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.
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.
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|