Geological Regions: Siliceous, Limestone, and Shale
Siliceous terrain comprises ancient rocks from the Precambrian and Primary eras. It is mostly in the western peninsula (Galicia, León, Extremadura) and has ramifications to the western part of the Cantabrian mountain range, the CS, Montes de Toledo, and Sierra Morena. It is also located in isolated areas such as the axial zone and some sectors of the Pyrenees, Iberian system, coastal mountain range, and the Catalan-Penibético system. The prevailing rock is granite, and the landscape is determined by the characteristics of this crystalline and rigid rock, which is sensitive to different forms of disturbance. In some cases, granite is chemically altered in depth by water: the crystals are decomposed and transformed into pardoamarillentas arenas. In other cases, altered granite occurs, starting with a network of joints which results in high and rocky crests, needles, and joints striking parallel in lowlands and scaling. Also, if a joint perpendicular occurs, scree and granite formations are formed.
Limestone Formations and Karst Relief
Limestone is formed by sediments of the Secondary era, folded during the Tertiary. The limestone soils form an inverted Z that extends from the Pyrenees, Basque Mountains, eastern sector of the Cantabrian Mountains, Iberian System, part of the Subbetic mountain range, and Catalan system. The prevailing rock is limestone, a hard rock that fractures, forming cracks or striking joints, but it dissolves easily in rainwater, especially through the joints. This creates a complex relief, karst relief, whose characteristics are the following:
- Sinkholes: Long and sharp grooves produced by the solution of limestone by the action of streams.
- Gorges, Gorges, Gorges: Narrow, deep valleys, surrounded by steep slopes, caused by rivers.
- Polje: Horizontal bottom closed depressions where water disappears to continue underground.
- Dolines or Torques: Cavities that arise where water stagnates, can take many forms and bond with one another.
- Caves: Created by water seeping through fissures in the ground.
- Depths: Narrow openings that connect the surface with the underground galleries.
Shale and Badlands Topography
Shale is made up of little resistant sedimentary materials (clay, marl, gypsum) deposited in the late Tertiary and the Quaternary. It also covers a large proportion of depressions in the northern and southern sub-plateau, the depressions of the Ebro and Guadalquivir, and the Mediterranean coastal plains. The clay is basically horizontal, as are areas not affected by subsequent folding. Its rapid erosion is due to the softness of the materials. The rivers open valleys that separate horizontal structures, which are soon eroded, resulting in undulating reliefs. In areas where long dry periods alternate between hot and heavy rains, short and try, and there is no plant protection (SE mainland), water erodes the sides of streams, resulting in alleyways or narrow and deep grooves separated by ridges, creating a steep topography like a miniature mountain. Its wide development over an area gives rise to the landscape called badlands.