Geology of Snowdonia National Park

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Geology of Snowdonia National Park (simplified)

Snowdonia’s rocks tell a long, dramatic story. Most of the bedrock came from the Cambrian to Ordovician periods, around 540 to 420 million years ago, when parts of the Earth’s crust formed shallow seas, rivers, and occasional volcanoes. These rocks were then squeezed, twisted, and broken as continents collided in a long, complex battle known as the Caledonian Orogeny. Later, as the North Atlantic Ocean opened, the land slowly rose again. The mountains we see today were carved by ice ages in the last 2-3 million years.

What you can see at the surface today
The oldest rocks you can walk on in Snowdonia are Cambrian rocks exposed in the Harlech Dome and along the park’s northwestern edge. The earliest layer is the Dolwen Formation (sandstones and mudstones laid down in a river delta), followed by the Llanbedr Formation (blue and purple mudstones), and then the Rhinog Grits (sandstones and pebbly rocks formed in underwater currents). The Rhinog rocks help form the rugged Rhinog Lae—Rhinog Fawr, Rhinog Fach, and nearby peaks.

Higher up in the Cambrian rocks are other formations you may hear about, such as the Hafotty, Barmouth Grits, Gamlan, and Clogau. The Mawddach Group includes the black mudstones of the Clogau Formation and other turbidites (sediments deposited by underwater landslides). In some places the same Cambrian rocks match younger parts of the sequence, and elsewhere the middle Cambrian layer is missing in places.

As you move into the Ordovician, rocks become a mix of marine mudstones, siltstones, sandstones, and, importantly, igneous rocks that intruded into the existing rocks. The lower Ordovician Dol-cyn-afon Formation shows up around the Harlech Dome, and the Nant Ffrancon Subgroup (shallow marine siltstones) stretches around the park’s northern and eastern edges. Large, hard dolerite intrusions (a type of volcanic rock) remain today as prominent cliffs at sites like Dinas Mot and near Tremadog.

Silurian rocks appear mainly along the park’s eastern and southern margins. These include sedimentary rocks and more intrusive rocks that wrap around the edge of the Harlech Dome. In the Dovey valley you can find mudstones from the Silurian.

Important notes about gaps and buried rocks
There are no late Paleozoic, Mesozoic, or most Cenozoic rocks on the surface inside Snowdonia. A borehole at Mochras, near Harlech, revealed hidden layers of Mesozoic and Cenozoic rocks beneath a thick cover of soil and rock debris. These buried rocks include a thick Jurassic sequence (Lower to Upper Lias) and layers from the Palaeogene to Neogene periods, all in a half-graben basin created by faults near the Cardigan Bay coast.

Glaciation and landscape shaping
Britain’s ice ages, especially the Devensian glaciation (the last one), carved Snowdonia’s landscape. Dozens of cirques and large U-shaped valleys—Nant Ffrancon, Llanberis, and Nant Gwynant—were formed. Lakes such as Llyn Peris and Llyn Gwynant sit in bedrock hollows carved by ice. Glacial till (unsorted rock debris), river deposits (alluvium), peat, and dunes near the coast and in valleys all tell the story of ice and wind shaping the land.

Structures and minerals
Snowdonia’s rocks were intensely faulted and folded during the Caledonian Orogeny. Major faults include the Bala Fault/Tal-y-llyn Fault and the Pennal Fault, which helped carve the Dovey valley. The region also shows many NE–SW folds, such as the Moel Hebog Syncline, Capel Curig Anticline, and the Snowdon Syncline.

The park has mineral stories too: copper, lead, zinc, iron, and gold have been mined in the past. The Sygun Mine near Beddgelert and other sites reflect this history. The Cambrian and Ordovician slates were quarried for roofing and building materials, fueling local slate-roofed houses and heritage railways. The Slate Landscape of North West Wales was recognized as a UNESCO World Heritage Site in 2021, highlighting Snowdonia’s famous slate heritage. The National Slate Museum in Llanberis and related heritage sites celebrate this history, along with geology-focused programs at Ogwen Cottage.

Protection and learning
Many parts of Snowdonia are protected for their geological value as SSSIs (Sites of Special Scientific Interest) and RIGS (Regionally Important Geological Sites). They help us understand how the park’s rocks formed, moved, and were reshaped by ice and water over hundreds of millions of years.