Spain’s Diverse Relief: A Geological Journey
Clayey Areas
Composed of Tertiary and Quaternary sedimentary rocks, these areas are located in the northern and southern sub-plateau basins, the Ebro and Guadalquivir depressions, and sunken areas within the Mediterranean coastal plains. The predominant clay rock, characterized by low resistance, results in a primarily horizontal relief. Where the land hasn’t experienced subsequent folding, two relief types emerge:
- Rivers carve valleys, separating horizontal structures.
- In areas with alternating dry, hot periods and short, torrential rains, and lacking plant cover, streams carve deep channels, forming gullies.
Relief from Differential Erosion
a) Horizontal Strata: When strata are horizontal with alternating hard and soft layers, rivers carve valleys that separate broad, flat platforms known as mesas, plateaus, or alcarrias. These horizontal platforms are formed by the hard layers, while the concave flanks coincide with the softer stratum beneath.
b) Inclined Strata: Gently inclined strata with alternating hard and soft materials form slopes. These slopes have a back or reverse side consisting of the inclined hard layer, and a front with a steep ledge and a concave bottom where erosion occurs more rapidly.
The Plateau
This high plateau, situated around 800 meters, formed during the primary era through ancient Hercynian erosion. Tertiary deformation and significant destruction during the Alpine orogeny resulted in the following distinctions:
a) The Old Paleozoic Basement: Tertiary erosion removed material covering the basement, exposing raw siliceous materials. The relief consists of peneplain surfaces, either gently undulating or eroded. Rugged island mountains, or residual reliefs of resistant rock, interrupt the peneplain.
b) Inner Mountain Ranges: The Central System and Montes de Toledo, formed during the Tertiary by the uplift of basement blocks due to the Alpine orogeny, are composed of primary bedrock with rounded shapes and flat summits.
c) Internal Sedimentary Basins: The northern and southern sub-plateaus formed during the Tertiary when the plateau’s basement subsided due to the Alpine orogeny. Initially lakes, these basins filled with horizontally arranged Tertiary materials, with softer layers at the bottom.
Mountain Ridges of the Plateau
a) The Galician-Leonese Massif: Formed in the Tertiary by the uplift of the plateau’s northwest corner during the Alpine orogeny, this massif consists of Paleozoic materials and features low, rounded hills.
b) Cantabrian Mountains: This range has two distinct sectors:
- The western sector, the Asturian Massif, formed during the Tertiary by the uplift of this section of the plateau’s basement. Its Paleozoic materials and varying hardness have led to an Appalachian-type relief. Its eastern end features a massive primary limestone outcrop, home to Europe’s highest peaks.
- The eastern sector, the Cantabrian Range, formed in the Tertiary by the folding of secondary materials deposited by the sea at the plateau’s edge.
c) Iberian System: This intermediate ridge formed in the Tertiary by the folding of secondary materials deposited by the sea on the eastern edge of the plateau’s basement. Its materials are primarily limestone.
d) Sierra Morena: Not a true mountain range, but a steep escarpment separating the plateau from the Guadalquivir valley. Likely a large flexure fractured in many places, it formed in the Tertiary due to the southward thrust that uplifted the Baetic System.
External Depressions of the Plateau
a) Ebro Depression: Parallel to and enclosed by the Pyrenees, this depression occupies the site of the ancient Ebro Massif, which subsided as the surrounding mountain ranges rose. Initially a sea, it became a lake until the end of the Tertiary, accumulating marine and continental materials.
b) Guadalquivir Depression: Parallel to the Baetic System, this depression lies between these mountains, Sierra Morena, and the Atlantic Ocean. Initially open to the sea, it transformed into a coastal lake or lagoon, then into marshes, filling with clay, limestone, and marl.
External Mountain Ranges of the Plateau
a) Pyrenees: These mountains have a complex structure:
- The Axial Zone corresponds to the ancient Hercynian Massif of Aquitaine, rejuvenated during the Alpine orogeny. It features Paleozoic materials and rugged relief.
- The Pre-Pyrenees, located south of the Axial Zone, formed during the Tertiary Alpine orogeny by the folding of materials deposited in the Pyrenean foredeep. They are composed of limestone and have a gentler relief.
- The Median Depression is a long, narrow depression separating the Axial Zone from the Pre-Pyrenees.
b) Basque Mountains: Largely an extension of the Pre-Pyrenees, these mountains are primarily composed of limestone, with gentle relief and moderate altitude. The axial Pyrenees, with Paleozoic bedrock, only emerge at the eastern end.