1911 Encyclopædia Britannica/Pyrenees
PYRENEES [Span. Pirinéos, Fr. Pyrénées], a range of mountains in south-west Europe, separating the Iberian Peninsula from France, and extending for about 240 m., from the Bay of Biscay to Cape Creus, or, if only the main crest of the range be considered, to Cape Cerbére, on the Mediterranean Sea. For the most part the main crest constitutes the Franco-Spanish frontier; the principal exception to this rule is formed by the valley of Aran, which belongs orographically to France but politically to Spain. The Pyrenees are conventionally divided into
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three sections, the central, the Atlantic or western, and the eastern. The central Pyrenees extend eastward from the Port de Canfranc to the valley of Aran, and include the highest summits of the whole chain, Aneto or Pic de Néthou (11,168 ft.), in the Maladetta ridge, Posets (11,047 ft.), and Mont Perdu or Monte Perdido (10,997 ft.). In the Atlantic Pyrenees the average altitude gradually diminishes westward; while in the eastern Pyrenees, with the exception of one break at the eastern extremity of the Pyrénées Ariégeoises, the mean elevation is maintained with remarkable uniformity, till at last a rather sudden decline occurs in the portion of the chain known as the Alberes. This threefold division is only valid so far as the elevation of the Pyrenean chain is concerned, and does not accurately represent its geological structure or general configuration. The careful examination of the chain by members of the English and French Alpine Clubs has since ISSO considerably modified the views held with respect to its general character; the southern versant, formerly regarded as inferior in area, has been proved to be the more important of the two. It has been recognized, as shown in the maps of MM. Schrader, de St Sand and Wallon, that, taken as a whole, the range must be regarded, not as formed on the analogy of a fern-frond or ish-bone, with thx lateral ridges running down to the two opposite plains, but rather as a swelling of the earth's crust, the culminating portion of which is composed of a series of primitive chains, which do not coincide with the watershed, but cross it obliquely, as if the ground had experienced a sidewise thrust at the time when the earth's crust was ridged up into the long chain under the influence of contraction. Both the orderly arrangement of these diagonal chains and the agreement which exists between the tectonic and geological phenomena are well shown in the geological and hypsometrical maps published in the Annuaire du Club Alpin fnmgais for 1891 and 1892 by MM. Schrader and de Margerie. The primitive formations of the range, of which little beyond the French portions had previously been studied, are shown to be almost all continued diagonally on the Spanish side, and the central ridge thus presents the appearance of a series of wrinkles with an inclination (from north-west to south-east) greater than that of the chain as a whole. Other less pronounced wrinkles run from south-west to north-east and intersect the former series at certain points, so that it is by alternate digressions from one to the other series that the irregular crest of the Pyrenees acquires its general direction. Far from having impressed its own direction on the orientation of the chain at large, this crest is merely the resultant of secondary agencies by which the primitive mass has been eroded and lessened in bulk, and though its importance from a hydro graphic point of view is still considerable, its geological significance is practically nil. . Geology.-The Pyrenees are divided by E. de Margerie and F. Schrader into a number of longitudinal zones. The central zone consists of Primary rocks, together with great masses of granite. It forms most of the higher summits, but west of the Pic d'Anie it disappears beneath an unconformable covering of Cretaceous deposits. On the French side the central zone is followed by (1) the zone of Ariege, consisting of Lower Cretaceous and Jurassic beds, together with granitic masses; (2) the zone of the Petites Pyrénées, Upper Cretaceous and Eocene; and (3) the zone of the Corbiéres, consisting of Eocene and Primary rocks. On the Spanish side, from north to south, are (1) the zone of Mont Perdu, Upper Cretaceous and Eocene; (2) the zone of Aragon, Eocene; and (3) the zone of the Sierras, Trias, Cretaceous and Eocene. In France the zones are clearly defined only in the eastern part of the chain, while towards the west they merge into one another. In Spain, on the other hand, it is in the central part of the chain that the zones are most distinct. Although the number of zones recognized is the same on the two flanks, they do not correspond. The zone of the Corbiéres has no equivalent in Spain, while in France there is no definite zone of Eocene like that of Aragon. The zone of the Petites Pyrénées, however, is clearly homologous with that of the Sierras. On the northern side granitic masses occur in the zone of Ariege amongst the Jurassic and Lower Cretaceous beds. On the southern side they are not found except in the axial zone, and the Jurassic and Lower Cretaceous deposits are reduced to a narrow band. ' in spite of these differences between the two flanks, the structure is to some extent symmetrical. On the north the greater number of the over folds lean towards the north, while on the south they lean towards the south. Thus the chain shows the typical fan-structure which has long been recognized in the western Alps.
