Source du Moulin Blanc

The Source opens in a pond with a muddy bottom. The flooded cavity is reached through a funnel 7 metres deep. The diameter reaches 13 metres at the top and 2 metres at the base. The funnel continues as a vertical neck that descends to a depth of 15 metres. Its diameter is just sufficient to let a diver pass with tanks on the back. A sloping gallery extends the cavity: 60 to 80 cm wide at the start, it can be traversed without great difficulty down to the -55 m level. The vault affords a glimpse of a chalk wall, white and cupola-shaped. The gallery runs south, that is, upstream of the rivers. An impenetrable fissure stops progress.

The entrance funnel, forming the bottom of the pond, is cut into an alluvium layer made up of one metre of vegetal and silty soil, then of mixed Clay and pebbles. The cavity develops essentially in Turonian chalk. This consists of several alternately compact and fissured layers over 6.50 metres. They rest on a more uniform level of marly chalk.

Visibility is reduced by the Clay in suspension that comes off the walls or the floor as the diver passes. Only a partial view of the cavity therefore remains. Note that in spring the water emerges turbid, whereas in periods of low water the surroundings of the funnel can be seen more clearly.

The flow of the Source varies from 43 litres/second to 254 litres/second. It is essentially fed by major losses located further upstream, in the very bed of the river. However, a dye test carried out in the losses of the ru du Cuivre showed that another relationship with the Moulin Blanc existed. The dye took 48 hours to cover the minimum distance of 4 kilometres, i.e. a speed slightly above 80 metres/hour. This is relatively low for the region and the ru du Cuivre certainly does not participate essentially in the feeding of the Source.

At least three other springs are distinguished further south in the same pond. The largest also takes the form of a funnel. We do not know whether it is penetrable.

Source: Site du Spéléo Club de Chablis (SCC) > "Source du Moulin Blanc (89358 - St-Martin-sur-Ouanne)" -- https://www.scchablis.com/Site/Speleo/Invent/89/89358/moulin_blanc.php