Collserola / Barcelona (ES-SNE) - Spain

The Serra de Collserola rises between two sedimentary basins: the Vallès trough and, to the southwest, the coastal plain. The moderate height and gentle forms of these mountains indicate the great age of the Serra, which is a geological unit formed almost exclusively by igneous and metamorphic rocks from the Primary Era (Palaeozoic) affected by movements of the Earth?s crust known as Variscan (or Hercynian) Orogeny. During the first phase of the Variscan Orogeny, the lutitic rock (rock formed from sedimentary mud) was transformed into slate and phyllite, which are those most abundant types of rock in the Serra. Most of the quartz veins here were also generated during this phase. Later, magma intrusion took place, producing granitic rock; the sudden, sharp rise in temperature and pressure caused these granitoids to be altered and transformed them into enclosing rocks in a phenomenon known as contact metamorphism. The intrusion of granitic rock was accompanied by the appearance of porphyry dikes along the fractures. The coastal mountains rose during the Alpine Orogeny, which took place in the Secondary Era. Later, the Serra de Collserola took shape as a heightened block delimited by normal faults. The sea invaded the Barcelona plain and entered the Llobregat Valley. During this period, marine sedimentation gave rise to the formation of reef limestone, such as that found in El Papiol. More recently, during the Quaternary Period, the Serra was subject to erosion, whilst the sedimentation of detritus was a constant occurrence in the more depressed areas, such as the valleys and streams.