When entering the municipal district of Villar del Pedroso from El Puente del Arzobispo, take a road on the right immediately after passing the bridge of the same name over the River Tajo; follow the path nearest the watercourse and you will finally reach the defile after a route of some 2 km. From its beginning at an old mill to its joining the Tajo the defile is some 750 m long.
To the east of the municipal district of Villar del Pedroso can be found granitic rocks accompanied by both filonian rocks and metamorphic rocks (slates and greywackes formed from sedimentary rocks subjected to high pressure and temperatures) from the Upper Precambrian period that are over 600 million years old. Structurally these rocks belong to the deepest part of the so-called Valdelacasa anticline, which has now been completely worn down by the erosive processes that acted as from its emersion from the ocean by Hercynian orogeny some 300 million years ago to form the current extensive peneplain.
Near the town of Villar del Pedroso the granite is coarse-grained with two micas and large crystals of orthose feldspar. However, where the Arroyo del Pedroso has dug its deep watercourse the granite is very fine-grained aplite (“microgranite”) that is very resistant to erosion. These are very hard rocks that have however been intensely fractured by two families of orthogonal diaclases which facilitates their fragmentation into blocks and the encasing of the Arroyo del Pedroso that seeks its “base level” in the Tajo trench. These aplites rise to form walls of rock many metres high, which have resisted erosion and maintain a delicate balance.
On these granites and on the Precambrian slates that occur between Villar del Pedroso and El Puente del Arzobispo, a thick layer of Pliocene sediments (some 3.5 million years old) has formed: it consists of quartzite boulders joined by a reddish-clayey matrix known as rañas.
We can observe the curious erosive forms that are the result of the chemical and mechanical meteorisation of the granite rocks (rounded blocks, “mushroom stones” and “tors“; and the various fracture planes (diaclases) that have determined the formation of blocks of different sizes within the massive granite rocks and in the aplite dikes.
The encasing of the Arroyo del Pedroso within these rocks is spectacular and has led to the formation of rapids, waterfalls, and “potholes” or pilancones. When the water of the Arroyo is running high it is not possible to cross to the other bank; the roar of the water falling into the deep canyon is awe-inspiring.
The excursion can be completed with a visit in the vicinity to the ruins of several old mills and the impressive fortress of Castros, its walls, the columns of the bridge over the River Tajo and the surrounding houses; and the Muslim fortress which was built in the 10th century adjacent to the nearby Islamic city of Vascos so as to defend the middle frontier of the Tajo.
In a plaza of Villar del Pedroso we can also admire several zoomorphic sculptures (“verracos”) executed by the Vettonian people of the Iron Age, together with numerous Roman inscriptions of great archaeological interest.