Grotte de la Borie 3

The cavity begins with a conduit cut into alluvium, followed by a succession of small pockets in the blocks separated by narrow passages.

At -6, a 4-metre step drops into a large joint inclined at 50° to the NE, in which two passages allow one to reach the terminal sump. Descending immediately below the step towards the SE, one arrives near the edge of the fissure where a stream flows during flood periods (section B-B). At -41, one lands in a gallery that joins the sump after about ten metres. Descending the large joint towards the NE, one reaches a concretion-covered platform that overlooks a clay slide of about twenty metres (rope essential) ending in a 10-metre vertical that leads to the sump (section A-A). Depending on the water level, one lands either on blocks three-quarters submerged (high level, elevation -46.5) allowing a parallel ascending fissure to be visited that emerges halfway up the slide, or on a beach of pebbles and sand (low level, elevation -51).

Climbing the fluted wall above the concretion-covered platform, one reaches a descending fissure followed by a vertical forced conduit of 3.5 metres calibrated to the dimensions of the human body (round people need not apply!). An ascending joint leads to a large boulder choke comprising several pockets, one of which holds the high point of the cavity at +8. At the foot of the boulder choke, on the right side of the access joint, one reaches a pocket where several leads present themselves. Upwards, a crawlway accesses two chambers separated by a step; on the edge of the lower one begins a downward-sloping joint (not surveyed) that joins the large joint of the sump through an impassable passage. Towards the NW, a 4-metre step gives into a steep meander that closes off at -14. Below the crawlway, a severe constriction followed by a vertical fissure opens into a lower clayey chamber, inclined, ending at -23.5 on a small gallery very close to the unsurveyed joint. This last chamber contains numerous lateral diverticula that are choked.

During the discovery and the work to open the cave, the explorers witnessed on many occasions the extremely violent air movements generated by the sudden rise in the level of the terminal sump. This was observed a few months later, in December 1986, causing some excitement among the participants.

Our purpose is not to construct a theory that would explain this phenomenon, at once original and very rare, very similar to that observed at the trou du vent des Caousous n° 1 by our colleagues of the Société Spéléologique du Plantaurel, but to describe its main phases based on observations made in situ by novices. Only the implementation of sophisticated instrumentation (water-level recorder, ...) would make it possible to quantify the flow and better appreciate the causes of the phenomenon.

In a relatively short time of 2 to 3 minutes, the sump level rises by 5-6 metres before falling back to its initial height within about ten minutes. The associated air movement reproduces this sudden fluctuation with a deformation caused by the compressibility of the air. This movement breaks down as follows: first there is a phase of acceleration of the air current for about thirty seconds, corresponding to the start of the water's rise, followed by a period of one minute during which the air current is at its maximum; the end of the rise and the start of the draining of the sump are characterised by a slowing of the air followed by a rest phase (pressure equilibrium reached) of about 60 seconds during which the internal atmospheric pressure reverses, causing an intake less violent than the exhalation for about ten minutes, the time it takes the sump to regain its base level.

In April 1986, these oscillations occurred four times in succession at 30-minute intervals and did not resume until after an hour and a half. They were perfectly synchronous with the intermittence recorded at the same period at the résurgence de la Doux.

These various observations call for a few comments: the pulsations observed have no regularity in time and are closely dependent on the water input; the phenomenon is best observable during medium-water periods; it seems to persist during heavy rainfall with a longer periodicity of the intermittence and a reduced level variation caused by the rise in the level of the phreatic zone; finally, the phenomenon tends to disappear during a pronounced low-water period (end of summer and before the first autumn rains). The cavity is not a window onto the phenomenon that generates the intermittence: it lies only on the path between the phenomenon and the résurgence de la Doux. The impossibility of evacuating the sudden influx of water as well as the loading of the cave over about twenty metres of height during the rare major floods of the ruisseau de Linze suggests the existence of a constriction point downstream of the sump.

The cavity lies at the centre of the Minervois aquifer, constituted as a whole by a normal flank of a plunging head of an aquifer overflowing to the South, emplaced during the Hercynian orogeny. The carbonate zone that contains the cave, known as the limestones with Archaecyathus, dates from the Upper Georgien. It consists of a set of alternating sandstone-limestone beds 100 to 200 metres thick, in which the source de la Doux and that of Bibaud spring forth, at the contact with the underlying sandstones of Marcory, and of an important dolomite level at least 400 metres thick, composed of massive dolomite with a reddish patina and then bedded or schistose dolomites in which the cavity develops along NS fractures inherited from the Hercynian period.

The geological structure described above induces an ENEWSW underground drainage. The water collected upstream on an impermeable compartment composed of sandstones of Marcory is lost at the level of the carbonate layers that it crosses lengthwise to resurge in the Argent-Double. There is no single loss point but a multitude of absorption points that function according to the flow of the ruisseau de Linze. During long rainy periods, the losses do not absorb the entire stream, which then flows as far as the Cros.

The exploration of the grotte de la Borie 3, in which the base level of the spring has been reached, makes it possible to affirm the conclusions of the hydrogeological study carried out in 1979 by the Compagnie Nationale d'Aménagement de la région du Bas Rhône et du Languedoc (CNABRL) on behalf of the Direction Départementale de l'Agriculture (DDA) of the Aude, namely the existence of a phreatic zone between the part of the ruisseau de Linze close to the ravin des Escaliers and the résurgence de la Doux and of a zone with more or less free and rapid flow to the north, near the contact with the sandstones of Marcory. The underground observations also showed the amplitude of the variations in the level of the phreatic zone, which is around 4.5 metres between the altitude of 293 m at low water, which is also that of the Doux, and the altitude of 297.5 m during the wet period.