Trauc ou perte de la Mandra

The cavity is interesting for its structural position on the northern flank of the syncline and for the morphology of its galleries. These follow the contact with the sandstone situated above, which is very often found in the ceiling and even in the wall over a certain height. The galleries are of vadose type and bear witness to the youth of this cavity, with very few speleothems and deposits that are only sandy, earthy and gravelly. They are often cluttered with blocks coming from the equilibration of the walls or vaults. Towards the back, small faults are observed in relation to an important break noted on the geological map. The first part of the cavity presents small labyrinths adjacent to the main gallery, often dug under the sandstone layer and presenting the same characteristics as those of the Veiraria, that is to say evolutions of anastomoses by a coring system, a conduit gradually gaining the upper hand over the others and becoming the main drain. Pillars and remains of pillars are also found. It should be noted that the galleries remain close to the sandstone, which is very frequently in the ceiling and even constitutes part of the walls, whereas at the nearby Trauc del Caucé the sandstone is in the floor, which is more logical.

The main gallery takes the form of meanders or wider but rather low galleries, with pillars. The main meander continues on the SSW axis but tends to narrow considerably, while remaining passable.

The walls of the meander situated after the narrowing are covered with speleothems and have been strongly eroded by floods; this period of speleothem deposition corresponds to a phase when the cavity was less active.

The stream comes up against a stratum; beyond it, it never flows. It rarely reaches the entrance because a perennial loss exists 200 metres upstream, but when the flow increases the water reaches the entrance, which can absorb several hundred l/s. The cavity is then swept by impetuous floods and many sections are drowned in it; caution is therefore required to visit it and stable weather is indispensable. The entrance, cluttered with blocks and branches, leads into a first floodable chamber (traces of floods).

The loss of la Mandra has been dyed twice as part of research on the source des Tourtes. Both tracings emerged at les Toutes in 48 hours in 1972 and in less than 6 hours in May 1974, for a height difference of 165 metres and a direct distance of 1250 metres. The relatively slow transit times would confirm the existence of a fairly large flooded zone in the bottom of the syncline.

On the right of the first chamber begins the Réseau Guy (marked in red paint by the first explorers): these are old conduits of the network coming from losses situated higher than at present. One climbs up through joints, then a route through small chambers separated by pillars leads to a narrower part rising towards the north. One then emerges into a spacious ascending gallery of fine workmanship; it ends at +17m on a scree cone. 25 metres before, on the left, a chimney stops at +15m; 5 metres before, another gallery with lateral branches leads to +16m after a collapsing chamber. Back at the access to the gallery, the downstream narrows rapidly and, after a serious squeeze, one arrives in another narrower conduit. On the right it rises to +3m. On the left, one descends a meander then low galleries return into another meander. This one then splits in two: on the left a dry gallery ending in a flat-out crawl, on the right a small meander with a trickle ending at -13.5m on a squeeze close to the terminus of the flat-out crawl. It is possible that this branch is the upstream of the Affluent des Dunes and constitutes a network parallel to the main branch. The Réseau Guy develops about 400 metres of galleries.

The continuation of the main gallery was named « Réseau des Trois » by the first explorers; it is the main axis of the cavity. We will leave aside the description of the secondary conduits, fairly numerous in the entrance zone, which sometimes form small labyrinths. A small active stream appears quickly. After about 150 metres of travel, a narrowing momentarily slows progression. One then crosses a collapse chamber. 20 metres further on, a small gallery rises on the left to +33m; an inscription (again!) bearing the name of the first explorers in red paint is found at this branch. Then the gallery resumes and, 20 metres after, one is 45 m in front of the beginning of a narrow zone 25 metres long. One exits it through a narrow meander which widens rapidly, then the gallery splits in two for 60 metres. One then traverses a very fine meander down to -85m. There arrives an affluent with about a hundred metres of development on a sandy floor: it is the Affluent des Dunes, which ends at -68 m.

One begins to find in the main meander numerous deposits of sand, gravel, mud, branches, leaves and sometimes a few toads; these deposits often change place, according to the floods. At -116 m, a low passage can sump or become blocked. Just after, a pocket precedes the sump at -120 m whose level is very variable: -120 m at low water and up to -116m when the hole is well active (much more in flood). When the level is low enough, the draught indicates that one can pass, the bath is often inevitable.

The continuation is at 130° and can be blocked by spoil. One climbs painfully and straightens up in a fine meander which immediately sets off in the opposite direction and descends. A chaos of muddy blocks marks the lowest point of the cavity at -122m. One then climbs between the worked vault and the shifting deposits. When one straightens up and it becomes horizontal again, one leaves a crawl rising in height over 15 metres. The continuation is spacious and ends after 30 metres on a squeeze which can also silt up and which gives access to an ascent where one finds the sandstone and the blocks which irremediably obstruct the continuation at -110 metres despite a good draught.

This fine cavity requires no progression equipment and proves very interesting and sporting, but beware of floods.