OUCC Proceedings 9 (1979)

by Martin Laverty and Pete Ireland

The Picos de Europa is an area of 'alpine karst' in that it is 'at a high altitude,..... at or above the tree line, .....where all the water is frozen for an extended period each year' (Werner, 1979). The main reference to academic work is Miötke (1968), and articles based on early OUCC expeditions are by Crompton (1964) and Walker (1966). Miötke is concerned almost exclusively with surface geomorphology, which he mapped, along with vegetation and snow cover from air photos. Most of the present landforms are glaciokarstic sensu lato (Ford, 1979), with fluviokarst at lower altitudes. The influence of pre-glacial forms is not clear, but post-glacial development is commonly seen, and scree deposits are seen overlying glacial drift, as at Largo Ercina. Glacial drift is present as superficial deposits on some valley floors and as moraines of impressive size. It contains small striated boulders in a clayey matrix and frequently gives rise to marshy conditions. Erratic boulders are common, but not exotic. The drift blankets a distinctive type of pinnacle formation of limestone at Buferrera, the pinnacles now having been exposed to extensive mining activities. It is difficult to know to what extent those features owe their form to fracturing and mineralisation as opposed to solutional processes.

The landscape of the Picos de Cornion is largely made up of closed depression of many different shape and sizes, separated by ridges and hills of likewise diverse form. Cliffs and steeply inclined slopes are common and can be attributed to glacial action. However, limestone pavements are not found because the strata generally dip too steeply. Valleys also occur, but are frequently dry over much, some, or all of their length, at least during the summer! The long profiles of these are often stepped, and glacial scour has often produced closed depressions along them which may now be bare or, if drift filled, marshy. Some of the closed depressions are 'true' poljes and are subject to flooding from the streams which traverse them, examples being the Las Reblagas depression into which some of the Ercina water drains, and the Jou Seco, where the stream may well be identical with that seen in the upstream and downstream active sections of Cueva del Frieru.

Cave Development

Walker (1966) considered that the caves then known in the Picos de Cornion fitted a developmental scheme comprising two cycles of erosion, each taking the form of solution followed by stal deposition followed by clastic deposition. One of these cycles would have happened in the Riss-Würm interglacial, and the other in the Holocene. Surface shafts were considered to continue developing under the action of snowbank meltwater.

With the development and application of accurate isotopic (and other) dating techniques such simple schemes are today being laid aside as intensively studied areas like the Dales and the Mendips, so it seems inappropriate to overinterpret the scanty evidence available today. However, it is possible and useful to point to the lines of enquiry which might be worth following up in much more detail than has so far been attempted. The clastic sediments will probably not be of great importance, because of lack of intelligible variety in the source areas and , also, their relative rarity, arising from the lack of surface drainage. Many pebbles and coarse sediments must be derived from the cave environment itself and modified therein, whereas finer material can enter through some of the fissures as the percolation water. Induration of deposits by calcite deposition are important in protecting deposits from later erosion and redeposition, and false floors may result from such a consequence of events. These features are very common in the caves of N Spain, and might well be worthy of further study to see whether it may be possible to correlate them from cave to cave, and perhaps to date them from associated stalagmite samples. Submerged or broken stalagmites may also be useful in studying the development of a cave. For example, the Martian Spaceship in Osu must have formed after the deposition of the sediments it rests upon, and before the downcutting of the stream undermined these. The layering of stal also contains the information on the periods when conditions favoured stal deposition. To illustrate this we can note that some 0.5m of recrystallised flowstone from the entrance to Pozu del Cantu del Hombre showed three main periods, defined by mud bands. At least 14 layers could be identified in all. Pozu del Xitu will, doubtless, be interesting to explore further as at least two separate periods of passage formation appear to be found here. Two periods can also be inferred in Osu and in Frieru. The draining of the phreatic conduits and initiation of vadose downcutting is to be related to the lowering of base levels by glacial erosion, but insufficient is known about this to be able to say much more.

The caves are clearly strongly influenced by geological controls in the form of faulting, jointing and bedding. Dominant trends show up clearly on, maps of both the surface and subterranean features, as illustrated by the Osu and Ario area surveys. The entrance series of pitches in Xitu occupy a fault breccia; those of Hombre, a massive calcite vein.

It is likely that the main time for cave development at present is during the spring melt period when considerable volumes of water must flood through the caves. The development expected in such 'flood phreatic' conditions is sketched by Palmer (1971). He says that the 'turbulent, solutionally aggressive water under unusually steep hydraulic gradients' (relative to the slow, laminar flow, low aggressiveness water typical of true phreatic conditions) leads to 'rapid development of diversion passages and blind solution pockets.....and the following features: (a) ungraded passage profiles, (b) spongework, (c) ceiling pockets, (d) large variations in cross-sectional area, even in zones of uniform solubility, (e) extensive and rapid deposition of clastic fill, (f) local maze patterns, and (g) extensive solution along exposed joints'.

These correspond to many of the features observed in the caves of this area, and with the annual regime expected.

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