Pyrenees

Geology

The Pyrenees represent the geologic renewal of an old mountain chain rather than the more recent and vigorous mountain-building process that characterizes the Alps. The Variscan (or Hercynian) orogeny, a mountain-building event that lasted from the late Devonian Period to the early Permian Period (a span of time extending from 370 million to 290 million years ago), generated the folded region that is now occupied by the present-day Pyrenees (see Hercynian orogenic belt). The remnants of the Variscan orogeny include the Massif Central in France and the Meseta Central in Spain. Although these other massifs have had a comparatively quiet history of internal deformation, or tectonism, since their emergence, the Pyrenean block was submerged in a relatively unstable area of Earth’s crust that became active about 225 million years ago.

The earliest formations, which were sediments severely folded over a granitic base, were submerged and covered by secondary sediments. They later were lifted once again into two parallel chains running to the north and south of the original Hercynian massif. These became the two zones of pre-Pyrenean ridges—of which the Spanish is the more fully developed—that are now great spurs of the main chain of the Pyrenees. This most-recent period of uplift was caused by the collision of the Iberian and European tectonic plates that began during the second half of the Cretaceous Period (100.5 million to 66 million years ago); however, the majority of the mountain building associated with this collision occurred during the Eocene and Oligocene epochs (56 million to 23 million years ago).

Under the forces of folding, the more recent and comparatively more plastic layers folded without breaking, but the original rigid base fractured and became dislocated. In the vicinity of the breaks, hot springs appeared and some metal-containing deposits formed. This upheaval affected chiefly the central and eastern regions. During this era, erosion continued incessantly, and, in the most exposed of the raised areas, weathering wore away the softer terrain and uncovered the old Hercynian sedimentary formations, occasionally reaching the deeper granitic bedrock.

Even today the old rocks, slates, schists, limestones transformed into marble (all of which come from old sediments transformed by great pressures and enormous heat), and granites of various kinds make up the spine, or axial zone, of the chain. The geologic phases of this zone, which rises and widens from west to east and ends by sinking, with a steep drop of nearly 9,800 feet, into the depths of the Mediterranean, have determined the evolution of the massif as a whole.

Physiography

The structure of the Pyrenees is characterized by patterns of relief and of underlying structure running in a north–south sequence (like the base rock); these alternate with depressions, some of which are the result of internal deformations, others of erosion of less resistant overlying deposits. In a cross section directly through the central area, where the tectonic activity reached its fullest width and development, it is possible to distinguish, from north to south, two strips of the comparatively recent pre-Pyrenean fold, one Spanish and one French, in juxtaposition with the axial massifs. An outer strip to the north consists of folds constituting the Petites Pyrénées. Cut into channels, they permit the passage of rivers. Nearer the middle of the range rise the Inner Ridges, represented by the mighty cliffs of the Ariège, which contain the primary, or granitic, axial zones. On the Spanish side the series is repeated in the opposite direction, but it is more highly developed and thicker. Thus the Interior Ridges—e.g., Mount Perdido and the massif of Collarada—are sometimes higher than the neighbouring primary axial peaks. They are followed, to the south, by a broad, pre-Pyrenean, middle depression, with a succession of marine and continental deposits of varying hardness that constitute the valleys of such tributaries of the Ebro as the Aragón. This depression continues across the rest of the pre-Pyrenean ridges, among which are new secondary outcrops that form the fringe of Exterior Ridges and the northern rim of the depression of the Ebro; they are not, however, as thick or as important as the Interior Ridges.

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From the structure of their relief and from the climatic conditions (especially on the south) that derive from the geographic situation of the chain, the Pyrenees have been divided into three natural regions: the Eastern (or Mediterranean), Pyrenees, the Central Pyrenees, and the Western Pyrenees. The different vegetation, the linguistic divisions of the people, and—to a point—certain ethnic and cultural distinctions appear to confirm this classification.

Drainage

The hydrographic system consists basically of series of parallel valleys that descend from the high peaks and from the passes. They are bordered by high, dividing ridges in a north–south direction, perpendicular to the axis of the chain. This type of valley produces short, torrential rivers that drop precipitously over short stretches; only seldom do these rivers flow, like the Aragón, through valleys that, as in the Alps, have both gentle slope and greater length. Their flow, extremely variable, especially on the southern side, is heavily influenced by the climate, as well as by the relief. Different maximum low waters occur in summer and winter; the spring, with maximum rain and melting snow, usually sees the greatest flows. In the Western Pyrenees and the northern zone, the rainfall pattern helps produce greater regularity; hence, flow is only slightly lower in summer. On the south a few torrential rivers are fed principally by melting snows, a few largely by rain, but most from a combination of sources. Heavy rains occasionally cause severe flooding in the region.

The river patterns and flow have been important since antiquity in human use of both the land and the rivers—from the floating of timber rafts downstream, which can be done only in the spring, to harnessing waterpower for industry and irrigation on the southern side by means of dams. The torrential flow of many of the rivers is the cause both of the purity of the Pyrenean waters and of their excellence and richness as fishing streams.

The present Pyrenean glaciers, perhaps more frequent on the northern than on the southern slopes, have been reduced to high basins—cirques or hanging valleys—at elevations over 9,800 feet. During and after the great Ice Ages (i.e., within the past 2.5 million years), however, especially in the Central and much of the Eastern Pyrenees, glaciers left widespread erosion and various important sediments. The present-day lower lakes and idyllic meadows with their winding rivulets are among their marks. Glacier tongues were also the main causes of the deep valleys containing the river system.

The fractured areas have many hot springs, both sulfurous and saline. The former are found throughout the axial massif, while the latter occur at the edges. These springs were popular in Roman times and reorganized and modernized toward the end of the 19th century. There are more than 20 famous spas on the French side; those in Spain are as numerous but are less fully exploited.