From Champagnole follow the D127 to Saym, turn left on D277, then rught on D277E to the parking lot at the Pont des Forges.
|Dimension:||L=200 m, W=2 m, H=15 m.|
|Guided tours:||self guided|
|Address:||Pertes de l'Ain, 39300 Bourg-de-Sirod|
|As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
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|1557||first forge and furnace established at this site.|
|1779||Forges de Châteauvilain built.|
|1800||purchased by Claude-Etienne Jobez.|
|1808-1810||completely rebuilt after fire, but without the blast furnace.|
|1898||first hydroelectric power station established.|
|10-DEC-1918||a prefectural decree authorizes the erection of a bigger power plant.|
|1922||new power station commissioned, two generating sets with Francis turbines.|
|1943||grorge, swallow hole and forge listed.|
|1946||after the Second World War many buildings of the old forge were razed, nationalized by EDF|
|1963-1964||modernized with semi-automation.|
|1976||automation of 2 groups.|
|1994||hydroelectric power plant inscribed on the Inventaire général du patrimoine culturel (General Inventory of Cultural Heritage).|
|2011||residence of a forge master listed as a historical monument.|
The Pertes de l'Ain (Swallow Hole of the Ain) are actually a gorge, not a swallow hole. Pertes is the French term for swallow hole or sink, so you could also call it sink of the Ain. There actually is a swallow hole, but you have to look for it, the spectacular gorge is much easier to see. The river vanishes underground and reappears 15 m further dow the gorge, it fills a pool and then overflows in a 15 m wider waterfall.
The river crosses a limestone ridge between two parallel valleys, a quite common thing in the Jura, but this gorge is not only spectacular, it is also well developed with a trail on each side. The narrow section is about 100 m long and ony 2 m wide at the narrowest point. It has 12 m to 15 m high vertical walls. So most people walk upstream along the river on one side, cross it at the next bridge, or actually a weir, and walk back the other. It makes no difference on which side you start. The trail is well developed and easy. There is a bridge in the middle of the gorge, which not only allows to change the trail, but also offers great views of the gorge.
The gorge contains only a part of the water it once had, which is a result of the hydroelectric power station on the northern side of the gorge. The weir where the trail crosses the river was built to divert a part of the water through a tunnel to the power station. Below the gorge it returns to the river. The Centrale hydraulique EDF de Bourg-de-Sirod is actually a technical monument and listed. The water flows through the tunnel into a reservoir, from which it falls 45 m through three massive steel tubes into the power station supplying three Francis turbine groups. The annual deliverability is 20 million kWh, the installed power 5.09 mVA. This was originally the site of the Forges de Châteauvilain (Forges of Châteauvilain), between the 18th and 19th century the water powered the hammers and bellows of the forge. There was also a furnace which produced 500 t of iron in 1789 and 900 t in 1800. The first hydroelectric power station was established in 1898. The current size with three turbines was built in 1922, but modernized numerous times.
After the narrow section of the gorge, if you take the southern trail, you will reach the Cascade de tuff. A small karst spring in the hillside produces limestone rich water which flows down the slope, deposits tufa, and thus creates a ledge with a waterfall. In other words the ledge is a result of limestone deposition which is a result of the ledge. The ledge continually grows and if it becomes unstable collapses, just to continue growing. There are also rimstone pools, small semicircular lakes with a thin wall or dam downstream, which also grow by deposition of limestone. Massive bulbous tufa deposits are continually overflown by water and thus covered by moss.