Subject to volcanism that has subsided in the last 10,000 years, these central mountains are separated from the Alps by a deep north-south cleft created by the Rhône River and known in French as the sillon rhodanien (literally Rhône furrow). The region was a barrier to transport within France until the opening of the A75 motorway, which not only made north-south travel easier, but also opened up the massif itself.