Fosse Dionne
| Fosse Dionne | |
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| Name origin | Fosse is the French word for "pit". The spring was dedicated to an ancient deity from which the name “Divona”, meaning “Divine” evolved into “Dionne”. |
| Location | Tonnerre, Yonne, Bourgogne-Franche-Comté, France |
| Coordinates | 47°51′24″N 3°58′14″E / 47.85654°N 3.970565°ECoordinates: 47°51′24″N 3°58′14″E / 47.85654°N 3.970565°E |
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The Fosse Dionne, is a karst spring, is located in downtown Tonnerre (Yonne Department). It is driven by the infiltration of precipitation on the plateau as well as drainage from at least one river. The Fosse Dionne is remarkable because of its outflow (reportedly at least 100 litres per second on average).
Its presence is at the center of the creation story of Tonnerre. An elaborate wash basin was built around the spring in the 18th century.
The Fosse Dionne is a hydrogeological focal point and exsurgent spring. It is fed mainly by the infiltration of precipitation in the limestone layers of the karstic plateau which dates to the Jurassic neighboring the city of Tonnerre. Colorations have demonstrated that part of the spring’s water is supplied by the Laigne River which is lost at Villaines-en-Duesmois in the Gouffre de la Garenne (English: Garenne Chasm), 43.5 kilometres (27.0 mi) from Tonnerre as the crow flies. There is also a relationship between the spring and the Gouffre d’Athée (English: Chasm of Athena).
The average flow rate is 311 liters per second, but in periods of flood it can reach 3000 liters per second such as on 15 Mar 2001. The average flow oscillates between a peak of 619 liters per second in January and minimum of 87 liters per second in August.
In the Gallo-Roman period, the Fosse Dionne was used to supply water to the Oppidum of Tornodurum installed on the Vieux Châteaux plateau which dominates the city. The modern city of Tonnerre is built around the spring.
In 1758, Louis d'Éon, father of the Chevalier d'Éon converted the spring into a wash basin. A 14 meter wide basin was built. The washerwomen were protected from the weather by a roof in the form of a “half rotunda” supported by a framework abutting a rubble wall. To avoid pollution, a wall separates the spring from the annular trough used for washing. Fireplaces located around the laundry made it possible to produce the ash needed for cleaning. The laundry has been classified as an historic place since 1920.
The spring consists of a deep basin (hence the name “fosse” [English: pit]). It opens into an embedded gallery with a 2.5 meter high entrance that is visible from the surface. The underground hydrogeological network was explored by divers despite the difficulties created by narrow passages and a succession of deep siphons requiring frequent decompression stops. The first known exploration was carried out in 1955. The gallery first sinks at an angle of 45º to the depth of 32 meters. To continue exploring it is necessary to cross a flap of 0.8 meters by 0.4 meters. Then it goes up twice to depths close to 0 meters before sinking gradually down to -70 meters at 370 meters which is the farthest divers have reached. As a result of fatal accidents, subterranean diving in the Fosse Dionne is strictly regulated.
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Wikipedia