Since the publication of the maps by de Margerie and Schrader it has been shown that the phenomena of “ recouvrement " play almost as large a part in the Pyrenees as in the Alps themselves. Large masses of rock have been brought upon nearly horizontal faults (thrust-planes) over the edges 'of either beds with which they originally had no connexion. In the region of Salies-du-Salat, for example, patches of Trias lie discordantly upon the edges of the Cretaceous and Tertiary beds. Several other similar cases have been described; but denudation has been carried further than in the western Alps, and accordingly the masses overlying the thrust-planes have been more completely removed (q.v.).
The earth movements which raised the Pyrenees appear to have begun in the Eocene period, but it was in Oligocene times that the principal folding took place. The Pyrenees are therefore contemporaneous with the Alps; but they appear to have escaped the Miocene disturbances which affected the latter.
The arrangement of the Pyrenees in chains gently inclined near the centre but longitudinal everywhere else, is illustrated by the courses of the streams which flow down towards Spain. On the French side most of the longitudinal valleys have disappeared; and this is why the range has so long been described as sending out transverse spurs, the more important slope remaining unknown. It is, however, still possible to distinguish some traces of this formation towards the east, where atmospheric denudation has been less active. On the south the principal streams, after cutting their way through the highest zone at right angles to the general direction of the range, become involved half-way to the plains in great longitudinal folds, from which they make their escape only after traversing long distances without finding an outlet.
The importance shown to attach to the Spanish versant has greatly modified the values formerly assigned to the area and mean elevation of the Pyrenees. Instead of the 13,440 sq. m. formerly put down for the total, M. Schrader found the area to be 21,044 sq. m. Of this total 6390 sq. m. fall to the northern slope and 14,654 sq. m., i.e. more than double, to the southern, the difference being mainly due to the zone of plateaux and sierras. The mean elevation, estimated by Élie de Beaumont at 1500 metres (4900 ft.), has been sensibly diminished by the addition of that zone to the system, and it must now be placed at only 1200 metres (3930 ft.) for the range as a whole; so important a part is played b the above-mentioned plateaux of small elevation in a chain whose highest summit reaches 11,168 ft., while the passes show a greater altitude than those of the Alps.
Four conspicuous features of Pyrenean scenery are the absence of great lakes, such as fill the lateral valleys of the Alps; the rarity and great elevation of passes; the large number of the mountain torrents locally called gaves, which often form lofty waterfalls, surpassed in Europe only by those of Scandinavia; and the frequency with which the upper end of a valley assumes the form of a semicircle of precipitous cliffs, locally called a cirque. The highest waterfall is that of Gavarnie (1515 ft.), at the head of the Gave de Pau; the Cirque de Gavarnie, in the same valley, is perhaps the most famous example of the cirque formation. Not only is there a total lack of those passes, so common in the Alps, which lead across the great mountain chains at a far lower level than that of the neighbouring peaks, but between the two extremities of the range, where the principal highroads and the only railways run between France and Spain, there are only two passes practicable for carriages—the Col de la Perche, between the valley of the Tet and the valley of the Segre, and the Col de Somport or Pot de Canfranc, on the old Roman road from Saragossa to Oloron.
Projects for further railway construction, including the building of tunnels on a vast scale, have been approved by the French and Spanish governments (see Spain: Communications).
