Routes to GeoMon – the A5
(note : this route describes travelling along a road whether by car, bicycle or bus – the later would would be simlest to start at Wrexham via Rhuabon and then follow the A539 to Trevor, joining the A5 at Llangollen)
We’ll start at the Welsh border on the A5. Thomas Telford was commissioned to build a turnpike road from London to Holyhead to enable rapid transport to Ireland. Despite modern improvements it is still possible to follow his original route for most of the way, especially from the Welsh border to Holyhead. Let’s start on the roundabout at the western end of the town of Chirk. From here Telford constructed a road along the Dee valley passing Llangollen, Corwen and up onto the Denbghsire Moors then down into the Conwy valley at Betwys y Coed. From there following the valley to Capel Curig, over the pass at Ogwen and down the Nant Ffrancon into Bangor. His brief was to avoid steep climbs enabling the coach horses to travel with speed. It is an amazingly scenic route, crossing some spectacular geology. We will describe the route with some deviations to take in some of the geoheritage along the way, especially the various UNESCO designated sites.
Travelling west from Chirk along the A5 we come to the village of Pontcysyllte where we turn right onto the B5434 towards Trevor. The Pontcysyllte Aqueduct World Heritage Site soon looms into view as the road drops down to the river Dee and climbs back up the far valley side. Naturally, the “canal in the sky” www.pontcysyllte-aqueduct.co.uk maintains a level course. This World Heritage Site’s most outstanding feature is Thomas Telford’s Pontcysyllte Aqueduct, carrying the Llangollen canal across the Dee valley and described as ‘a masterpiece of creative genius’. Spanning 11 miles of stunning canal and countryside and 32 listed structures, the World heritage site is more than just a bridge!
Every 10 yeas or so the aqueduct is drained for inspection and any required maintenance by the Canal and Rivers trust. This video shows the drained aqueduct
After exploring the area and perhaps walking across the aqueduct, leave via the A539 towards Llangollen to rejoin the A5.
Once on the A5 continue for 36 miles [an hour’s drive] to Betwys y Coed following the Dee valley towards Corwen. The river follows a strange route which has been ascribed to diversions caused by a glacier blocking the original valley – for example the road often follows a higher ‘valley’ abandoned by the river which has cut a much deeper meandering valley below.
Approaching Llangollen, a very prominent escarpment of Carboniferous limestone dipping eastwards forms the landscape to the north, these rocks sitting uncomformably over the Silurian greywackes. A ruined castle, Dinas Bran, sits atop an isolated hill of Silurian rocks between the old and new valleys.
High above the road to the left slate tips can be seen. The uppermost Silurian beds were fine shales, metamorphosed to slate in the Caledonian orogeny and quarried in the hills south of the road and to the north along the A542 Horseshoe Pass. Our route crosses high moorland on these Silurian greywackes (which can be seen exposed in the roadside on the left hand side of the road on entering Llangollen] and glacial deposition features before descending into the Conwy valley.
Approaching Betwys y Coed there is a turning to the left (B4406) towards Penmachno; this is on the Snowdonia Slate Trail which circumnavigates the Slate landscape of Northwest Wales a UNESCO designated Heritage Site and follows the A5 to Bangor.
Entering Betwys, continue on the A5 into the town over a bridge which proudly proclaims it was built in the same year as the battle of Waterloo was fought! Betwys has a fine geoheritage; slate quarries, stone quarries and metal mines abound. A peculiar quirk is building with enormous stones, seen best in the “Ugly House” a couple of miles north along the A5. You can turn right up a small road which leads up into the hills full of old lead and zinc mines. Be warned, this is a narrow road with passing places. The road passes several mines which are easily accessible on the roadside, Cyffty, Hafna and Parc. Lead and zinc minerals, with quartz, can be found here with some interesting industrial archaeology.
After exploring the mines, return to the A5, turn right and continue towards Capel Curig where the A4086 branches off to the left. You have two choices, stay on the A5 over the spectacular pass at Ogwen passing between Trfyan and the Glyders to the left and the Carneddau to the right or turning off down the valley with euqally spectacular views to wards Snowdon.
