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The Geology of the Pyrenees

The geology of the Pyrenees

The 430 Km long mountain chain of the Pyrenees was uplifted (starting 65 million years ago) by the collision of the tiny Iberian and the Eurasian plate, followed by intense erosion from ice and water.

However, you can find rocks over 500 million years old in some areas and the formation of the Pyrenees wasn’t quite as simple as one big uplift that created the mountains.

Ramond de Carbonnières was officially the first man to climb the summit of Monte Perdido (3355m) in 1802. There, he made an amazing discovery, encountering fossils of mussels, oysters and other shells high on the massif that proved that the sea once covered the Pyrenean range.

The formation of the Pyrenees

Here’s the quick version of the formation of the Pyrenees – for more detail carry on reading below!

Softer layers in the limestone can be eroded away creating the famous faja paths. The Faja de la Flores in the Ordesa Canyon is one of the most famous and dramatic – a narrow balcony path only a metre or two wide that winds around the tops of the cliffs with the valley floor over a kilometre below. A must for the adventurous hiker!

350 Million years +

Emerging lands in the world came together into the “Pangea”. The immense forces began to push up the sea bed giving birth to a huge range known as the Hercynian mountains.

The formation of the Pyrenees

350 Million years +

Emerging lands in the world came together into the “Pangea”. The immense forces began to push up the sea bed giving birth to a huge range known as the Hercynian mountains.

300 Million years ago

The Hercynian range was huge – wider than the current day Himalayas and of similar altitude. Of course as soon as the mountains were formed they began to be eroded flattening the land.

250 Million years ago

A relaxation of the pressure between the plates meant that a shallow sea flooded the now flattened land. This new sea deposited sediments on it’s bed over millennia creating limestone layers.

250 Million years ago

A relaxation of the pressure between the plates meant that a shallow sea flooded the now flattened land. This new sea deposited sediments on it’s bed over millennia creating limestone layers.

65 Million years ago

The African plate pushed north, pushing the Iberian micro plate into the large Eurasian plate. This lifted the sea bed once again forming the Pyrenees mountain chain. The high mountains were now formed but we wouldn’t have recognised these mountains and there was one final stage in the process.

65,000 years ago

Glacial and fluvial erosion occurred, has continued to the present day and is still shaping the landscape. Huge glaciers carved through the soft limestone forming valleys such as Ordesa, Pineta and the Serrablo. Rivers did their work too creating the Añisclo Canyon and many others and finally forming the Pyrenees we see today.

65,000 years ago

Glacial and fluvial erosion occurred, has continued to the present day and is still shaping the landscape. Huge glaciers carved through the soft limestone forming valleys such as Ordesa, Pineta and the Serrablo. Rivers did there work too creating the Añisclo Canyon and many others and finally forming the Pyrenees we see today.

Cross section of the Pyrenees

This diagram shows a north-south cross section of the Pyrenees and the types of rocks that form each zone. This is specifically the Valle de Tena and the pre-Pyrenees but most areas will have a similar make-up.

The Ibones de Arriel lie in one of the exposed granite areas on the French/Spanish border. As the granite is not as porous as the limestone which dominates many areas of the Pyrenees, it allows these beautiful mountain lakes to form.

The Hercynian range

Actually, the Pyrenees have risen from the sea twice in the geological history of these lands. The first chain was formed in the Carboniferous period (more than 300 million years ago), when all the emerging lands in the world came together into the “Pangea”. This gave birth to a huge range known as the Hercynian mountains which rose as high as the current day Himalayas.

Taking advantage of this movement, large masses of magma ascended and slowly cooled down under the Earth’s crust, which now that the layers above have eroded, you can see forming the granite rocks of some of the highest peaks like Aneto, Neuvielle or the Panticosa batholith. The heat of the magma also transformed the surrounding sedimentary rocks into metamorphic rocks, like the marbles you find at Vignemale or covering the slopes of Picos del Infierno. Both of these 3000 metre summits have very distinct white marble faces rising to their summits.

 

The Tethys sea

During the Permian and Triassic periods, the Hercynian mountain chain was strongly eroded, producing typically red sediments of sandstone and conglomerates. At the same time, the relaxing tectonic environment reduced the upwards pressure and also helped to flatten the range and allowed some volcanic activity.

This period is the origin of the volcanic horns of Midi D’Ossau and Pico Anayet. Both of these peaks are surrounded by wonderful red sandstone and conglomerate that can be seen on the Vértice de Anayet and Pic Ayous. On these peaks you walk across incredibly bright red shale which is very striking.

