Geoparque Mundial UNESCO
Geoparque Villuercas > Nº11 – Source of the River Almonte
The source of the River Almonte is a large pedrera (block slope) on the western slopes of La Villuerca, the highest peak in the Geopark. Access is afforded by means of a cement road that turns off the Navezuelas-Cañamero road some 3.5 km from Navezuelas. 4 km along on the left. The pedrera is about 2 m from this road. It can also be reached by the military track that ascends the Risco de la Villuerca.
The River Almonte is one of the most important tributaries of the left-hand bank of the Tajo in Extremadura. The river’s source can be traced to a large pedrera, a chaotic deposit of large angular and irregular fragments of rock. Rainwater easily penetrates between these rocky blocks until it encounters a less permeable layer, the shales of the hillsides. The water flows inside the pedrera according to the slope of the hillside, to emerge finally on the surface in the form of a spring at its foot.
Pedreras (locally casqueras) are very widespread on all the slopes of Las Villuercas, and are always located below the quartzite outcrops which represent their area of origin. Geologically they are hillside debris, but when the rocky fragments making up a pedrera are several cubic metres in size they are also known as block slopes.
The origin of these blocks is related to the characteristics of the Armorican quartzites of the Lower Ordovician period (480 m.y.) that are visible on the crests of the sierras. These rocks are very hard although they show numerous fractures known as diaclases (generated by a mechanical meteorisation process known as the “ice-wedge effect”: the water enters the fissures and expands them on freezing to fragment the quartzites). The latter intersect with each other and with the stratification planes to form an orthogonal network. It is along these fractures where the slow disintegration of the quartzite outcrops is initiated, which leads to the formation of large blocks that become detached from the crags by gravity.
The blocks fall at the foot of the quartzite crest, and slide or roll down the hillside depending on the slope and their weight.
When the block deposits form steep slopes they are very unstable and further secondary movements occur. As a final result these deposits form a pedrera with slopes of up to 20% and a variable thickness that may exceed 10 metres.
The blocks are currently covered in lichens and no detachments are being observed, which indicates a certain stability and also that the pedreras cannot have formed recently. It is very probable that this occurred under very cold and rainy conditions, which were typical of the periglacial climate of the Pleistocene (from 1.8 m.y. to 10,000 years ago).