A5 Route along the Nant Ffrancon
This route climbs gradually (remember those coach horses!) across the moorland and broad valley towards a summit immediately south of Llyn Ogwen. The massif to the left finally gives way to a view of the isolated peak of Tryfan and beyond to the twin peaks of Glyder Fawr and Glyder Fach. To the right is the massif of the Carneddau with several peaks above 3000 ft. The rock sequence here is mid-Ordovician with a series of ignimbrite ash-flow tuffs, some welded some not. The unwelded volcanic material was subject to reworking by water and redeposited as sedimentary layers. Hence we have a mix of shales and mudstones, sandstones, conglomerates and true volcanic rocks, but all of the same acid volcanic composition. The variation in hardness and resitance to erosion leads to sharp ridges and peaks, waterfalls and glacially eroded features standing out. Any protruding rock on the valley floor has been carved by ice into a roche moutonee – smooth and rounded on the upstream side, steep and jagged on the down stream side.
The sequence has also been folded; keep looking to the left along this stretch, a prominent slab of rock dipping steeply to the south stands out, then the whole face of Tryfan can be seen to be a sequence of beds steeply dipping to the north. You have just crossed the axis of an eroded anticline.
The road skirts around Llyn Ogwen passing various buildings at the far end – there is a small visitor centre with a geology display. From here the official path up into Cwm Idwal starts. This is a superb walk with fantastic views of the synclinal structure at the head of the cwm. Careful examination of the rocks as you walk round the lake reveals that it is not actually a syncline but a pericline.
Carrying on along the A5, the valley floor drops away very steeply beyond the head of the lake, the river cascades down in a series of waterfalls, and the rock ridge across the valley has been polished smooth as the glacier flowed down over it. At first glance this looks like a textbook glacial valley, straight, flat floor, steep sides but look carefully, the steep side to your right is actually a massive scree slope, the valley side to your left is a series of small tributary glacier valleys [‘hanging valleys.] and the floor is flat because of a deep sequence of sediments infilling a lake basin.
At the end of the Nant Ffrancon a hard rocky ridge crosses the valley and resisted glacial erosion. It is rounded and polished, with the river passing through narrow gorges in short rapids and small waterfalls. The dominant feature of the landscape here is a set of flat topped terraces – the waste tips of the massive Penrhyn slate quarry. The quarry has been redeveloped as a tourist adventure site with a variety of zip wires – you get a really good view of the quarry from here !!!!
We have reached the Ogwen Valley, a location within The slate landscape of North West Wales www.llechi.cymru
A4086 Route via Capel and Llanberis
Turning left at Capel Curig the road passes the twin lakes in Dyffryn Mymbyr with Moel Siabod to the left and the Glyders to the right. However most eyes are looking forward for views into the Snowdon horseshoe directly ahead.
The rock sequence here is mid-Ordovician with a series of ignimbrite ash-flow tuffs, some welded some not. The unwelded volcanic material was subject to reworking by water and redeposited as sedimentary layers. Hence we have a mix of shales and mudstones, sandstones, conglomerates and true volcanic rocks, but all of the same acid volcanic composition. The variation in hardness and resitance to erosion leads to sharp ridges and peaks, waterfalls and glacially eroded features standing out. Any protruding rock on the valley floor has been carved by ice into a roche moutonee – smooth and rounded on the upstream side, steep and jagged on the down stream side. Look carefully across to Moel Siabod and notice the lower slopes are rounded, smooth and polished by the passing ice, the upper slopes are jagged – the summit level of the ice can be determined by this change.