The metallic ores of the Pyrenees are not in general of much importance, though there are considerable iron mines at Vic de Sos in Ariège and at the foot of Canigou in Pyrénées-Orientales. Coal deposits capable of being profitably worked are situated chiefly on the Spanish slopes but the French side has numerous beds of lignite. Mineral springs are abundant and very remarkable, and specially noteworthy are the hot springs, in which the Alps, on the contrary, are very deficient. The hot springs, among which those of Bagnères de Luchon and Eaux-Chaudes may be mentioned, are sulphurous and mostly situated high, near the contact of the granite with the stratified rocks. The lower springs, such as those of Bagnères de Bigorre (Hautes-Pyrénées), Rennes (Aude) and Campagne (Aude), are mostly selenitic and not very warm.
The amount of the precipitation, including rain and snow, is much greater in the western than in the eastern Pyrenees, which leads to a marked contrast between these sections of the chain in more than one respect. In the first place, the eastern Pyrenees are without glaciers, the quantity of snow falling there being insufficient to lead to their development. The glaciers are confined to the northern slopes of the central Pyrenees, and do not descend, like those of the Alps, far down in the valleys, but have their greatest length in the direction of the mountain chain. They form, in fact, a narrow zone near the crest of the highest mountains. Here, as in the other great mountain ranges of central Europe, there are evidences of a much wider extension of the glaciers during the Ice age. The case of the glacier in the valley of Argelès in the department of Hautes-Pyrénées is the best-known instance. The snow-line varies in different parts of the Pyrenees from 8800 to 9200 ft. above sea-level.
A still more marked effect of the preponderance of rainfall in the western half of the chain is seen in the aspect of the vegetation. The lower mountains in the extreme west are very well wooded, but the extent of forest declines eastwards, and the eastern Pyrenees are peculiarly wild and naked, all the more since it is in this part of the chain that granitic masses prevail. There is a change, moreover, in the composition of the flora in passing from west to east. In the west the flora, at least in the north, resembles that of central Europe, while in the east it is distinctly Mediterranean in character, though the difference of latitude is only about 1°, on both sides of the chain from the centre whence the Cobières stretch north-eastwards towards the central plateau of France. The Pyrenees are relatively as rich in endemic species as the Alps, and among the most remarkable instances of that endemism is the occurrence of the sole European species of Dioscorea (yam), the D. pyrenaica, on a single high station in the central Pyrenees, and that of the monotypic genus Xatardia, only on a high alpine pass between the Val d'Eynes and Catalonia. The genus most abundantly represented in the range is that of the saxifrages, several species of which are here endemic.
In their fauna also the Pyrenees present some striking instances of endemism. There is a distinct species of ibex (Capra pyrenaica) confined to the range, while the Pyrenean desman or water-mole (Mygale pyrenaica) is found only in some of the streams of the northern slopes of these mountains, the only other member of this genus being confined to the rivers of southern Russia. Among the other peculiarities of the Pyrenean fauna are blind insects in the caverns of Ariège, the principal genera of which are Anophthalmus and Adelops.
See H. Beraldi, Cent ans aux Pyrénées (1901), Les Sierras, cent ans après Ramond (1902), Après cent ans. Les Pics d'Europe (1903), and Les Pyrénées orientales et l'Ariège (1904); P. Joanne, Pyrénées (1905); H. Belloc, The Pyrenees (1909); for geology, in addition to the papers cited above, A. Bresson, Études sur les formations des Hautes et Basses Pyrénées (Paris, Ministère des Travaux Publics, 1903); L. Carez, La Géologie des Pyrénées françaises (Paris, Min. des Tr. P., 1903, &c.); J. Roussel, Tableau stratigraphique des Pyrénées (Paris, Min. des Tr. P., 1904); and for climate and flora T. Cook, Handbook to the Health Resorts on the Pyrenees, &c. (1905), and J. Bentham, Catalogue des plantes indigènes des Pyrénées et de Bas-Languedoc (1826).