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.

Scratching the surface or scratching my head?

Fourteen of us stepped out of Pen Ceunant Cafe on the slopes of Snowdon for a guided geology walk led by Paul Gannon. I’d read parts of his excellent book, Rock Trails Snowdonia, but I understood more in five hours of walk and talk than any amount of reading. At least I thought I did; the more I try to write it up the more I find I’m scratching my head.

Paul's ripples

Paul gave a quick briefing into the history of the world and how rocks are formed to set the scene. A long long time ago Snowdonia was underwater, on the edge of a continental plate that crashed into an oceanic plate, triggering volcanoes that formed rocks. When our continental plate crashed into another continental plate those rocks were squeezed up into mountains, higher than the Alps but lower than the Himalayas. Since then constant weathering, including many ice ages, has eroded our mountains to a fraction of their former height.

Streams were pouring off Snowdon, full from the previous day’s downpours, taking with them tiny particles into the rivers, lakes and sea where they will re-form into sedimentary rock. Smallest fragments convert to mudstone, larger ones to siltstone and even bigger ones to sandstone building up at the rate of 0.1 millimetre a year or 100 metres in a million years.

Across the valley was the mayhem of Dinorwig which began as mudstone then morphed into slate through intense pressure from colliding plates. We were introduced to examples of ‘slaty cleavage’ which I think can occur in all (?) sedimentary rock.

Brittle deformation

From a vantage point overlooking Nant Peris there was a layer of rock at a 45 degree angle with shelf-like gaps hollowed out of it – this was ‘brittle deformation’ not to be confused with an example of ‘plastic deformation’ a bit higher up. These deformations were caused by plates squeezing layers of rock into folds until they deformed. The plastic one would have occurred when the rock was deeper, maybe 15km inside the earth’s surface, where things are a lot hotter and more malleable or less brittle. 

Plastic deformation

We would only be looking into sedimentary rocks on our walk but there was an erratic volcanic rock where erratic means out of place, dumped by a glacier on its way to the sea. I preferred the erratic dolphin on top of a hill.

Erratic dolphin

Walking towards us a visitor had just taken a photo. When asked by Paul whether he’d been photographing a geological feature he replied it was a sheep and came back with us to see what was so special. This was the boundary where Cambrian met Ordovician.  On our right towards Llanberis were Cambrian rocks and on out left towards Snowdon were rocks (with slaty cleavage) 50 million years younger.  Why the sudden leap? For some reason this part of Snowdonia had been above water for 50 million years so no new rocks were formed until it sank again and sedimentation could continue.

Cambrian meets Ordovician

Earth slips, glacial cwms, moraines and other geo titbits were pointed out but for me a highlight was the ripples. Sedimentary rock which had been formed into a series of ripples whilst in shallow water with strong currents flowing over – just like the effects of water on sand.

Thank you Paul for your patient and thoughtful explanations and thanks to the Snowdonia Society (in conjunction with Discover Gwynedd) for organising this brilliant event. My head is full of many more questions than I had at the start of the day; time to re-open that excellent book.