OUCC Proceedings 9 (1979)

Cueva del Osu  [survey]

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John Singleton and Martin Laverty

(See also article in OUCC Proceedings 11, with redrawn survey)

Although less challenging than some of the other caves in the area, Osu is certainly worth a visit. The trip down to the final sump is largely a pleasant stroll along a fine streamway, introducing many of the typical features of the caves of this area.

The cave has been known for a long time and has probably been surveyed at least twice before. However, as many side passages had not been pushed, despite the rather obvious remains of an underground camp, and the previous surveys seem unobtainable, one expedition objective was to survey Cueva del Osu.

Derouet et al. (1955) recorded a biospeleological collecting trip in Osu but they did not go beyond the heads of the 30m pitch, from which they reported hearing a stream. More recent visitors have included the Speleo Club Alpine Languedocien (1964), SIE (a Barcelona group still active in the area) and the Grupo Espeleologica de Gijon.

The main entrance to Osu is in a small rocky bay, where cattle often lie in the cool breeze from the SIE initialled entrance. The upper entrance is almost directly above this in a small pile of boulders on a ledge. The leafy main entrance leads to a complex series with a maze of little oxbows where the unwary may find themselves going round in circles or ending up against a blank wall. However, the route is reasonable obvious if the old survey marks are followed. After a 3m climb down between boulders a short rift leads to a 4m descent into an aven, at the top of which the upper entrance emerges. A hand-line is sufficient for the 4m descent, as we found after some unknown person stole our ladder. Another short climb down through boulders, which intimated their unstable nature by falling on John, leads to a short section of high passage, with a boulder floor, and the head of a 30m pitch. The first attempt at inserting a bolt here failed miserably as the rock disintegrated, but the next one was much more convincing and provides a free hang onto the impressively large boulder pile below. The pitch should be descended as far as the top of the pile and not followed down the 5m deep blind hole below.

The boulder pile ends in a drop of 15m, which can be descended safely via a small crawl down through the boulder floor, ending over a nasty looking but easy climb onto a slope of loose till. It is advisable to stay clear of the base of the pile while people are ascending or descending as they, or small boulders, are apt to come thundering out of the crawl.

After encountering an extremely large aven to the right, the cave bends to the left and narrows down to meet a small steam. The many false floors and well preserved pebble beds which are a distinctive feature of Osu begin to occur here, and some pretty helictites are seen in the roof. About 5m up in the left-hand wall is one end of the small section of passage discovered by the expedition which eventually emerges high in the wall opposite the 30m pitch. This inlet consists of a series of short thrutchy climbs with some curious 'gruyere cheese' phreatic development. The passage degenerates to muddy crawls, finally opening out at the base of an aven about 15m high. A window, created by touching the wall 4m up the aven, looks out over the 30m pitch, an impressive vista. A small secondary inlet leads off between the muddy crawls, which starts as an amazingly thrutchy climb up through a tight hole. After this there is a little standing room on fine mud formations. An incredibly slippery climb with some fine helictites probably leads to a too tight hole, but we all fell off!

The stream continues through many piles of pebbles and finally joins the main streamway at the 'T Junction'. Above, on the upstream side of the T Junction, is a large chamber with lots of stal. Unfortunately, the many strongly draughting ways leading upwards from this chamber were too tight to follow.

The upstream passage passes several small tunnels to the right, which are again too tight to follow, and then twists and turns past several inlets and a few avens, one of which contains a bat skeleton. Another aven, just before the prominent cracked and tilted column nicknamed 'the Martian Spaceship', is the point of entry of Stone-Lid Cave, also known to its explorers as 'Dave's Entrance'. A rising sump, thoughtfully inscribed with the legend 'sifon', is eventually encountered and has not been passed.

Downstream, the stream meanders through more raised pebble beds and smashed false floors in a high, spacious passage. Behind the first large broken section of false floor a small inlet stream flows out from a very tight wet squeeze, into which Skippy plus wetsuit were shoved. A climb up for about 10m, from where the drysuited cavers were watching Skippy's semi-aquatic antics, led to a moderately decorated chamber that Skippy eventually reached via his squeeze. The stream splatters into this chamber from a truly minute hole.

To the left the streamway opposite another trickling inlet, a small passage leads up to an aven with very loose sides. This was climbed to a limited extent by Ian and John, but no way upwards was seen. The passage then narrows beyond a broken rock bridge which no longer merits the word 'peligrosa' daubed on it. A climb down into a pool follows and the way on follows the water. However, clambering over boulders to the left reveals part of a large old passage. This was one of the sites used for bug collecting, and also has some large stal formations, which have, in fact, blocked the passage completely. Its continuation, Giga-stal chamber, is reached either by climbing up a prominent overhanging stal flow before, or up at, Windy Corner. Continuing easily downstream the upper levels are not seen again but a large chamber is soon reached which has a flat mud floor embellished with rubbish left from an underground camp. However, the stones, spelling out 'OPERATION ENTRELAGOS', seen by the Oxford visitors in 1976, have been removed. At a sharp left-hand bend, a large inlet enters straight ahead, and it is easy to stumble up this on return rather than follow the stream. The passage assumes a 'Keyhole' section and passes an inlet to the right and a distinctive calcite vein in the floor before reaching the first sump. The bypass is opposite a small inlet, and leads up over some boulders to a car number plate marked 'O', which indicates the way on. After a short stretch of mud floor the streamway is regained via a small hole in the base of the passage. The streamway continues under a low flowstone arch to a 2m waterfall, which is easily descended in normal water conditions although a wire hand-line is provided. The passage here is a narrow vadose trench but a climb of about 20m leads to a large high level. Unfortunately this was muddy and unstable, and was not extensively explored, although it could lead to a bypass to the terminal sump. After a small chamber, the going becomes easier, and the stream joined by the large inlet rushes joyfully down to the left over a series of three cascades. The last two cascades were rigged with wires which were of little use, and one eventually broke under the weight of Mike. These cascades could be very difficult in wet conditions, particularly for cavers in dry kit.

Eventually after a long uneventful section, the passage becomes very spacious and muddy before narrowing down to a duck, followed by a deep pool. The terminal sump is in the pool, which contains a muddy ledge on the right hand side. No bypass was found, even after extensive, and often hair-raising, climbing in the avens and on the mud slopes. [see OUCC Proc 12 for info on diving.]


4m climb down aven 10m handline belayed to flake
30m pitch 40m SRT rope to bolt (hanger and bolt removed, and anchor left ungreased.) 4m wire tether for back-up belay to large rock arch.