British Speleological Expedition to the Cantabrian Mountains of Northern Spain, 1965
in conjunction with the "Espeleologos de Penalba", Leon
|British Speleological Expedition 1965 Report|
Earlier studies on the Enol Platform in the Picos de Europa (Asturias) had shown that cavern formation was largely formed in response to a fall in base-level caused by a reduction in pressure on the area, following the melting of 1000 m. thickness of ice, It had been observed by Spanish workers that this corresponded to a relative rise of 100 m, of the massif c Post-glacial nivation melt-waters were doubtless also responsible for this extremely rapid cave development throughout the Picos de Europa. However, large systems were only found by previous expeditions where influent streams fed the caves continually, This situation had only applied in about 6 caves. Therefore the expedition chose to visit first in 1965 the mountains flanking the canyon of the Rio Torio in Leon, an area of limestone dipping at over 50 degrees 7, but nevertheless less steeply than the vertically bedded strata of the Picos de Europa, Unlike the Picos de Europe, this group of Leonese mountains had escaped the Alpidic mountain-building phase, and there was a chance that caves might still be found which owed their origin to much older erosion cycles than can now be discovered in the high Picos de Europa.
One cave, indeed, seemed as if it might bear out the hypothesis. This was the Cueva de Valporquero, reported to receive an influent stream by the Espeleologos de Penalba, The stream drains a wide area of the Sierra del Gato three or four kilometres to the west of the cave entrance. The hills rise to about 1700 me, and the cave entrance was at 1309 m above sea-level. Unfortunately from the sporting point of view, the cave broke out into the side of the Torio canyon about 1 km to the east. The cave is developed along the strike of Devonian limestone lying adjacent and to the north of the Namurian beds. The vast proportions of the chambers of this cave, and the large amounts of stalactite formations and other deposits in the upper series of the cave, suggest a very much longer period of evolution for the Cueva de Valporquero than known for any of the caves visited in previous years in the Picos de Europa The re-excavation of the deposited material in the upper part of the system suggests that the cave is at present undergoing a new and second solutional phase.
The smaller Fly Pot Cave discovered above the entrance to the Cueva de Valporquero in the same valley was presumably also related to the drainage of the area. It is now no longer an active system. To the south of the entrance to the Cueva de Valporquero are various large poljes. Some have been subjected to terrace cultivation, but in one uncultivated polje the expedition base camp was set up. A small shaft on the northern edge of this polje led down to water usually, according to the Spaniards, but in 1965 it was dry.
A large doline whose floor is at 1500 m above sea-level occurs half a mile to the south of the base-camp. On the NW lip of the doline, a shaft was descended to -8 m. The SE edge of the doline abuts onto a minor fault which passes from the BE flank of the peak called Moneca across the north slope of Pena Viva. On the S side of Pena Viva, and associated with this fault, are three dolines of 'jou' type of Llopis Llado, formed by nivation. These are each -16 m deep and in places vertically walled. Their bases are intercommunicating via short passages. Around their lips occur limestone pillars showing marked lapiaz, a feature not encountered hitherto in the Cantabro-asturic mountain chain by British speleologists. Nearby, and associated with the same fault, is a shaft of -45 m which was descended to a snowbank. It had not been expected that snow would be found preserved in shafts at such comparatively low altitudes in this area, although it has frequently been noted in the Picos de Europa, where surface snowfields lie throughout the year. No passages led from this shaft, called locally the Pozo de los Grajos, on account of the birds of that species which inhabit it. The shaft showed another very interesting feature. Its mouth was offset, there being a roof of rock partly overhanging the shaft. This type of shaft, now more and more frequently seen in Alpine-type areas, was first described by the Italian caver Maucci. It indicates that the pothole was formed by radial solution, as well as vertical solution down a joint. In other words, the solution was by a standing column of water rather like a phreatic tube may be formed by a horizontal column of water under pressure. Such shafts rarely lead to passages because the argillaceous material deposited on the floor by the water which fills the cave in the Spring when the snow melts, is not later taken up again into solution and removed as conditions are too arid in the Summer for influent streams to occur, Probably a similar explanation may be given for the deeper shaft, more obviously utilising a joint, known as the Sima Grail on the side of Pena Cimera, reaching almost -100 m of depth. An open shaft on the north side of Pena Viva of about -25 m of depth was also descended, occurring in a doline.
To the east of the Rio Torio canyon (Hoces de Vegacervera) another influent cave was explored, the. Cueva del Valle del Marques. This cave also was formed in Devonian strata, its entrance lying in the Valle del Marques to the east of the peak called Monto Prieto, (1669 m. above sea-level). This cave also has undergone considerable deposition recently. No stream was entering the cave when the present expedition visited it, but stagnant pools in the stream bed in the valley on the surface indicated that the cave receives a stream of sorts. Further to the east still, on the aide of the Pico Polvoredo, (1998 m high), another large shaft vas noted, called locally El fil de la Colurabina, and by us "Ghyrrt Cavern"- This cave is in Devonian limestones, and is a dry system, It is, indeed, hard to envisage where the water which formed this system might have derived from, as the entrance is a large sloping cleft on a steeply dipping limestone pavement.
This brings us to the question as to whether there was glaciation of any sort in the mountains around the Torio canyon. Some members of the expedition felt that there might have been. F.E.T. Sanders notes stone polygons in the Valle del Marques, and J.N.S. Schofield noted 'boulder clay' near the peak of Moneca, A German worker, Lötze, has reported traces of glaciations also in the Leon mountains. It seems most likely that any glaciation in the area was the penultimate Riss phase, owing to the greater extent of that glaciation than the Wurm, and the paucity of glacial vestiges, both on the surf-ace and related to cavern development. Whether any post-glacial uplift of the type known to have occurred in the high Picos de Europa and to have triggered off a phase of surface rejuvenation of valleys and also of cave development underground cannot be determined. However, many resurgences appear close to the present level of the Rio Torio. One such resurgence cave could be followed some way, the Cueva del Pozo do Infierno. Other small caves can also be climbed to easily from river level in the Torio canyon This would support a relatively quiescent period of recent speleogenesis. On the other hand, the cave of Covona, the exit from the Cueva de Valporquero, is elevated several hundred feet above the Rio Torio. This author is inclined to think that this is due to an accidental cause connected with the development of the Cueva de Valporquero along the band of Devonian limestone.
In other words, there is good reason to suggest that around the Hoces de Vegacervera cave development has been in progress at least since the Miocene peneplanation of the western part of the Cantabro-asturic chain, involving a time-span of something like the past ten million years. By contrast, the Picos de Europa show that cave development has occurred only over the past 20,000 years (or in inter-glacial epochs of time measured in mere tens of thousands of years, not millions), It is not my intention to discuss the geomorphology of the caves discovered by the present expedition in the Picos de Europa, in 1965 as an extension of the discussion given as an appendix to the re port of the 1963 expedition may appropriately referred to those caves, and they are mentioned also in another recent paper.