Geoparque Unesco

3 An ocean of rocks

An ocean of rocks (Guaso)

Localización: Mirador del Tozal de Guaso, junto al esconjuradero e iglesia.

Coordinates . 30T0677471 UTM 4616060

Textos Esperanza Fernández (Geopage)

This viewpoint is an excellent place from which to observe how the geological substrate, the product of a long and complicated history, has determined the shape of the Sobrarbe region. Looking around us, we can see an ocean of rocks, within which we can distinguish various landscapes with characteristic geological and anthropic features.

Vista panorámica del lado oriental, valles del Ara y el Cinca. Fotografía: Ander Gómez.

1) Inner mountain range

This forms the wall of grey rocks (limestone) and ochre rocks (sandstone) that we can see in the background. It contains some emblematic, mostly limestone, mountains such as the Macizo de Cotiella, Monte Perdido or, closer to us, Peña Montañesa.

These are formed of ancient rocks from the Palaeozoic, which are very hard and resistant to erosion. In fact, they are the ruins of an old mass that has now been almost completely destroyed, and which stand out in relief because the surrounding rocks have eroded away. The range can be identified by its height, the escarpments and the fact that it often lacks plant cover.

Peña Montañesa, un relieve de referencia en Sobrarbe. Fotografía: Esperanza Fernández.

As Tres Serols (al fondo) con Sestrales (en la parte anterior). Fotografía: Ander Gómez.

2) Mid-mountain range

This consists of younger, mostly Eocene, materials of the kind called turbidites or flysch. Two alternating types of rocks are present in turbidites: marls, which erode easily when they are not protected by vegetation, creating a landscape full of gullies, and sandstones, which are more resistant.

They create gentle reliefs cut through by narrow valleys, with plant cover and occasional headlands (such as the one on which we are standing) when a sandstone layer protects the marl that lies beneath.

We are standing on the western shore of the Ara River, again in the mid-mountain range. Looking to the right (south) and left (north), we can see a succession of peaks; these are the result of the presence of turbidites, with their alternation of hard sandstones and soft marls. On this side, there are also deposits of marl with occasional layers of limestone, where surface runoff has produced a landscape marked by gullies and bare rock.

Detalle de laderas formadas por rocas de tipo flysch y erosionadas de tal forma que se ha generado un diseño geométrico de cárcavas. Fotografías: Esperanza Fernández

Fotografías: Esperanza Fernández

La colina de Boltaña, donde se edificó el castillo, está formada por la presencia de una capa de arenisca resistente a la erosión y que protege de la misma a las margas que hay bajo ella. Este tipo de colinas es relativamente frecuente en las Sierras Medias. Otros ejemplos sons el lugar donde nos encontramos, en el cual se ha edificado la iglesia y el esconjuradero de Guaso, o el castillo de Morillo de Tou. Fotografía: Esperanza Fernández

3) Central zone


This is the lowest area, through which the Ara and Cinca rivers flow. Both rivers have taken advantage of a stretch of very clayey materials to carve out their riverbeds. Above them are located the fluvial terraces: deposits from the rivers themselves that form areas of different sizes and heights. It is difficult to distinguish these at first glance but they usually have a red outline and can be identified by the fact that they are used for growing crops. Among them is situated the wide, gravel-covered riverbed of the Ara River.

Las terrazas fluviales son depósitos dejados por ríos a lo largo de una historia de excavaciones y depósitos, usualmente debida a los cambios climáticos acontecidos durante el Pleistoceno. Hay varios niveles de terrazas y no siempre es fácil reconocerlas a simple vista pero cuando están seccionadas destacan por su perfil ocre-rojo, que contrasta con las tonalidades oscuras de las turbiditas en las cuales el río ha excavado su cauce. Fotografías: Esperanza Fernández

Fotografías: Esperanza Fernández

Las terrazas son lugares habituales de cultivo. Fotografía: Ander Gómez

4) Outer mountain range

These are the mountains at the end of the horizon to the West. They are formed by the Sierra de Guara, consisting mostly of Eocene limestones, where the rivers are carving out deep canyons.

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.