Geoparque Unesco

2 A turbulent history (Boltaña Castle)

A turbulent history (Boltaña Castle)

Mountain building involves enormous compressive forces that are generated by the collision of tectonic plates, which in turn are driven by the Earth's internal heat. This collision directly affects both the rocks which form the plates themselves and those which are squeezed between them. These compressive forces fold, break, shift and heap up the rocks, shortening the horizontal surface and lifting the materials vertically.

From this viewpoint, we can see a striking structure that clearly shows the part played by compressive forces in the formation of the Pyrenees: an anticlinal fold.

Gran pliegue anticlinal de Boltaña visto desde el Esconjuradero de Guaso. Fotografía: Ander Gómez.

Detalle del anticlinal de Boltaña en el que se ha marcado la diferente inclinación de una capa cualquiera de roca. Esta diferente inclinación nos indica que los estratos o capas de rocas, originalmente horizontales, han experimentado esfuerzos compresivos que las han plegado. Fotografía: Ander Gómez.

El río Ara corta el anticlinal de Boltaña. Esta sección nos permite observar la inclinación de las capas más externas del pliegue, por ejemplo en el impresionante desfiladero de Jánovas, donde los estratos horizontales son ahora verticales. Fotografía: Luis Carcavilla.

The history of this landscape consists of three episodes in which processes occurred which were very slow on a human scale.

Episode 1

The rocks that form the mountain in front of us were generated from sediments deposited in horizontal layers on the bottom of an inlet that connected the Atlantic Ocean and the Mediterranean Sea between 55 and 35 million years ago.

Episode 2

At the same time that these sediments were being deposited, a collision was occurring between the Iberian Peninsula and Europe, closing the inlet and compressing the materials forming the foundations of the terrain. This compression broke, bent and pushed the stratified rocks, which ceased to lie horizontally. And this was how the Boltaña anticline fold was formed.

Episode 3

Since it was formed, water, ice, wind, gravity and living beings have been eroding the mountain range and have lifted deeper rocks to the surface. In the case of Boltaña, the Ara River and has also been gouging out the valley for millions of years, allowing us today to observe the folds in the strata.

Esquema de la formación del anticlinal de Boltaña.

Geological clue

  • Tectonic plate: each of the huge sheets which make up the lithosphere.
  • Lithosphere: the outermost layer of the Earth, characterised by rigid behaviour.
  • Fold: bends in the rocks, usually caused by compressive stresses.
  • Anticlinal fold: a fold with the oldest materials in the core or inner part. Anticlinal folds are usually convex, with sides which slope in opposite directions. Stratified rock: rock which is arranged in layers or strata.

Did you know .....?

When tectonic plates collide, they create a mountain range. The rocks which are most strongly affected are those which are tens or hundreds of kilometres below the surface of the Earth. Just as with iron when it is forged, high temperatures make rocks malleable, as if they were made of plasticine.

Recomentations in your visit to Sobrarbe-Pirineos Geopark

  • Safe visits, unforgettable experiences. There are companies who collaborate with the Geopark where you can hire specialized guides to make your tour a unique and safe experience.
  • Respect the geological outcrops. Do not break off samples of rock, mineral or fossils.

Avda. Ordesa 79. 22340 Boltaña. Huesca. 974 518 025.