Basque Country Relief: Pyrenees, Mountains, and Ebro Depression
The Basque Country’s diverse relief is a result of alternating periods of orogenic calm and activity, erosion, sedimentation, and ice ages. It can be divided into three main units: the Pyrenees, the Basque Mountains, and the Ebro Depression.
The Pyrenees
Within the Pyrenees, three distinct areas can be identified: the Axial Pyrenees, the Southern Pyrenees, and the Northern Pyrenees.
The Axial Pyrenees
Composed of ancient Paleozoic massifs like Larrun, Five Villas, or Peñas de Aya, the Axial Pyrenees are primarily formed of Paleozoic siliceous material uplifted during the Hercynian orogeny. This period of intense tectonic activity caused the uplift of various massifs across the Iberian Peninsula, including those in the Basque Country. These formations were further elevated during the Alpine Orogeny. Their hard, fractured materials resulted in a horst and graben structure. Igneous and metamorphic rocks are common. Periglacial processes, particularly gelifraction (freeze-thaw weathering), significantly shape the landscape.
The Southern Pyrenees
This area comprises the Inland Mountains, the Interior Depression, and the Outer Ranges.
The Inland Mountains
The longest and highest sector of the Basque Pyrenees, the Inland Mountains are predominantly limestone, uplifted during the Tertiary period, and characterized by karst landscapes. Surface features include sinkholes, while caves and potholes dominate the subsurface.
The Interior Depression
Located in the Alava plains, stretching from Lumbia to the Araquil Corridor, this depression consists of Tertiary clay materials.
The Outer Ranges
Formed by the Izku and Leire mountain ranges, these are a continuation of the Cantabrian Mountains and the Urbasa Range. Limestone is the dominant material, resulting in prevalent karst landscapes.
The Northern Pyrenees
Situated in the French Basque Country, this area features gentle topography, broad valleys, and river-eroded landscapes. Sedimentary materials predominate.
The Basque Mountains
The Basque Mountains are divided into two sectors based on the watershed: north and south. Rivers north of the watershed flow into the Cantabrian Sea, while those to the south flow into the Mediterranean.
North of the Watershed
This area includes the Tertiary Coastal Range, the Northern Coastal Anticlinorium, the Biscay Synclinorium, and the Biscay Anticlinorium. The Tertiary Coastal Range extends to the Jaizkibel alignment and consists of alternating layers of limestone and quartz.
The Northern Coastal Anticlinorium
Extending from Five Villas to Cape Matxitxako, this area is composed of Tertiary limestone, resulting in a predominantly karst landscape.
The Biscay Synclinorium
Located between Punta Galea and Five Villas, this area features rounded peaks, wide valleys, and river gorges.
The Biscay Anticlinorium
Running from Five Villas to Ordunte, this area is characterized by karst landscapes with underground limestone formations.
South of the Watershed
This area comprises the Alava Plains, the Western Highlands, the Central Highlands, and the Southern Mountains.
The Alava Plains
Composed of sedimentary materials.
The Western Highlands
A series of hills surrounding the Alava Plains, composed of limestone.
The Central Highlands
Separating the Alava Plains from the Ebro Depression, these mountains are formed of limestone uplifted during the Alpine Orogeny.
The Southern Mountains
Separating the Alava Plains from the Rioja Alavesa region, these mountains are primarily limestone.
Two distinct morphogenetic systems operate within the Basque Mountains. North of the watershed, abundant rainfall leads to solifluction processes. South of the watershed, a drier climate and successive ice ages have resulted in river terraces.
The Ebro Depression
Located amidst the Pyrenees, Basque Mountains, and Catalan Coastal Range, the Ebro Depression in the Basque Country lies between the Southern Pyrenees and the hills south of the watershed. Formed from the sunken Paleozoic Ebro Massif during the Hercynian and Alpine orogenies, it became isolated from the sea and filled with water. During the Quaternary period, sediments from surrounding mountains accumulated while the water evaporated. Clay and sedimentary materials predominate, creating a tabular landscape easily eroded by water, resulting in gullies, hills, and badlands. Over time, river terraces have formed.