The 'Route Forestiere (Forest road)' in the Pay-de-Gex in eastern France stretches over about 30 km from Farges to Gex, at an altitude between 840 and 1170 m. Due to the forest trucks and the cold winters the road is in rather poor condition, but OK on a gravel bike. Enjoy!!
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You can also do this ride in opposite direction (a bit more difficult), see Gex-to-Farges