The Sierra de Guadarrama has a wide geological diversity, according to its soil composition and its origin. The range goes from 500 million years old rocks, as gneissic ones, to more recent sedimentary accumulations, as for example the limes, sands, and gravels the water courses system provides, or the Quaternary glaciers formations, that modelled the landscape on circuses, moraines and high mountain lakes. In addition, the granite batholith from La Pedriza, is very relevant, since it has been designated a Geological Point of Interest and a Biosphere Reserve, remaining a unique and singular formation in the Iberian Peninsula.
Although these sites currently belong to the Sierra de Guadarrama National Park, their earliest protection status goes back to 1930, when La Pedriza along with the Summit, Peñalara Cirque and Lakes were declared Natural Sites of National Interest.
During the Paleozoic, 380 to 280 million years ago, the collision of tectonic plates caused a mountain genesis global event, the Hercynian orogeny. This geological process gave rise to the formation of mountain ranges, and the subsequent formation of many Sierra de Guadarrama rock grounds, as granitic ones. The origins of gneissic rocks are previous, but they emerged to the earth´s surface by reason of the genesis event. During the Mesozoic (252 to 72 million years ago), the Hercynian system eroded almost entirely and provided the sedimentary rocks -made of sands, limestones and dolomites. Then a new tropical sea appeared. By the end of Mesozoic era, about 80 million years ago, another mountain formation process occurred: the Alpine orogeny.
The Sierra de Guadarrama was formed during the latter orogeny, when the Central System emerged as a mass of sunken and raised blocks (Grabens and Horsts) further to the African and Iberian plates collision.
Subsequently, the mountains eroded one more time, creating the Sierra de Guadarrama water courses system, and providing new sediments. These processes, along with the Quaternary glacier activity and the periglacial processes -started 2,6 million years ago, and still active- have given rise to the current topography of the Sierra de Guadarrama, such as the Peñalara Massif and Cuerda Larga.
Types of rocks
The geological substrate of the Sierra de Guadarrama is made of different types of rocks having very diverse origin -plutonic, metamorphic, sedimentary and philonian-. Most are very ancient rocks from Paleozoic and Mesozoic- the eldest being gneisses, marbles and schists.
Gneisses are metamorphic rocks whose original minerals have undergone many changes as a result of extremely high temperatures and squezzing inside the earth’s crust. The minerals composition is quartz, feldspar and micas, similar to granite, but with a narrow bands structure, made of different minerals. Sometimes, nodules and feldspar or quartz-feldspar crystals -porphiroblasts- are observed, in which case they are known as nodular gneisses.
They are the dominant rocks in the Mountain range of Guadarrama, and they are among the most ancient ones, about 450 million years old.
According to its origin, the Sierra de Guadarrama gneisses of are the orthogneis type, that is, they are igneous or magmatic, and non-sedimentary origin. The nodular orthogneiss is the predominant in the Sierra de Guadarrama National Park; Siete Picos, Cuerda Larga and Montes Carpetanos, including the Lozoya and Eresma valleys, are mainly made of this type of rock.
The schists are another type of metamorphic rock, not as common as the preceding. They are made of mica and minerals such as quartz and feldspar. These rocks hold a property known as schistosity, which consists of splitting along irregular and more or less parallel planes. They are common in the middle and lower Lozoya valley grave, and particularly in the North-East area of the Sierra de Guadarrama. Other minerals, such as garnets and staurolite appear as part of the micaceous schists.
There are small outcrops of marbles, such as the Peñalara Massif, whose origin is limestone -so its predominant composition is calcium carbonate- and is usually associated with a singular basophilic flora.
The metamorphism of all these rocks is associated with the Hercynian Orogeny, at a depth over 30 km, pressure about 10 kb and temperatures exceeding 750 °C.
The granites of the Sierra de Guadarrama are igneous rocks that formed during the Carboniferous, in Hercynian Orogeny, as a result of the solidification of the magma on the terrestrial crust. Its great depth and slow cooling process produced mineral crystals. This type of rocks, along with the gneisses, are prevalent in the Sierra de Guadarrama.
Granites are composed of quartz, feldspar and mica -usually biotite- but also other minerals and they classify depending on minerals and temperature. The ones prevailing in the Sierra de Guadarrama are the leucogranites and the monzogranites. They can be found in Siete Picos, La Cabrera and La Pedriza. The ones from La Pedriza are coarse grain and clear coloration leucogranites, with variable concentration of biotite, whose oxidation gives the granite a reddish tonality.
The philonian rocks originate in the interior of the terrestrial crust, by solidification of the magma ascending to the outside by cracks or fissures. They usually form two types of tabular intrusions: dikes and reefs. They usually are intrusive igneous rocks.
In the Sierra de Guadarrama, there are philonian rocks in La Pedriza, as dikes within the mass of granite rock.
The Sedimentary rocks are made by the sediments accumulation, with a subsequent process of compaction, and a more or less consolidated structure. Until the end of the Cretaceous, sands, limestones and dolomites settled on the coasts and tropical seas that existed at that time.
The Sierra de Guadarrama holds three types of sedimentary rocks: marls, limestones and dolomites, aged from Cretaceous era. Marls are sedimentary rocks composed of a mixture of calcite (calcium carbonate) and fine sediment, usually clay. Limestones are another type of sedimentary rocks composed of calcite (calcium carbonate with rhombohedral crystalline structure). Dolomites are sedimentary rocks formed by dolomite -mineral made of calcium carbonate and magnesium-.
These rocks can be found in valley graves, such as Pinilla del Valle, having been exposed on the surface due to river erosion and also on the South-East slope of the Sierra de Guadarrama.
In connection with processes of erosion and water transport and settlement, stand out the fluvial formations of the Quaternary -gravel, silt and sands- from rivers flood terraces and plains.
Other processes influencing the topography of the Sierra de Guadarrama have been the Quaternary glacial and periglacial ones. They are rather sedimentary processes, and not rock formation as they were unconsolidated. These processes, particularly bold in colder times -in periods of glaciations- had a deep influence in the Sierra de Guadarrama topography and landscape. Within the National Park, the glaciology of the Peñalara Massif -with its circuses, moraines and lakes- must be emphasized. Periglaciation is still active in the peaks of the Sierra de Guadarrama.