer if I buy the revised 2^nd edition of Paul’s book The Rock Trails of Snowdonia?  Maybe Paul could take me on this walk for a 3^rd time?

Thanks very much to Paul for donating his time and to the Snowdonia Society for organising this geology walk. Write ups of previous geology walks are here and here.