Trou de Balbonne

The cavity begins with a vertical fissure down to -6 m, then a 4 m drop leads into a small pocket. To the south, a 6 m passage is now practically filled by spoil. One then passes under a block held by a metal bracket, then a squeeze (-12 m) leads into a shored section with uprights (R 3). A narrower and more tortuous section is then negotiated to emerge into a wider part (-19). The next descent, not very committing, is also equipped with uprights. A final drop leads into a collapse chamber (-27) where several passages arrive. Three are ascending and all end in choked shafts at -19, -21 and -24. Two are descending to the north of the chamber, after a somewhat unsettling pile of blocks that must be crossed, and are blocked at -28 m and at -30 m (trace of a small stream infiltrating into the sand). A final draughting passage at the level of the block pile leads towards -30.

A trace of a small stream infiltrating into the sand is visible in one of the descending passages north of the collapse chamber, blocked at -30 m.

Geology: Lochkovian black crinoidal limestones (Devonian).