OUCC Proceedings 11 (1983)

The Topography of the Western Picos de Europa, Asturias, Northern Spain

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Stephen Gale

The western Picos de Europa is one of the world's classical areas of alpine karst. Its topography has been previously described by Crompton (1965), Miotke (1968), and Laverty and Ireland (1979, 30). The region broadly consists of a series of east-west aligned ridges, rising to altitudes of up to and over 2500 m. The high -mountain karst of the area is largely composed of bare rock surfaces which have been stripped of any superficial sediment, presumably by glaciation. Consequently, the micro- relief is highly structurally -controlled. Since the limestone is steeply dipping, bedding planes in particular are etched by solution to give the characteristic sharp-edged, vertically-aligned micro-relief of the area. Elsewhere, solution by meltwater and rainfall on the exposed limestone surfaces has been gravity-controlled and has resulted in well-developed rillenkarren, maanderkarren and kamenitza. On a larger scale, the area is pockmarked by closed depressions ranging from a few metres to several hundred metres in diameter. These are especially well developed on the northern sides of the mountain ridges where glacial act~ion has given rise to deep cirques backed by cliffed headwalls. These cirques retain snow for most of the year, particularly as plugs within fissures. Consequently, nivational processes, meltwater and rainfall all contribute to the further development of closed depressions within the cirques. These depressions often have vadose shafts in the bottom, although it is rarely possible to descend them to depths of greater than 20 in before they become choked by angular limestone boulders and cobbles. It is clear, however, that many of the closed depressions found in the area, both in the high-mountain karst and at lower altitudes, may have existed prior to at least the last glaciation, their form having been subsequently modified and exploited by glacial activity.

At lower altitudes, the limestone tends to be veneered by moraine and, as a consequence, streams are occasionally found. Nevertheless, such streams tend to be seasonally intermittent and to sink in blind or semi- blind valleys.

The entire range of the western Picos de Europa is bisected by the deeply-incised gorge of the Rio Cares, which reaches depths of up to 1000 in below the general surface of the mountains. Similar deeply -entrenched valleys are found along the courses of the Rio Dobra and the Rio Casafto (Frontispiece). According to Crompton (1965, 21), these valleys are the result of incision by glaciation and by glacial meltwater. On the basis of observations in the Rio Cares and the Rio Casafto, however, the former process appears unlikely. The valleys display all the characteristics of rapid fluvial incision; they are not tributary to the presumed ice source- areas around Pefia Santa de Castilla, and their own tributary valleys do not display hanging characteristics. Further evidence in support of the fluvial origin of the Rio Cares gorge is provided by the existence of fluvial, clast-supported cobble deposits on the wall of the gorge below Puente Bolin at about 50 in above the level of the present stream. These can only have been laid down by a river flowing approximately 50 m above the present level of the Rio Cares.

Exposed in the sides of the Rio Cares gorge are numerous cave entrances, most of which display phreatic origins. These can only have been formed when river and resurgence levels were much higher than those of the present. Some of these caves appear to be fossil resurgences, that is, ancient analogues of those resurgences such as Fuente Qiliembro and Fuente Puente Balm which are found at river level at present. A study of such caves and their infill may well elucidate the nature and rate of incision of the gorge across the entire massif of the western Picos de Europa.