Geoparque Mundial UNESCO
Geoparque Villuercas > Nº02 – Cerro de San Cristóbal
The isolated hill known as the Cerro or Sierra de San Cristóbal has a maximum altitude of 680 m and is some 2500 m in length; it rises southwest of the town of Logrosán. Its granite dome stands some 200 m above the surrounding peneplain.
Easy access is afforded by a wide road starting from the Chapel of the Virgen del Consuelo on the southern slope of this sierra; it is possible to descend by a steeper route on the northwest side.
The granitic and filonian rocks of the Sierra de San Cristóbal and their rugged relief as a result of their resistance to erosion should be observed.
Attention should be paid to the remains of the mining deposit where the best cassiterite crystals in Europe have been found. These crystals (tin dioxide) were exploited from the earliest times until very recently. Trenches, galleries, and mining implements from the late Bronze Age related to the legendary kingdom of Tartessos (7th century B.C.) have been found on the summit of the hill. Tin was highly prized by the ancient peoples of the Mediterranean for the fashioning of bronze, but by the mid 20th-century these lodes of Logrosán cassiterite were being exploited exclusively for the manufacture of tinplate containers for the meat products of the industrial slaughterhouse of Mérida.
The summit affords to the north a view of the massive sloping walls of the sierra that extends from Zorita to Cañamero, and opposite of the Sierra de los Poyales and the Sierra de las Paredes. These sierras are in fact the upper or raised block of the lower or sunken block of the peneplain, which is the result of movement along a great fault that was produced by the last tectonic manifestations of Alpine mountain building. The upper block is the so-called Raised Block of Garciaz and the lower is part of the depression of the River Guadiana.
If we look to the east we can make out Cañamero, the Risco Gordo, and the Sierra del Pimpollar, and to the southeast the Rañas de Cañamero and Rañas de Logrosán (see the leaflets of these two geosites).
The Cerro de San Cristóbal is formed by a great variety of different types of granitic rocks or “piedras berroqueñas“. Geologically they can be classed as plutonic rocks, i.e. igneous or magmatic rocks that have solidified within the earth’s crust by the slow cooling of molten magma. When the latter rose towards the surface under great pressure, its temperature gradually lowered and the minerals of which it consists (essentially silicates) became solid and crystalline.
This granite outcrop is also known as the Batholite of Logrosán; it increases in size towards the inner crust in such a way that its depth cannot be known with any certainty. For some 300 million years erosion has worn down the shales that covered it. Today it stands out above the peneplain because its granitic rocks are more resistant to this erosion than the surrounding shales.
Within the granitic rocks of the batholite crystals of orthoses, plagioclases, quartzes, micas, tourmalines, apatites, etc., are abundant. Numerous veins of pegmatites, porphries, aplites, and quartzes with cassiterite can also be found, which are genetically related to the fluids emanating from the magma that gave rise to the batholite.
