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 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 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.

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!