Jura (mountains), mountain range, straddling the border between France and Switzerland. Beginning in eastern France on the northern bank of the Rhône River in Ain Department, the range extends northward, forming the western bank of the river until it reaches the Swiss frontier near Geneva. From that point it extends through the Jura and Doubs departments, following the boundary line between France and Switzerland in a long curve toward the northeast. The range finally passes wholly into Switzerland and terminates on the southern bank of the Rhine River west of its confluence with the Aare River. The range thus defined is the Jura proper, but many geographers apply the name also to the mountains north of the Rhine and south of the Rhône, considering that these rivers merely make two breaks in an otherwise continuous chain. South of the Rhône the chain is known as the Jura Alps and merges with branches of the western Alps. North of the Rhine an irregular chain extends east of the Black Forest in the state of Baden-Württemberg, Germany, and through the state of Bavaria as far as the Main River. This chain, called the German or Swabian Jura, is similar to the Jura proper in the character of rock formations, but different in structure, being formed entirely by faulting.

The Jura proper consists of a series of parallel folds in the strata, forming together a plateau about 320 km (about 200 mi) long and 32 to 56 km (20 to 35 mi) wide. These folded ridges have in many places suffered transverse fractures, which, in the form of steep gorges known as cluses, add greatly to the picturesque character of the landscape. The general height of the range is from 910 to 1520 m (about 3000 to 5000 ft). It is highest near the southern end, west of Lake Geneva, where the Crêt de la Neige has an altitude of 1,718 m (5,636 ft). The Jura are formed of limestone rich in fossils; for this reason the Jurassic period of the Mesozoic era was named after the range.