The mountains of the Swiss Jura were formed relative recently, about 65 million years ago. However, the limestone originates already from the Jurassic Era, 145 to 200 million years ago. During that time the area was located at the northern margin of a shallow tropical sea called Tethys. Upon death of sea-dwelling creatures, they were quickly buried in the seabed, and compressed over millions of years.
The rocky amphitheater has been created by a glacier in the last ice age about 18’000 years ago followed by erosion through water and ice, and cold and heat. This weakened the rocks causing a section of the mountain to fall into the Areuse river valley leaving the exposed rocks you see in the Creux du Van today. At the bottom of the depression, the subsoil below the debris remains frozen during the whole year [permafrost]. Above it grows a forest of arctic-alpine flora.