Geology of Gelli Iago – Llyn Llagi

After a brief introduction to rocks we headed out of the quarry near Gelli Iago, on the short cut road from Llanfrothen to Nant Gwynant. Our guide Clive Hudson, with long wooden staff in hand, looked a bit like Gandalf but without the beard.

Measuring the dip

His staff was not for conjuring up spirits or smiting rocks – this staff would help indicate the angle at which rocks dipped into the earth and another gadget was used to measure the angle of dip. At the start of our walk they were all dipping towards the south east and later on to the north west which meant we were walking through a basin – strong forces had squidged the rocks into a U shaped fold.

We walked over slate, crossed a slither of sandstone (and a bit of yellow tuff / volcanic ash) and were introduced to a dolerite intrusion; magma at high temperature and under high pressure, which had been squeezed between sedimentary rock and solidified slowly, deep underground. This rock has no slaty cleavage, tends to be blocky (breaking up into blocks) and covered in vegetation such as lichens.

Clive explains the process of bedding in sandstone

Dolerite was butted up against sandstone and, whereas in the past I would have called them all rocks, I like to think that I could now tell the difference. Sandstone is built from layers of sediment accumulating underwater and this was obvious to see with larger granules in places, where stronger currents had carried heavier bits. On the face of some rocks we could see the ripple effect, with heavier granules on the ridges and finer sediments in the troughs.

This area is obviously a well used outdoor classroom; every now and again we would notice a chipped off piece of rock where budding geologists had taken a sample. Newly exposed rock is a very different colour to the weathered rock.

Not a contact lens but a fossil!

Along the way through the basin, Clive turned over a rock to illustrate some fossils. Nothing with great jaws, wings or long tails but a few tiny, finger nail shaped hollows formed by seashells. Upside down (round side on top) shells are less likely to be dislodged by moving water so we could assume which way up this boulder had been created. As we got closer to Llyn Llagi we found what we were looking for, the dolerite rocks coming up at the far side of the basin.

From the shores of Llyn Llagi we stood on a hollow-sounding structure of slate in the middle of a bog which we surmised to be a platform for drying peat. But who wanted the peat up here? It was handy to have Kathy Laws in our party, the archaeologist from the National Trust, to point out the site of an iron-age settlement just a hundred metres or so away – maybe they would have needed a supply of peat.

The buoys in Llyn Llagi are part of the monitoring of this and 20 other lakes in Britain to measure acidity. Since 1988 the acidity of Llyn Llagi has fallen with PH rising from about 5.3 to about 5.9 – the main factor being the reduction of acid rain caused by sulphur dioxide from coal-fired power stations. The other lake monitored in Snowdonia is Llyn Cwm Mynach.

From here we walked to the top of a microgranite sill to eat our lunch beside a dolerite erratic. I’m not sure I could identify microgranite – it’s like dolerite but formed with the addition of quartz. On a sunny day it made for a great picnic site.

Our last mission was to climb to the top of Castell, a rhyolite intrusion - a ‘volcanic plug’. Clive explained the different viscosity of lava (which is magma that has broken through the earth’s surface). Volcanoes erupting on, or close to the edges of continents have sticky lava whereas in the open seas they have thin, fast flowing lava, which is why Hawaii is the shape it is. When lava spewed out of Castell it was the sticky stuff that stayed upright and slowly moulded over time. The rhyolite was a light, whitish colour with lots of fragments solidified in wavy bands.

On top of the volcano

It was a wonderful day led by Clive and organised by the Snowdonia Society. I feel I know a lot more than when I started but the number of questions buzzing around my mind makes me realise the extent of my ignorance on this fascinating subject.

Clive Hudson is an enthusiast for local pre-history and maybe organising a day school to look into subjects such as: flint technology and lime bast string and other cordage. Another possibility is the extraction of birch bark tar which was the glue to stick flint onto shafts; Clive’s prototype involves a biscuit tin and a baked bean tin in a fire! If you would like to know more or take part in an interesting day please contact Clive at cliveshan2@gmail.com

How it all happened over millions of years

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.