Geoparque Unesco

8 A channel under the sea (Morillo de Tou)

Castles on vantage points (Morillo de Tou)

Localización: Mirador sobre el pantano de Mediano, en Morillo de Tou.

Coordenadas. 265971 4695727 31 N

Textos Esperanza Fernández (Geopage)

The castle of Morillo de Tou is built on a hill which is formed of two types of sedimentary rocks:..

  • -a protruding top layer with an ochre colour and a hard appearance. This is composed of sandstonewhich has resisted erosion while the materials that surrounded this layer have eroded away.
  • -a lower, blue-grey layer with a worn appearance. This is mainly composed of marlstone, a very soft rock which is eroded by rain water, generating small gullies. Embedded in these rocks are thin layers of sandstone.

Colina donde se asienta el castillo de Morillo de Tou. Se observa que el propio castillo está edificado en una capa de materiales de aspecto duro y consolidado. Son las areniscas y microconglomerados depositados en los canales submarinos. Bajo ella, la zona sin vegetación nos deja observar una roca blanquecina, muy erosionable y modela con pequeños barrancos. Se trata de margas y arcillas, depositadas en el abanico submarino, fuera de los canales. Fotografía: Esperanza Fernández.


These are sedimentary rocks formed from sand-sized mineral grains that have been bound together by cement. If the grains and the cement are rich in silica, these rocks are very resistant to erosion and stand out in relief.

Aspecto de rocas de tipo arenisca y, a la izquierda, microconglomerado. Fotografía: Esperanza Fernández..


These are sedimentary rocks consisting of a mixture of clays and limestone (calcium carbonate, often in the form of tiny shells of living beings). The higher the percentage of clay, the more plastic the rock and the more easily eroded.

Aspecto de rocas de tipo margas. Fotografía: Esperanza Fernández.

...and submarine canyons

Deep, steep-sided valleys known as submarine canyons cut into the sea floor. The rocks on which Morillo de Tou sits were formed in one of these canyons, on the floor of the sea which covered Sobrarbe about 42 million years ago.

On land, rivers act as huge collectors which carry materials to the sea, where they are deposited. Under certain circumstances (earthquakes, climate changes, the discharge of large amounts of material, etc.), the deposited materials are stirred up and begin to move, carried in the water, flowing through the canyons at high speed and forming what are called turbidity currents.

When these materials reach the base of the slope, the current is halted and the sediment is deposited by gravity. The largest particles (gravel and sand) settle first, and over time this produces layers of sandstone, which fill the channel through which they were carried. The finer sediments (clay) may remain in suspension for years and when they finally settle, they are mixed with particles of calcium carbonate and the remains of creatures that fall slowly but steadily to the sea floor. These deposits are very extensive and generate marlstone.

This process is repeated throughout geological time. The result is a regular sequence of rocks known as turbidites.

Esquema de un ambiente submarino en el que se depositan estas rocas especiales, las turbiditas. Dibujo de Albert Martínez.

Did you know….?

The hills on which the castle of Boltaña or the esconjuradero de Guaso are located, among other viewpoints, have a formation similar to the hill on which Morillo de Tou sits: a layer of sandstone, formed in an ancient submarine canyon, protects the marls located beneath from erosion.

En diferentes puntos de Sobrarbe, las capas de arenisca sobre materiales más erosionables destacan en el relieve generando promontorios. Estos han sido utilizados históricamente para la edificación de torres vigía, castillos, esconjuraderos y otras construcciones que requieren una localización en zonas de gran visibilidad.

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.