Eventually you come to a fork in the road at Pen y Gwryd, the A498 straight on and eventually down to Beddgelert, the A4086 turning right up to Pen y Pass and on to Llanberis. Both are fantastic routes to follow, we will take the A4086 but not until we have ventured a short way down the A498 to a prominent viewpoint looking directly up into the Snowdon Horseshoe. With a deep valley developing beneath to the left, a steep valley side leading up into the broad Cwm Llydaw in front and a hint of the higher Cwm cut directly below the peak, and the two ridges of the horseshoe, Yr Aran to the left and Crib Goch & Crib Ddysgyl to the right, this is again a classic glaciated landscape with all the text book features. The pipe down the steep valley side to a small stone building on the valley floor is a very old hydroelectric power station constructed in 1905- the oldest and at the time the largest in the UK.
Returning to Pen y Gwryd and taking the A4086 up to Pen y Pass, where the Miners Path and Crib Goch paths set off from the car park (either pre-book in the car park or use the park and ride scheme from Nant Peris) and then the road descends into the Llanberis Pass. The Snowdon massif to your left, the Glyders to your right. The geology is as described previously with one noticeable difference.
The name Crib Goch translates as the ‘red ridge’ and it is noticeably reddish brown compared to the grey of the acidic volcanics and the sandstones. The ridge is capped by a sequence of basaltic rocks, mainly re-worked ashes and tuffs but with some small lava flows often with pillow structures (best seen at the top of the Devil’s Kitchen in Cwm Idwal). These basaltic rocks are nutrient rich and host the various Arctic Alpine plants such as saxifrage, roseroot and the Snowdon lily. Two massive boulders on the roadside are examples of these basaltic pyroclastic rocks.
And so the road descends to the valley floor and skirts around Llyn Peris backed by the massive slate tips of the Dinorwig quarry. To your left on the hillside the waste tips of a small copper mine can be seen. Llyn Peris is the lower lake for the Dinorwig pump storage power station [housed inside the mountain].
Water falls from a lake high up on the mountain through the power station generating electricity when needed at peak times and is pumped back up again – originally using the output from Wylfa nuclear station – at time of low demand.
The lake was drained during construction of the power station and the lake basin lined with slate waste to prevent sand and mud being entrained in the water flow and damaging the turbines. While it was drained, glacial seatures that had not been seen since the end of the Ice Age were exposed such as the roche moutonee pictured abvoe – the fine detail of glacial scratches is amazing, and the presence of pyrite cubes acting as minute crag and tail features.
We have reached the Dinorwig quarry a location within The slate landscape fo North West Wales www.llechi.cymru
Beyond the end of Llyn Padarn, the route crosses a jumbled landscape of glacial debris with some rocky outcrops and rivers chaotically making their way to the sea. Fields are small wet pastures, it seems that every one has rock protruding, with scrubby trees. The road makes its way to Caernarfon, home to The Castles and town walls of king Edward I World Heritage Site (www.cadw.gov.wales/visit/best-history/castles-town-walls-king-edward-gwynedd ) which includes Caernarfon, Conwy, Harlech and Beaumaris and demonstrates medieval military architecture at its most impressive. Built by Edward I (1272-1307) during turbulent times when the English king demanded submission of the Welsh princes, the scale of construction was unprecedented. Caernarfon Castle is most magnificent and impressive of the castles (Beaumaris is the most perfect design).
Like Beaumaris, Caernarfon castle was also built with Carboniferous rocks – those quarries at Penmon must have been teeming with stone masons. Here, however, the protective wall encircling the town is almost complete. New Red sandstone has been used for door and window frames. Whilst here, don’t miss the Roman fort at Segontium and try to find the last remaining piece of Roman town wall. (photo JConway)
Taking the old road towards Bangor (A487) you will pass the old fishing port of Y Felinheli (which translates as the salt or tidal mill) which was massively redeveloped in Victorian times as Port Dinorwic to receive the slates from the enormous quarry of that name at Llanberis. This old road leaves Port Dinorwic and skirts around the Vaynol Estate, obvious by its high encircling wall, home to the owner of Dinorwig Quarry, (the Assheton Smiths, later the Duff family), then climbs the hill to meet the A55 dual carriageway just as it approaches the Britannia Bridge.
