Sunday 19 August 2012

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.

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