Geoparque Mundial UNESCO
Geoparque Villuercas > Nº29 – Fracture System of the Sierra de La Breña
On the EX-386 road, from Deleitosa towards the A-5 motorway, just at the exit of the village (kilometre point 9.5) a road called Camino de Romangordo, which starts, after going up a small slope, takes us to the Llanos del Pino. This road runs parallel to the mountain range and over the foothills between it and the Deleitosa raña. From here you can access the quartzite crests of the Sierra de la Breña, where you can see a formidable system of fractures.
Observe how the rock formations lose longitudinal continuity due to the effect of transverse fractures that move the quartzite materials laterally in opposite directions on either side of the fracture plane.
On a hill in the sierra is the La Breña hermitage, from where you can enjoy a magnificent panoramic view of the Extremadura peneplain and the Deleitosa rañas , which extend from the sierra to the vicinity of the Almonte river. Among the projections of the Armorican Quartzite of the Sierra de la Breña, we can also observe good examples of ichnofossils: Cruziana and Skolithos.
It should not be forgotten that this area of the geopark also belongs to the Monfragüe Biosphere Reserve. The continuity of this mountain range to the northwest forms part of the Monfragüe National Park, which contains a forest that serves as a refuge for a wide range of biodiversity, with a large number of protected species.
The Sierra de la Breña mountain range stands out to the north of the village of Deleitosa, on the eastern edge of the Trujillo peneplaoin, consisting mainly of shales and greywackes, and conglomerates deposited in the Ediacaran and folded during the Cadomian orogeny (Ediacaran-Lower Cambrian).
At the top of this mountain range is visible the Armorican Quartzite sedimented during the Ordovician. These are the most competent and hardest rocks in the relief, which is why they form the crests of the sierras of Las Villuercas. There are also two other very recent geological formations in the landscape, the result of the erosion of these rocks: the pedreras (slope landslides) and the rañas (see the origin of these formations in the respective geosites of the source of the Almonte and the rañas de Cañamero).
All the rock formations of Las Villuercas are intensely folded and fractured due to the pressures created during the Variscan and Alpine orogenies. These fractures are especially visible in the quartz-arenites, where they generally do not occur in isolation, but form parallel systems of numerous faults that give rise to transverse steepening and displacements that interrupt the continuity of the quartzite crests.
The parallel fractures found in the Sierra de la Breña are strike-slip or transverse faults, in which the fault plane is vertical and the movement of the blocks is horizontal.
Geomorphologically, these faults are manifested in Las Villuercas as displacements in the Armorican Quartzite. In some cases, the incisions are sufficiently wide and deep for the watercourses to be embedded in them, forming the typical portholes (e.g. Almonte apreturas: narrow passes), or deep gorges (e.g. Estrecho de la Peña Amarilla), or in other cases, abandoning their linear course, so that they are incorporated, by “fluvial capture”, into another water course into which they did not previously flow (e.g. confluence of the Viejas river with the Ibor river).
