Le Castor Fou

Le Castor Fou sends runners into the woods of the Vallée de la Bièvre, around Vauhallan, Bièvres, and Igny, with extra trail sections on the Saclay plateau. It is a spring nature race that starts in the late afternoon, and its main choices are a 13 km route with 230 m of climbing, a 25 km route with 620 m of climbing, or a two-person relay over the 25 km distance. The race is organized by L’Etoile Sportive de Vauhallan and draws about 900 runners as part of the Challenge Essonne Nature. The paths run through Bois Brûlés, Bois des Caves, and Bois de Normandie, so the day is more about wooded hills and changing trail rhythm than flat road speed. The relay option lets pairs split the longer race into roughly 13 km and 12 km legs, which gives the event a team format alongside the solo runs.