Chaîne de l'Épine

The Chaîne de l'Épine and the Mont du Chat separate the Lac du Bourget (to the east and north) from the Lac d'Aiguebelette (to the west and south). The highest point is Mont Grelle (also spelled Mont Grêle) at an elevation of 1,425 metres (4,675 ft). Another notable peak is 1,232-metre (4,042 ft) Gratte-Cul at the northern end of Mont Grelle.

The Chaîne de l'Épine is an anticline, part of the "upper ridge" geological structure of the Jura Mountains.[3] Situated between two Miocene molasse basins, the ridge is chiefly composed of limestones from the Jurassic and Cretaceous periods. The western escarpment, between the Col de l'Épine and the Col de Saint-Michel, is composed of limestones from the Kimmeridgian and Oxfordian formations, with scree at the base. The portion south of the Col de Saint-Michel consists mainly of scree with a Jurassic limestone ridge. The east escarpment, is composed of Valanginian marls and marbles, with scree and Cretaceous limestone at the base.[4]

South of the Col de Saint-Michel pass, a change in the strike-slip fault deflects the ridge line to run northeast–southwest. Another slip fault crosses the ridge perpendicularly at the northern tip of the peak Gratte-Cul. The fold extends across the Guiers river at Entre-deux-Guiers to join the Mont Tournier anticline and form the Grand-Ratz plateau above Voreppe, forming the southern terminus of the Jura Mountains.[4]

Sign at the top of the Col de l'Épine.

There are three principal passes across the ridge. A paved road (D916) runs over the 987-metre (3,238 ft) Col de l'Épine. The two other passes, the 915-metre (3,002 ft) Col du Crucifix and the 903-metre (2,963 ft) Col Saint-Michel, overlook the Lac d'Aiguebelette and are crossed by hiking trails. Some of these trails follow the routes of earlier Roman roads.

Stele near Saint-Christophe in Savoie, France, at the southern end of the Chaîne de l'Épine (45°26′57.0″N 5°47′13.3″E), commemorating the opening of Charles Emmanuel II of Savoy's royal road from Turin to Lyon in 1670.

The Voie Sarde skirts the southern end of the ridge, near Saint-Christophe, northeast of Les Échelles. The Voie Sarde or Sardinian Way was originally part of the Roman Via Agrippa road network. In 1670 the road was rebuilt by Charles Emmanuel II, Duke of Savoy with the aim of connecting his western landholdings in Savoie to Turin, capital of the Duchy of Savoy.[5] In 1720, following the acquisition of Sardinia in the 1718 Treaty of London, the duchy officially became the Kingdom of Sardinia, leading the road to become known colloquially as the Voie Sarde.

A century later, Napoleon judged this narrow route to be an impediment to his military campaigns. After 13 years of construction (1804–17), the Tunnel de la Grotte bypassed the Voie Sarde route. The tunnel was nearly 1,000 feet (300 m) long and 25 feet (7.6 m) wide and high, sufficient for two laden wagons to pass. The English artist J. M. W. Turner sketched views of both the Voie Sarde and the new tunnel in 1819.[5][6] The modern-day Route nationale 6 uses this tunnel to cross the southern end of the Chaîne de l'Épine ridge.

There are tunnels through the main bulk of the ridge in two places, both named Tunnel de l'Épine. The A43 autoroute runs through a pair of parallel tunnels under the Col de l'Épine. The northern tunnel is 3,182 metres (1.98 miles) long and was completed in 1974; the southern tunnel is 3,157 metres (1.96 miles) long and was completed in 1991. This is the 14th-longest road tunnel crossing in France. About 5 kilometres (3.1 miles) to the south is a single-track tunnel completed in 1884 for the Saint-André-le-Gaz–Chambéry railway line. This 3,076-metre (1.91-mile) tunnel begins east of the station of Aiguebelette-le-Lac and runs under the ridge, south of the Col Saint-Michel, to emerge just west of the now-closed Saint-Cassin-la-Cascade station, in the valley of the Hyères river, a left tributary of the Rhône.