Rock School

Posted by Paul Gannon on 02/06/2008
Llanberis Pass. Photo: Messenger.

Born from fire and sculpted by ice - Paul Gannon is your guide to the violent geological childhood of Llanberis Pass.

Llanberis Pass is one of Snowdonia’s scenic gems, gateway to climbing Mount Snowdon and home to some of the very best rock climbing in Britain. A fine forecast sees Pen y Pass lined with walkers’ car and climbers flocking to throw themselves at the daunting sweep of Dinas Mot, the sheer walls of Dinas Cromlech and the pocketed bulges of the boulders littering the valley floor.

Gasping for breath on Grib Goch or hanging from a stance high on Clogywn y Grochan, many a visitor will have paused to admire the view. But have you ever wondered just how this dramatic landscape was created? The answer is a rather violent youth.

The scenery and climbing opportunities in Snowdonia are the result of an intensely dramatic period of geological history. Extremely violent volcanic eruptions produced very tough rocks which have survived everything that nature could throw at them for over 400 million years. Long periods of erosion followed, most spectacularly by glaciers in recent ice ages, carving the rocks into their present much-loved shapes.

The Llanberis Pass was smack in the centre of all this drama, the biggest and the most explosive pyroclastic eruptions took place around here. Then in the ice age a massive outlet glacier from an ice sheet centred in the Migneint area smashed through the rocks at Pen y Pass then ran down the Pass carving the deep trench we see today. There’s plenty of evidence of all this geological activity in the Pass to this day, easily spotted when you’re out walking and climbing. So, here’s a quick tour of some of the features that you can spy around the area of Dinas Mot and the Cromlech boulders.

Volcanic rocks
A good starting point for the geological tour is the Cromlech boulders themselves. Wandering round the large blocks of stone you can see that they contain lumps of rock, of all shapes and sizes, often forming the handholds on various crimpy boulder problems. Some of these lumps are easy to see – even plastered with climber’s chalk – others are hidden by a covering of lichen. But look closely and they will become increasingly evident. This rock is what is known as a volcanic breccia.

It was produced by a massive pyroclastic eruption which occurred under sea level and took place through fissure vents. Pyroclastic means the eruption of fragments of molten magma and gas, rather than a flowing molten lava. This eruption was so big that a large caldera (a roughly circular depression) was created, extending from here all the way to Moel Hebog. This was the major event in the Snowdon volcanic cycle and the presence of the breccia at the Cromlech indicates that the actual eruption took place very close to here.

The fissure vents were probably located around the present day ridge of Cyrn Las and Cwm Glas Bach. The finer material would have been carried farther than bigger lumps, creating a type of volcanic rock known as tuff. The crags on either side of the Llanberis Pass, all the way up from here up to Pen y Pass, are mainly tuffs, although of many different types. Have a rummage around the mass of smaller boulders behind the bigger Cromlech boulders and you can find a whole variety of tuffs and breccias.

Look up from the boulders and above the high, sweeping expanse of Dinas Mot and you’ll see a different type of intrusive volcanic rock – dolerite. Unlike the magma which was erupted to create tuffs and breccias, this molten magma never made it all the way to the surface, but got stuck in the vents and cooled there. The lighter coloured rocks on Dinas Mot are tuffs and breccias, while the darker rock above is dolerite. Dolerite is usually a good rock for climbers, but not here and it’s actually the lower tuffs and breccias that are climbed.

More intrusive rocks can be found higher up in Cwm Glas Mawr and are spectacularly well exposed in the crags of Cyrn Las. Among these crags you can find a fascinating feature known as flow-banding, recording evidence of the flow and cooling of the molten magma in the vents. It’s these tough pyroclastic and intrusive volcanic rocks which form the heart of the area and provided the raw material for later geological forces to work on and shape into today’s scenery.

