Grand Thur, Mountain summit in Izieu, France

Grand Thur is the summit of the Izieu mountain in the southern Bugey, standing at 758 metres on the commune of Izieu. Its northwest face drops steeply above the Gland valley, while the slopes in the other directions are gentler. A band of pale grey layered limestone runs across the face, with bare brownish woodland above and leafless forest below.

The limestone of the Izieu mountain, including Grand Thur, has been quarried since Antiquity; the Roman quarry of choin de Fay, near the hamlet of Fay, is the best-known site. The forest on the slopes is still worked today.

The long-distance hiking route GR 59 crosses the Montagne d'Izieu between Peyrieu and Izieu, but it does not pass over the Grand Thur summit itself. Walkers on this path cross the wooded slopes of the massif rather than the 758-metre top.

Trails start from Route de la Grande Montagne and follow marked paths that climb gradually across several kilometers to reach the peak. Proper hiking boots and water are helpful, especially for the steeper sections on the northern side of the slope.

The adret, or sunny southern slope, of Grand Thur is classified as a zone naturelle d'intérêt écologique, faunistique et floristique (ZNIEFF), a French inventory of areas with notable flora and fauna. The summit itself is not on the GR 59: that long-distance path crosses the mountain of Izieu between Peyrieu and Izieu but bypasses the top.