The gorges du Guiers Vif (or gorges du Frou), linking Saint-Christophe-sur-Guiers and Saint-Pierre-d’Entremont, form the departmental divide between Savoie and Isère. The route du frou, built in 1865, features an impressive 300m-long overhang above a 150m precipice, known as the Pas du Frou (“dreadful passage ” in Dauphinois dialect).
This road is a reminder of the difficulty of accessing mountainous regions, and of similar routes in the Isèroise region that required extensive engineering work (Les Grands Goulets (1851), Gorges de la Bourne (1872), Route de Presles (1885)).