Réserve naturelle nationale du ravin de Valbois, National nature reserve in Chassagne-Saint-Denis, France.

The Réserve naturelle nationale du ravin de Valbois is a 235-hectare national nature reserve created in 1983 in the Doubs department. It occupies a Jura reculée, a V-shaped ravine cut into the first plateau of the Jura massif, bordered by abrupt Rauracian limestone cliffs. Its floor lies on impermeable Oxfordian clay, its slopes are covered in scree, and a stream crosses the site in a 40-metre waterfall.

Until the late 19th century, the sunny slopes were used for growing grapes, but phylloxera disease and wine imports caused this activity to stop. The land then regenerated naturally, with trees and vegetation returning to the former vineyard areas.

The ravine's name, Valbois, refers to the wooded valley itself. The reserve is crossed by a 4 km discovery trail that starts at the Ferme du Pater, below the Château Saint-Denis, and returns past the castle. Group visits are allowed on request with the managing body.

A 4 km discovery trail starts at the Ferme du Pater, below the Château Saint-Denis, and the return route passes back by the castle. Group visits are allowed only on request to the managing body. The ravine runs 3 km between 330 and 550 m in altitude, so expect steep sections.

Two forest parcels in the reserve have not been logged since 1910, so old trees and a high proportion of dead wood dominate them. The reserve also records 262 fungus species and more than 700 butterfly species, including the woodland brown and the woodland grayling, both now scarce in France.