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.
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| 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.
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| 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.
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| 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.
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| 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.
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| 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.
