Oxford University Cave Club

Geology and Geomorphology

M..J. Walker

Part I The Limestone massif around the Hoces de Vegacervera

The Hoces de Vegacervera is the name of the canyon in which the Rio Torio flows from north to south between the villages of Felmin and Vegacervera, in Leon province, northern Spain. The length of the defile is about 3 kms. From its floor at roughly 1000 m. above sea-level, the walls rise for between 300 - 400 m, and the highest peaks of the flanking massif rise to over 1600 m above sea-level. Laterally the massif extends between 2 and 3 kms. on either side of the canyon.

The limestone is principally Upper Carboniferous Namurian limestone, known somewhat confusingly for British investigators as "caliza de montans" or mountain limestone. To the north there is a prominent outcrop of Devonian limestone, and Devonian limestone outcrops also to both east and west. Although the geology appears somewhat complex, the tectonics are in reality less complicated than those of the higher Picos de Europa to the north-east. This is because the area around the Hoces de Vegacervera represents the southernmost group of mountains of the Cantabroasturic mountain chain, which thus escaped the intense Alpidic thrusting which was responsible for the more impressive dominance of the Picos de Europa.

As Wagner (1963) has shown, most of the present features of the mountains around the Hoces de Vegacervera owe to Hercynian tectonic movements, occurring mainly during the Carboniferous. These are characterised by isoclinal folds with overthrusts which link the isoclinal synclines, eliminating the anticlinal structures. The southern flanks of the synclines have also been eliminated by the overthrusts, leaving the northern flanks stacked up against each other, and dipping steeply. The angle of dip varies between 55 to 70 degrees, being considerably gentler than in the Picos de Europa. The main trend of the structures so produced was E - W or WNW - ESE.

It is now necessary to move from the astronomical time scale on which such orogenic changes occur, to the more recent geomorphological. history of the region. It has been demonstrated for the Picos de Europa that active uplift of the post-Hercynian peneplain with faulting and erosion during the Tertiary formed the basis on which the present erosion cycles take place (Llopis Llado, 1954). It is possible that some uplift occurred in the massifs around the Hoces do Vegacervera with some rejuvenation of the valley systems, however, it is certain that this area did not take part in the Quarternary uplift in response to melting glaciers which Oakley and Walker (1965) have shown to be responsible for the present pattern of surface and subterranean erosion of the Enol Platform in the Western Massif of the Picos de Europa.

No records of glaciation arc given in the literature. However, traces of boulder clay were reported from the Valporquero side of the canyon near the peak of Monoca by J.N.S. Schofield, and stone polygons of the glacial type were reported from the Valle del Marques on the other side of the Hoces by F.E.T. Sanders during the present Expedition.

The speleogenesis of the massif suggests a far slower erosion process than in the Picos de Europa (see below). The cave of Valporquero foil owe the Davisian picture of a cave in steeply bedding limestone tending to grade and formed along the strike. Various depositional phases have been distinguished, but solution uninterrupted over a long period of time must have been required to form the enormous chambers, some of which are between 50 - 100 metres in height. Likewise the large galleries of the Cueva del Valle del Marques and the Pozo Sil de la Columbina near Correcillas, point to erosion over a much longer period than occurred in the caves of the Enol Platform to the north-east.

The antecedent gorge of the Torio testifies to the drainage of the basically E - W tending Hercynian mountain chain. Presumably the present level of the river some 600 m. below the flanking peaks demonstrates the amount of post-Hercynian uplift. This must have been mainly during the Tertiary, since the major slice of the 1000 - 1200 in, of Carboniferous limestones claimed to have been deposited, must have been eroded during the pre-Triassic peneplanation of the Cantabroasturic region. However, even to permit the entire Tertiary period for the formation of such caves as Valporquero, and for the dissection of the massifs, gives a much longer time span than is available in the Picos de Europa for speleogenesis. This vindicates one of the premises on which the present Expedition was held, namely that the absence of Alpidic movements in the massif around the Hoces de Vegacervera has permitted more extensive speleogenesis than is known from the Picos de Europa.

Other potholes, especially at higher altitudes, show features in common with those of the Picos de Europa, especially concerning vertical solution. The eccentric entrance of Pozo los Grajas is typical of this genre, indicating undercutting of the rock when the pothole is (or was) filled with tester due to melt-waters following the Winter snows. Sima el Solitario may also be such a shift, formed largely by the melt-waters from snow-hanks. Now, deprived of surface water by drier conditions, these. shafts are dry and inactive, thus preventing lateral continuation arid extension.

The poljes and underground drainage of the Valporquero area show a much more highly developed karst landscape than is known from the Picos de Europa. Nor is the lapies on the southern side of Moneca paralleled in the entire Picos de Europa, indicating the highly developed karst of the Valporquero region. The recent history of the area cannot have included the 100m uplift which occurred in post-glacial times in the Picos, for a well-developed cave occurs sit the water-table which must have a considerable under-water extension - the Cueva del Pozo del Infierno which opens in the bottom of the Torio canyon.

Part II The limestone of the Picos de Europa

Since the geomorphology of this area has been dealt with in some detail by Oakley and Walker (1965), only the broad outlines will be mentioned here. The intense Alpidic thrusting which occurred in this area of the Cantabroasturic chain in the Tertiary was superimposed on the E - W tending Hercynian mountain chain. The drainage of the former is still manifested in the antecedent canyon of the Rio Cares. However, apart from this, most of the present day drainage is directed to the north-went off the Enol Platform, to the Rio Dobra and Rio Sella, a block of limestone the west of the Bella is drained by the Rio Ponga, a tributary of the Sella.

Not only did the intense movements of the Tertiary prove unconductive to extended cave development, but even more rapid changes, have occurred (as in Cathedral Cave or El Burdio la Pena).

Many of the caves so formed, whilst quite interesting, lack the size and majesty of the Cueva de Valporquera. Some indeed are quite interesting, and have been noted by previous expeditions. Many shafts perched above the ice were formed by melt-waters from snow banks, and enlarged by vertical solution. The rejuvenation and the arrival of a drier climate removed whatever source of surface water might have re-dissolved the argillaceous material deposited by the vertical solution of Spring melt-waters. Thus these deep shafts are often blocked and offer little to the potholer. The fall in base-level and therefore the fall of perched water-tables has prevented lateral extensions from such potholes. Only in rare cases such as Pozo Palomeru has surface water been retained and adequate development occurred to excite the explorer.

Typical of those deep shafts is the Jou Cabau descended by the present Expedition. Many others have been noted by previous expeditions in the area.

Where surface water is retained, at any rate for a part of the year, extensive development of caves may occur One such cave is the Cueva de Don Pelayo some 3 kms east of Cangas de Onis, which still retains a stream active for most of the year. Whilst the stream was dry during August, 1965, the cave contained large pools, especially below the pitches.