The bridge now crosses the Tarn valley above its lowest point , linking the Causse du Larzac to the Causse Rouge , and is inside the perimeter of the Grands Causses regional natural park . The bridge now traverses the Tarn valley above its lowest point , linking two limestone plateaus , the Causse du Larzac and the Causse Rouge , and is inside the perimeter of the Grands Causses regional natural park .