Geoparque Mundial UNESCO

Nº25 – Fault Mirror – Cancho de las Narices

Geoparque Villuercas > Nº25 – Fault Mirror – Cancho de las Narices

LOCATION AND ACCESS

On the Castañar de Ibor to Robledollano road approximately halfway between the two towns. The fault mirror is in a trench that has been dug in the road under the Cancho de las Narices on a bend with poor visibility, which makes it necessary to stop before reaching the bend when coming from Robledollano on a track to the right that descends to the River Viejas. From here the platform created behind the handrail is accessible, from where we can enjoy this geosite in safety. To contemplate the Cancho de las Narices it is best to continue along the road, ford the River Viejas, and go up immediately opposite. The site is a fantastic Portuguese oak grove that affords magnificent views of this phantasmagorical place.

ATTRACTIONS OF THE VISIT

As well as the above the River Viejas syncline can be contemplated from this site; it can be identified by the dip (inclination) of the materials on the hillsides and the crests on both river banks (on the western slope of the Sierra de Viejas and the eastern side of the Sierra Alta). We can also identify in the vicinity the typical lithologies (rock types) of the Ordovician and Silurian periods with their characteristic fossils of trilobites, graptolites, and spiriferids.

There is a magnificent Portuguese oak grove on the hillside of the Sierra Alta facing towards the west, while the riverside vegetation is dominated by alder, ash, and Portuguese laurel. This is also a good ornithological site for observing black storks, Egyptian vultures, griffon vultures, booted eagles, eagle owls… and numerous forest and riverside bird species.

GEOLOGICAL INTERPRETATION

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This site represents the transition of the Ibor anticline to the Viejas syncline. It is here that the River Viejas makes its way towards the River Ibor by crossing the Sierra del Viejas. The river flows in a deep gorge and changes its longitudinal course to a perpendicular direction, making use of a fault that is perpendicular to the alignment of the mountains.

One of these faults can be identified at this point as a result of the displacement that has occurred in the quartzite formation of the Sierra de Viejas, which is visible on both sides of the watercourse. At the base of the crag of the Cancho de las Narices the fault mirror can be observed on the quartzite as a dark, polished, and shiny surface with direction striations that originated from the friction between two blocks of quartzite rock that have become displaced.

As a consequence of the tectonic pressures brought to bear on these quartzites minor folding and fracturing has also occurred. To the right of the fault mirror a chevron fold (in the shape of an accordion) can be observed, and moreover the rocks of both blocks have been partially crushed (mylonitised) which favours minor block displacement.

Immediately above stands the Cancho de las Narices, a steep quartzite outcrop with a high concentration of iron oxides that erosion and mining operations have given a singular cavernous shape with an entrance resembling a nostril.