Grotte du Cinquantenaire

The cave begins with a small olive doline. From -4 M, the effective entrance, down to -25 M, one progresses through an incredible tangle of blocks that gives a few scares. At -25 M, at the bottom of a R 2 of puddingstone, one emerges into a sloping boulder-strewn chamber followed by a R 6. A small shaft before the chamber communicates with the underlying gallery through a series of narrow squeezes. One reaches the often dry bed of a small stream that disappears 5 metres further on in a crawlway. A passage above this point leads into a chamber with no continuation.

UPSTREAM: one ascends a 40-metre gallery (minimum width of 4 metres) followed by an 8-metre step in three parts (delicate passage). A pretty meander crosses the puddingstone layer; 10 metres further on, one finds a 45-metre gallery (3 M x 2 M) very boulder-strewn at the start. One climbs a R 2 and arrives in a small chamber. In the ceiling, one can ascend a narrow meander for 30 metres. At floor level a wet flat-out squeeze begins. 15 metres further on, one finds a section of gallery that ends at a choke after 30 metres. At the entrance to the gallery, two chimneys lead to a short upper level.

DOWNSTREAM: one reaches the stream bed at the bottom of a clay slope. 20 metres further on, one enters a collapse chamber at whose entrance, on the right bank, arrives the main tributary of the cavity. Following the left wall, one comes to a very tight crawlway that guards access to a bell-shaped shaft rising 8 metres to an impenetrable top. Back in the watercourse, one slips into a low gallery (dam of fertiliser bags) followed by a narrow passage giving, a few metres further on, onto a 25-metre duck (-55 M). Oriented towards the East for 20 metres, it turns to the S before ending at a sump.

TRIBUTARY: ascending the tributary, one reaches a boulder-strewn triangular chamber 10 metres on a side by 3 passages. The route continues in the S-W direction. One finds the water flowing in a gallery that makes fine loops. On the right side arrives a trickle of water coming partly from a cramped hole, partly from a shaft rising about 12 metres whose top seems impenetrable. The ceiling lowers inexorably, the water seeps from a flat-out squeeze a few centimetres high. A muddy, wet crawlway allows one to bypass the obstacle and one straightens up at the bottom of the vastest chamber (25 M x 20 M). Entire sections of ceiling form a circular barrier 7 metres high. On the other side, one finds the stream flowing in a straight corridor. A tributary appears on the left. A low passage at water level, or a brief flat-out squeeze above, lead into a chamber (water pool) from which a 10-metre meander departs. After a dogleg, the stream cascades over a small step behind which it emerges from an impenetrable passage at -48 M. Before the terminus, a chimney leads into an upper level of about 10 metres, with no continuation.

The stream is formed by the union of several small rivulets: access gallery, upstream gallery, small tributary of the collapse chamber, the main tributary, itself composed of numerous water inflows. The low-water flow is almost nil, about 1 l/mn. The flood flow can exceed 50 l/s (observation in March 2011).

The waters emerge at the spring of Coumescure located 150 metres lower and 1.5 kilometres further from the injection point (CDS tracing 11 of 2011).

After the crawlway, one crosses two pockets in the blocks, then after a squeeze cut in a puddingstone layer, one arrives on a balcony in the active gallery at -48 M.

After a few squeezes one reaches a small active stream at the marls/puddingstones contact.

A special cavity carved into the puddingstones and the marls. The entrance zone that crosses these formations sometimes reserves surprises (collapses), one must remain very cautious in this part. The deep galleries deserve a visit for this geological particularity.

Geology: variegated marly terrain of the Cusian (Eocene, about 60 M.A.). The cave does not follow the direction of the dip and is carved perpendicular to it. Tectonics plays no great role. Presence of two puddingstone beds crossed several times.