The Jura gave it is name to the geologic Era Jura, which lasted from the end of the Trias 205 Million years ago to the begin of the Cretaceous 141 Million years ago (See Geologic Time Scale ). This is the era of the dinosaurs. But central Europe was a shallow and oxygen rich sea at this time, and the climate was much warmer. One reason were different climatic conditions, another one the more southern location of the rocks which moved north since then.
At the beginning of the Jurassic the sea was deep and low on oxygen. At this time bitumen rich black or dark grey limestones and marls were sedimented. But then the sea became shallow and pure white limestone was formed. The oxygen allowed bacteria to feed on all remains of plants and animals despite the lime, so this was sedimented nearly pure.
The pure limestone in the Jura is very characteristic. And it is the precondition for the development of Karst. The orogeny of the alps, a result of the collision of African and Europen plate, had important aftermaths on the formerly horizontal layers. The were squashed from the south and so the layers were folded. The folds form ridges and valleys running from south-west to north-east.
The area has a lovely landscape, many deep river valleys and lots of caves and karst springs. But as the waters drain north west to the Saone valley, the rivers have to cross the ridges. And there are two possible ways, the cluses, narrow gorges cutting through ridges, or underground as described in the Pernod Accident below.
The river Doubs is the most weird river in the Jura. From his spring he flows east, then throug a cluse northwards, back west, again a cluse, once again east, a last cluse and then finally west again. This looks like a big M on the map, and is the reason why the river is 250 km long, but the distance from source to embouchure - as the crow flies - is only 50 km.