This now flat land, was slowly invaded by shallow waters again, becoming part of the Tethys Sea – a huge ocean that separated the continents of Gondwana and Laurasia. For millions of years deposits accumulated on the ocean floor creating the now abundant limestone in the area.

 

The white marble summit ridge of Picos los Infiernos is simply stunning to walk across. You can clearly see the difference between the marble and the darker schist beyond. In the distance to the left you have the ancient volcanic horn of Midi d’Ossau, one of the most famous and recognisable peaks in the Pyrenees. On the right you have the granite ridges and summit of Balaitous.

Here you can see a good example of folded flysch layers. You find these around Biescas – alongside the road between Biescas and Torla you will find wonderful examples of flysch layers. The traditional roofing material here is ‘losa’ – layers of flysch that are excavated. They quite literally weigh a ton! Interestingly head north just 10 kilometres to Panticosa and the rock type changes and there’s no flysch – here the traditional roofs are slate.

This is the gorge of Os Lucas, the soft flysch erodes easily creating gorges which are great for canyoning.

Alpine Orogeny

Some 65 million years ago, the African and Indian continents drifted north, slowly pushing the Iberian microplate against the Eurasian plate once again. This raised and folded the ‘new’ ocean floor sediments creating the Pyrenees mountain range.

The Alps, Apennines, Himalayas, Hindu Kush and Karakorom ranges were all created by the same tectonic movements, in a period known as Alpine Orogeny. The effects were felt as far as Britain, where the north and south downs were formed. In many places the newer rocks were driven above the older layers in so called thrust sheets.

The pushing raised mountains while deepening the seas, collapsing coastal slopes and in doing so create turbidity currents which deposit the biggest grains first (sandstone) and then the tiny ones (clay and marls) in alternated horizontal layers called Flysch. This easily folded rock can be seen in many places such as the Sorrosal waterfall, near Torla or along the road between Biescas and Ordesa.

The Alpine Orogeny period is almost over, but the mountains don’t look as they do today until erosion has done its work. Glaciers covered the range, reaching their maximum 65,000 years ago, and lasted for thousands of years, advancing and retreating during the different ice ages and leaving behind deep U-shaped valleys with flat bottoms (like the Ordesa and Pineta Canyon), cirques, horns (pyramid shaped peaks such as you see from the Respomuso hut) and mountain tarns (locally called ibones) under the highest peaks. At the height if the ice ages, Biescas was under 200 metres of ice.

After the glaciers melted, erosion was continued by rivers (fluvial erosion) and the periglacial processes (freeze-thaw), but the abundance of limestone in the Pyrenees gives an important role to a very special effect of the waters: Karst.

The karstification of the land is produced by limestone dissolving in water, which creates small solution flutes, limestone pavements and the extensive cave and cavern systems that you can find all over the limestone areas of the range. On the surface you can find sinkholes and entrances to these systems or deep narrow canyons, such as the impressive Añisclo Canyon one of the deepest in Europe.

After tens of thousands of years of erosion from ice and water we are left with the Pyrenees that we know and love today!

Find out more at the Pyrenees Geopark

The Pyrenees/Sobrarbe geoparque is based at Ainsa and has an excellent visitors centre in one of the towers of the old castle. It’s only small but is packed full of information about the geology of the Ordesa National Park and the Pyrenees. Most of the info is in Spanish but there is an excellent audio-guide available in English. The views from the top of the tower over the surrounding mountains are superb and worth the entry alone! If you’re visiting, don’t forget to also visit the excellent Ecomuseo, also based in the castle, where you can learn about the wildlife of the Pyrenees and see the raptor rescue centre where they have bearded vultures, eagle owls and other raptors that have sadly been injured and can’t live in the wild any more.

The Geopark’s website is an excellent resource for those wanting to find out more about the geology of the Pyrenees.

A very informative video of the geology of the Pyrenees-Monte Perdido area made by the Geopark:

Geology walking trails

The Geopark has a network of thirty geology walking trails exploring the most interesting geological areas of the Ordesa National Park and the Sierra de Guara. Routes are of varying lengths to suit all levels of fitness. There are informative panels along the routes explaining the geology that you are passing through. You can find route details, descriptions and download GPS tracks from the Geopark’s website.They also have 13 geological driving routes and 13 routes for mountain bikes also with information panels. Again route descriptions and GPS tracks can be downloaded from their website.

The Ordesa NP

Guide to hiking the spectacular canyons of the Ordesa National Park

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