Geology of North Wales

One of the reasons why I run climbing courses in North Wales is that the area offers so much variety. There are so many different types of rock, from sandstone and slate to granite and rhyolite.

Snowdonia has always been an important area for geologists. Many fossils have been found in this area, which helped the first geologists to date the age of our planet. In fact the terms Cambrian, Silurian and Ordovician, all epochs used throughout the world, were named after Welsh tribes.

The history behind this massive variation in rock types is due to the area’s complex geology. To simplify things, the sequence of events that formed Snowdonia’s landscape can be split into 4 distinct stages. The first stage was sedimentation, followed by volcanic eruptions, a mountain building stage and then finally erosion.


Although there are some pre Cambrian rocks in Snowdonia (around Llyn Paddarn) these have little influence on the dramatic scenery. The story begins when the whole area was at the bottom of a shallow sea around 500 million years ago. During this period there was a large amount of erosion in the areas above sea level, this washed grits, sand, muds and silts into the sea. This sediment settled and formed many of the rocks that now make up the Northern Gylders.

If you were to stand on Tryfan or the Griban ridge and look towards Foel Goch you will be viewing the results of this process.

North Wales mountains

This view also shows the succession in which the different types of sedimentary rocks were laid down. The first rocks to be laid down can be seen on the far right of the photo at the spot height of 712m. Here you will see the harder gritstones known as Bronlwyd Grits. These tough grits have resisted erosion which is why the high point exists today. To the right of this you will see the next set of rocks to be laid down, the softer Marchlyn Flags (a type of siltstone). These are then followed by the Carnedd Grits. These grits also make up most of Elidir Fawr which is Snowdonia’s only 900m peak composed of sedimentary rock.

Scrambling in North Wales

Here is a photo of a scrambling route called Undulation (grade 2) near Foel Goch. Here you can see the distinct waves in the sedimentary rocks. These are caused by strong currents in the water, like the ripples found in sand after the tide has gone out.

Plate tectonics and volcanoes in Snowdonia

During the Ordovician period, around 450 million years ago, Wales was on the border of two ancient tectonic plates. During this time Wales was 30° south of the Equator. A continental plate called Laurentia was crashing into an oceanic plate called Iapetus. As oceanic plates are heavier than continental plates, Iapetus was slowly pushed underneath Laurentia. This subduction of the oceananic plate resulted in massive volcanic eruptions in Wales.

Types of volcanic rocks

Before I explain the sets of eruptions that caused Snowdonia’s landscape, it is worth understanding how different types of volcanic rocks are formed.

Rhyolite is the most common rock type in Snowdonia but globally very rare. Most volcanic rock in the world is Balsitic rock. The difference between the two is due to the mineral content. Rhyolite is high in Silica, giving the rocks a very acidic ph. Balsitic rocks are low in Silica and have a higher ph.

Balsitic rock

You can often tell what type of rock you are climbing on by the amount of vegetation. The high ph of Rhyolite stops many plants from growing. Conversely, the ph of balsitic rock is much more friendly to plant life. Within Snowdonia the Balsitic rock is darker. There are many places where these type of rock are lying right next to each other and the contrast between the two types of rock can easily seen.

Within these two groups there are further variations in rock type. The type of rock depends on its mineral content (rhyolitic or Balsitic) and how close to the surface the lava reached before it started to cool. Dolerite and Basalt are both Balsitic rocks but Dolerite never reached the Earth’s surface so it cooled very slowly and slow cooling allowed large crystals to form. On the other hand Basalt reached the surface and cooled rapidly. Rhyolite and Granite are both rhyolitic rocks but granite never reached the Earth’s surface whereas Rhyolite did.

Pits head tuff north wales

The volcanic cycles in Snowdonia

This eruptive period of Snowdonia’s history can be split into a number of stages. Names for all these stages often come from where the lava is to be found, which for a climber can be very confusing. For example the first set of eruptions was centred on Capil Curig and was therefore named the Capil Curig Volcanic group and this lava was mainly rhyolitic. The Pits Head Tuffs are also Rhyolitic rocks, these are named after a farm which is now close to the centre of these eruptions.

If you were to walk up Snowdon via Grib Goch, parking at Pont y Cromlech, you would be able to walk up a geological time line showing the different types of volcanic rocks that form most of Snowdonia. The rocks on the south side of the Pass at Pont y Cromlech are Pitts Head Tuffs.

From here you could walk or catch a bus to Pen y Pass and start walking through rocks called Lower Rhyolitic Tuffs. As the name suggests, these are also Rhyolitic rocks.

flow banding north wales

Once you start to make your way towards Crib Goch and reach the high point of 923m you will be walking through Intrusive Rhyolite ( Rhyolite that did not reach the surface). Look out for a feature called flow banding. This looks like layers in the rock that have been caused as the lava pushed its way through harder rocks, leaving grooves as it cooled.

As you walk further on, you will notice the rocks change colour to a darker grey. At this point you will have walked into an area dominated by Bedded Pyroclastic Formations (BPF). This is a fancy geological name for the type of Balsitic rock that was ejected towards the middle of the Snowdon Volcanic cycle. When this rock erupted from the volcanoes it flowed over the land. As the lava flowed the larger and heavier blocks sank to the bottom. As the lava cooled it formed what looks like layers that could be confused with sedimentary rocks.

At Crib y Ddysgl you will be walking and climbing on the rocks formed during the final stage of the eruptive cycle, Upper Rhyolitic Tuffs. These are also rhyolitic rocks but are not as common as Lower Rhyolitic Tuffs.

If you decide to carry on, you will mainly be walking through BPF until you reach the Summit, which is made of Upper Rhyolitic Tuffs.

Mountain Building in Snowdonia

This video shows what happens when two continental plates collide. Around 400 million years ago the oceanic plate Iapetus had completely subducted underneath the continental plate, Laurentia. Now Laurentia was being pushed against another continental plate called Baltica. As these plates collided with each other they were pushed upwards and formed a massive mountain range.

This mountain range was of a similar height to today’s Himalayas. These mountains have now been worn by millions of years of erosion. The folding of the land that caused this massive mountain range can still be seen in a few places. Cwm Idwal shows once of the best examples of this folding in the bedrock.

The final stage of the Snowdonia’s geology is Erosion. Over many millions of years the mountains have been worn away by wind, rain and ice. There have been many glacial periods in Snowdonia, each one eroding more rock. The last one was around 20000 years ago. Massive sheets of ice collecting high in the mountains would have worked their way down the slopes, carving out rock as they went. It was these that have formed the steep ridges and towering cliffs that climbers and walkers enjoy today.


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