The granite boulder area or berrocal of Peraleda de San Román extends from the River Gualija to the west to the vicinity of Valdelacasa de Tajo to the east. The River Tajo crosses it between the Arab fortresses of Espejel and Alija to form a deep canyon, cleft, or defile of almost vertical walls that are today mainly covered by the Valdecañas Reservoir. The “Cancho Castillo” is a huge “tor” situated in the berrocal of the common grazing land of Peraleda. It can be reached by a winding road some 5 Km long that leaves from the junctions of the Calvario de Peraleda towards Laguna Vieja, which is also located on the common grazing ground.
A berrocal is the landscape that results from the slow erosive process known as chemical meteorisation; this is caused by the chemical components of the atmosphere acting statically to destroy the granitic rocks, and is characterised by the formation of a large number of rounded blocks, boulders, “mushroom rocks” and “tors” scattered all over the surface of the batholiths. The granitic rocks have been superficially eroded following the fracture planes (diaclases) enlarged by the action of the atmospheric water that attacks and breaks up the minerals that make up the granite (quartzes, feldspars, and micas) to shape the berrocal over time. In Extremadura berrocales are known as berruecos or barruecos.
The granites of the Berrocal de Peraleda are essentially fine-grained with two micas, although others can also be found with large crystals of orientated feldspars and lodes of aplites, pegmatites, and quartzes. They are related to the numerous parallel fracture lines in a northeast-southwest direction that limit and compartmentalise the interior of this huge batholith, which are made use of by the streams that originate on the Sierra de la Breña and flow into the nearby River Tajo. Tertiary materials from the Pliocene age (rañas) have been deposited on these granites; they consist of clays, sands, and quartzite boulders from the sierras of Las Villuercas. The rañas platforms of El Madroñal, the Dehesa Boyal (common grazing ground), Las Cuevas, and Las Porquerizas are significant; they are separated from each other by the encasing of the river network along the fracture lines.
We can observe the curious eroded shapes of the berrocal that are the result of the chemical meteorisation of the granitic rocks: the rounded blocks, the “mushroom rocks” and the “tors”, the caves, etc., together with the various fracture planes (diaclases) that have determined the formation of blocks of varying sizes within these massive granitic rocks.
It is particularly interesting to make an archaeological analysis of the “magic place” of the Cancho Castillo, the large cavity in the lower part of which could have served as a shelter for shepherds and was perhaps a rock sanctuary for the first colonists of this area. This is borne out by pottery remains and numerous cave engravings at its base: pans, anthropomorphs, zoomorphs, soliforms…, that are witnesses of the passage of Neolithic and Chalcolithic peoples and even Romans with a Latin inscription. With its characteristic shape of a huge scallop or the prow of a ship, it is not surprising that it has attracted religious devotion and been a place of mystery since prehistoric times. The Cancho Castillo gives the berrocal of Peraleda de San Román great archaeological value together with its curious geomorphology.