Rivière souterraine des Fourneaux

The access shaft, 9.13 metres deep, opens onto the abstraction installation itself and determines an upstream course and a downstream course.

The upstream part is passable for about 90 metres; it is narrow and generally low. In places, the ceiling rises to 2 metres in height thanks to collapses. After thirty metres, the ceiling lowers.

The part that begins downstream of the access shaft is more spectacular: it is higher and more varied, 102 metres long. This part of the river is sinuous and its dimensions irregular; the ceiling sometimes rises to 6 metres while the width can reach 3 metres.

The cavity develops in the chalk of the Cenomanian or the Turonian.

There has never been any study of the hydrology of the cavity and the origin of the water remains unknown. However, it is observed that the cavity develops in the bottom of a large dry valley, at only 10 metres depth. Springs located downstream of this valley and of the village of Fourneaux are certainly outlets of the cavity.

The entire river is captured. The daily pumping capacity is 1,500 m3 with a flow rate that seems to vary little: 0.017 m3/s in September 1972 and 0.0167 m3/s in December 1971. According to Maingonat and Chabert, the water, one of the coldest in the region (10° C), has a resistivity of 2,600 ohms/cm. It has the particularity, rare for waters of this type, of containing no nitrates.

The downstream part is fed by the overflow of the abstraction. In periods of low water, it is first dry and then receives water inputs through small fissures. In places, rockfalls obstruct its bed. Beyond the potholes, the watercourse becomes a little calmer: clay deposits reappear.

In the upstream part, after thirty metres, the ceiling lowers and one must crawl in the water to progress. One stands up again in a short gallery that leads to the sump; the latter is shallow and 7 metres long, interrupted by a small bell. Beyond, the ceiling of the gallery rises progressively: one can then progress standing up to a small collapse chamber, which marks a change of direction. The gallery continues comfortably and can be traversed without difficulty to a new sump that has not been dived.

In the downstream part, soon, one encounters potholes in which one immerses oneself up to mid-thigh; the largest is 1.10 metres in diameter. After a final widening of the gallery, progression is done by crawling in liquid mud for about ten metres. A squeeze formed by the presence of a block separates this passage from a small alcove which is the passable terminus of the cavity. From there, in periods of low water, one can see the start of a duck, but the ceiling is very low: this is the known terminus of this part of the cavity.

The presence of clay on the walls proves the pressurization of the river.

Source: Site du Spéléo Club de Chablis > "Rivière souterraine des Fourneaux (89436 - Venizy)" -- https://www.scchablis.com/Site/Speleo/Invent/89/89436/fourneaux.php