SNPA Strapline

Ogwen

Ogwen Lake

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Darwin’s journey through Snowdonia

The Darwin wall, which is located in front of the Ogwen centre below Cwm Idwal, provides a geological record of Charles Darwin’s 1831 journey across Snowdonia. His journey took him up Nant Ffrancon to Ogwen, around Cwm Idwal then on via Capel Curig to Ffestiniog and across the Rhinogydd to the coast, finishing at Barmouth. Exact details of the entire route are not available, but the rocks in the Wall represent those he crossed, passed close to and, in some cases, examined.

The rocks were originally collected as 30-40cm sized boulders, from roadsides, quarries, forestry tracks and beaches. Welsh Slate, Natural Resources Wales and First Hydro kindly granted vehicular access to some remote localities, greatly assisting an already arduous task.

The rocks were then taken to Cerrig Granite & Slate Ltd’s workshops at Pwllheli. Here they were cut, polished and embedded into the sections making up the top of the Wall, which was constructed by Maentwrog Masonry. You may notice that some rock-types appear repeatedly along the Wall: this is done to illustrate the fact that, due to the geological faulting and folding that occur throughout the district, any long traverse such as that undertaken by Darwin will see the sequence of strata repeat itself in places.

Darwin's Wall, Ogwen

Darwin's Wall (© SNPA)

Darwin's Wall, Ogwen

Darwin's Wall (© SNPA)

The various sections of the wall’s side were built using stone local to the different parts of Snowdonia that were crossed during Darwin’s journey: they demonstrate how the diverse geology influenced the varied appearance of stone walls across the district.

1. The rocks begin with some glacial erratics of granite, commonly found along the Menai Strait.

2. Inland, Darwin would have gone over the outcrop of the late Precambrian Arfonian volcanic rocks before reaching the red, green and purple Cambrian slaty mudstones, still worked at Penrhyn Quarry, and the overlying Bronllwyd Grits with their distinctive pebbly bands.

3. As he went up to Ogwen he would have seen the overlying grey Ordovician mudstones and sandstones, intruded by micro-granite at Blaen-y-Nant, before observing the mid-Ordovician volcanic rocks that accompany the sediments around Cwm Idwal.

4. Continuing on towards Ffestiniog, he would have recognised how extensive these sediments and volcanic rocks are. Reaching Blaenau Ffestiniog, he could not have helped noticing that the thick Ordovician mudstones of that area are also worked for slate, and that the crags are made up of further volcanic and intrusive igneous rocks.

5. Walking on from Ffestiniog, he would have noticed a change, as he literally walked back in time down through the Cambrian sedimentary sequence of the area known as the Harlech Dome. It included the coarse sandstones and conglomerates that make up the distinctive landscape of the Rhinogydd, around whose flanks bedded manganese ore was worked and gold was obtained from quartz veins.

6. Finally, on descending to the coast, he would have noticed, along the beaches of the area, glacially derived boulders of local and perhaps more exotic origin.

Darwin's route through Snowdonia

Darwin's route through Snowdonia

Key

 Cenozoic clays, silts, sands and gravels west of the major Mochras Fault
 Middle Silurian sandstones and conglomerates of the Conwy Valley
 Lower Silurian mudstones, siltstones and sandstones of the Conwy Valley.
 Late Ordovican mudstones and siltstones of the Conwy Valley
 Felsic intrusive igneous rocks of late Cambrian to late Ordovician age
 Mafic intrusive igneous rocks of late Cambrian to late Ordovician age
 Upper Ordovician mafic eruptive igneous rocks - the Bedded Pyroclastic Formation
 Ordovician felsic eruptive igneous rocks including the major rhyolitic tuffs of Snowdon and Cwm Idwal
 Upper Ordovician: Cwm Eigiau Formation including fossiliferous sandstones
 Middle Ordovician: Nant Ffrancon Formation, dominated by mudstones including the slates of Blaenau Ffestiniog
 Lower Ordovician: sandstones including the Garth Grit conglomerate
 Lowermost Ordovician: fossiliferous mudstones and siltstones
 Upper Cambrian: mudstones, siltstones and sandstones
 Middle Cambrian: sandstones and conglomerates including the Bronllwyd Grits
 Middle Cambrian: mudstones, siltstones and sandstones
 Lower Cambrian: sandstones and conglomerates including the gritstones of the Rhinogydd
 Lower Cambrian: mudstones and siltstones including the famous Llanberis Slates
 Late Neoproterozoic (Arfon Group): sandstones and minor volcanic tuffs
 Late Neoproterozoic (Arfon Group): Padarn Tuff Formation - felsic volcanic rocks

Reproduced by permission of the ‘British Geological Survey © NERC. All rights reserved. [CP14/023]
Please note map colours have been changed from the standard BGS Colour Pallet.

  • One of the many sampling trips - at the Stwlan dam

    One of the many sampling trips - at the Stwlan dam (© SNPA)

  • The final haul, samples are ready to be sent for cutting

    The final haul, samples are ready to be sent for cutting (© SNPA)

  • Cut and polish

    Cut and polish (© SNPA)

  • Samples ready for the wall’s capping

    Samples ready for the wall’s capping (© SNPA)

Header image - Cwm Idwal (© Crown copyright (2014) Visit Wales)