La Pedriza: a particular granite batholith
La Pedriza is a particular mountain landscape, unique in the Iberian peninsula. Located in the Southern slope of the Sierra de Guadarrama, its topography is defined by a collection of amazingly shaped rocks and crags. It consists of a granite batholith, about 3.200 h wide, which formed in Paleozoic era, -in the carboniferous period- as result of the Hercynic orogeny (280 to 345 million years).
La Pedriza can be considered as a sierra having its own organization. The Eastern section is bordered by the Collado de la Ventana and the Collado de la Dehesilla hills. Its Western limit are the Cuerda de las Milaneras, La Pedriza circus, the Dehesilla stream source area, and the Peña del Yelmo, which is the highest spot in the South Western block. The Riscos de las Torres (2.029m) is the maximum height in La Pedriza.
La Pedriza holds a wide geomorphological diversity. The tectonic plate has split its surface in compartimental blocks, boosting fractures and diaclases. Their consolidation inside the bark land, the subsequent elevation and the erosion and alteration processes have contributed to form the current topography. These granite blocks include characteristic shapes such as: panholes, kettles, tafoni, alveoli, vaults, domes, crests, arêtes, rock pile formations, etc.
The panholes are depressions with a centimetre or decimetre depth and a circular or elliptical shape, caved on a more or less horizontal surface having decimetre to metre long axes. They are similar to the giant kettles, although there is no water flow arising them. The ones from el Yelmo are noticeable. The grooves are furrows appearing on slopes or inclined tracks, which are not completely vertical.
The tafoni are cavities in the inclined or vertical walls, with variable dimensions. When they are small they are called alveoli, and if their organization is more or less geometric, they are honeycomb highlights. When a tafone forms a basal concavity, it is called a vault, while when it belts a rocky block it becomes a lump, a pedestal or a button shaped rock. The pile formation crowning zones on the face of rock -as domes- are typical, (such as El Yelmo semi dome), and they hold a curvy morphology. On top of them can remain boulders, rocks detached from higher areas, ridges or tors. The rocky slopes modelled with curvy boulders, holding clasts from higher areas erosion, are also noticeable.
Though there are nodular gneisses -very typical of the Peñalara Massif- in the surrounds of Cuerda Larga area, the granite blocks clearly predominate in La Pedriza. They are coarse grain clear toned leucogranite, with variable biotite giving the rock a reddish tone as result of oxidation. There are also philonian rocks, organized as docks, which are distributed within granite masses and are made of microdiorite, aplite and granite porphyry. The most noticeable are the ones from La Charca Verde, which outcropped further to the Manzanares river erosion, and the ones from El Jaralón quarry (Eastern area from the La Dehesilla hill).
La Pedriza soils have a variable performance to erosion resulting in contrasts which give rise to a unique genuine granitic forms natural museum. Because of their peculiar shapes, the rocks hold names evoking common forms, as the Helmet (el Yelmo), the Bird (el Pájaro), the Seal (la Foca), the Camel (el Camello), the Little Elephant (el Elefantito), the Pig Stone (Canto Cochino), etc. Thanks to this geomorphology, La Pedriza is the most important climbing area from the Community of Madrid.