REnowned Geology

Complex, varied and renowned geology, which has been vital in influencing the disciplines of geology and geography internationally.

The landscape of Snowdonia and the nature and activities within, are founded on its geological history. It is a complex detective story of colliding continents, volcanoes, mountain-building, changing sea levels and glaciations that were involved in creating Eryri as we know it today. 

It is difficult to completely unravel our mountain scenery, to understand how this haunting landscape came about. However, with a knowledgeable guide or even a good guide book or app, we can start to recognise clues and understand why and how
the landscape developed over millennia. We can see some of the rocks created by volcanoes or sedimentary deposits. We can learn how to interpret features and signs of mountain-building and glaciations on the ground…evidence is all around when you know what to look for!

Ice Age activity has done much to shape the landscape in Snowdonia. The glaciers that were at their peak 18,000 years ago scoured out great U-shaped valleys including Llanberis and Nant Gwynant in the north and Tal y Llyn in the south.

Snowdonia presents a richly varied landscape composed of mountain peaks and ridges, deep valleys and moorland plateaux. The bedrock geology is dominated by Cambrian, Ordovician and Silurian sedimentary and volcanic rocks. A large area of Cambrian sandstones and shales, known as the Harlech Dome, underlies the Rhinogydd and extends west into Coed y Brenin and south to the foothills of Cader Idris. Coarse-grained sedimentary rocks form some of the higher peaks in the Rhinogydd and are some of the oldest rock formations in the UK.

Around the Harlech Dome, major volcanic centres developed at different intervals during the Ordovician period and these erupted vast quantities of lava and ash that are now preserved in upland areas such as Rhobell Fawr (705m) and Cader Idris (893m) in the south, and farther north around Yr Wyddfa (1085m) and the Carneddau. This volcanic activity was also accompanied by the widespread emplacement of igneous intrusions of granitic and basaltic composition that now form distinctive, erosion resistant features throughout the landscape.

There was a major phase of mountain building during early to mid-
Devonian times, the consequence of which was to create the important slate belts of the Bethesda, Llanberis, Nantlle, Blaenau Ffestiniog and Corris districts. The industrial exploitation of slate from the mid-18th to the mid-20th centuries transformed the landscape of these districts to leave a legacy of slate quarrying and mining remains that are of international renown.

Ancient volcanic activity created several types of mineral deposit in the area and copper, lead and zinc mineralisation is a particular feature of Snowdonia. These deposits too became the focus of much exploration causing characteristic remains to be left within the landscape. Manganese was mined from around the flanks of the Harlech Dome, whilst copper and gold were won from quartz veins that outcrop to the west and north of Dolgellau. Collectively known as the Dolgellau gold-belt, this mining district experienced a major gold-rush during the last half of the 19th century when large gold deposits were found at the Clogau and Gwynfynydd mines.

The glacial history of the area has resulted in a very rich and often
spectacular upland scenery that we see today, with over-deepened U-shaped valleys caused by glaciers radiating out from the upland core down slopes and valleys such as Nant Ffrancon and Nant Peris. There are a wealth of classic features such as high cwms or cirques, pinnacled‘knife-edge’ ridges, moraine dammed lakes, glacial striations, roche moutonees, dramatic waterfalls and so on.

Snowdonia played a key role in the development of geology as a science and the very challenging terrain has had a profound influence on the pattern of transport, farming, industry, tourism and culture throughout the area.


500 million years ago Snowdon looked very different…it was on the seabed as shown by fragments of shell fossils that have been found at the summit. 

  • Crib Goch the famous knife edged ridge is an arete and was created as two glacial hanging valleys were FORMED back to back.
  • Gold from the Clogau Mine near Dolgellau has been used in Royal wedding rings since the Queen Mother’s wedding in 1923.
  • The tiny particles that created first mudstone that then morphed into slate through intense pressure from colliding tectonic plates, was likely deposited at approximately just 0.1 mm per year, that’s at least 1 million years to create 100 metres!