 

Syncline
The Snowdonia volcanoes occurred as a result of the collision of tectonic plates. This collision of the plates in the area went on for many millions of years, initially prompting the volcanic activity, but later leading to the creation of a large mountain range. You can see evidence of this a short distance down the Pass. Looking up into Cwm Glas Bach, a syncline (downfold in the rocks) is visible just below the ridge. The same syncline is also seen in other popular climbing areas, Cwm Idwal and Cwm du’r Arddu - although in this case the symmetry is distorted by a fault which forms the east terrace.
This syncline was formed by the collision of two tectonic plates, folding and crumpling the rocks, pushing them up into massive mountain ranges. There followed millions of years of erosion which ground the mountains down to the last remaining stumps that we see today. Some of the erosion was so intense that the intrusive rocks - which had not originally reached the surface - became exhumed, now exposed at ground level.

Glacial trench
Whilst volcanic activity provided the raw material, the ice ages of the last two million years are responsible for much of the spectacular scenery in the area. The Pass itself was cut by an outlet glacier from an ice sheet centred in the Migneint area. This glacier was pushed out from the centre of the ice sheet with so much force that it eventually smashed through a pre-glacial col at Pen y Pass, deepening it and carving out the Pass. The glacier in the valley lower down would have been 400 or more metres deep at the height of the ice age. Today the Pass has the classic glacial profile - a flat bottom and steep crags on either side. The vast scree slopes that have accumulated since the ice melted tend to hide the full extent of the steep side walls, but if the scree were removed then the crags would actually extend all the way to the valley floor.

Glacial cwms
When the temperature began to rise the main ice sheet retreated, but it was still cold enough for small glaciers to develop in the higher recesses. This was when glacial cwms (also known as corries or cirques) such as Cwm Uchaf (Upper Cwm) and Cwm Glas were active. Ice would have accumulated in such spots, cutting deeper and wider into the cliffs, then flowed out of the cwm, tumbling over the edge of the cliffs in an ice fall. The ice also cut back into the mountainside to create the very characteristic steep back and side walls.

Arêtes
An arête is created when two glacial cwms are cut so deeply into the mountainside that they only leave a narrow wall of rock between them. And it is these arêtes, or sharp ridges, that now provide such wonderful routes to the summit of Snowdon for climbers and adventurous walkers alike. The arête forming the north ridge of Crib Goch provides a good alternative route onto the main Crib Goch ridge and is accessible from the Llanberis Pass by plodding up the steep slopes of Cwm Glas Mawr and Cwm Glas.

In general, glacial cwms tend to collect on the eastern or north-eastern side of mountains in spots protected from the wind and sun. Hence the cwms and their corresponding arêtes only appear on the Snowdon side of the Llanberis Pass. There are none on the Glyderau side, although there are spectacular cwms and arêtes on the other side of the ridge, such as Cwm Bochlwyd and Tryfan. Snowdon itself was high enough that cwms developed on all its flanks.


 

Boulders
Our rapid tour of the Llanberis Pass started at the Cromlech boulders and so will end there too. At the moment the lower sides and the bottom of the Pass are smothered by thick curtains of scree and large boulders. All of this material collapsed to the ground after the glaciers had melted. The ice had cut so steeply into the mountainsides that once the ice had gone, the rock faces were unstable and started to peel off, aided by repeated freezing and thawing of water in any cracks in the rocks. The weird shaped rocks on the summit plateaus of Glyder Fawr and Glyder Fach are also a product of such freeze-thaw action.

Humans
The present day floor of the Llanberis Pass is evidence of the full range of geological forces that have affected the area in the last half-a-billion years. And of course us humans that have also made our presence felt in the last brief moment of time. Although only being here for the merest blink of an eye in geological terms, we’ve undertaken some erosion of our own in the slate quarries just down the road. That’s also left some fine rock faces for climbing, but is of course another story.

Paul Gannon is a keen hill walker and a science and technology writer. He has written for many newspapers, magazines and websites, including the Guardian, the Independent, New Scientist and Times Online. His latest book is Rock Trails Snowdonia: a hillwalker’s guide to the geology and scenery.

 



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