About 185 million years ago, during the Toarcian period, the sea covered the area what we call today the area around Fontbelle. In its waters lived an abundant fauna: belemnites, ichthyosaurs and ammonites. The rise and creation of the Alps and subsequent erosion have uncovered this ancient sea bed on which thousands of ammonites have been fossilised. About twenty different species have been recorded here. Four of them allow the dating of "horizons". The "horizon" is the smallest unit of geological time; its average duration is about 10,000 years. If four "horizons" are present on the same surface, this means that the surface has not received any sediment but only shells for at least 400,000 years.