The ammonite slab
Exceptional witnesses of the marine history of the Alps: ammonites
A link in the history of the Alps….
At the beginning of the Jurassic, around -200 million years ago, Pangea is still present. But the stretching of the continent that began about 80 million years ago continues. The sea is thus gaining ground, submerging more and more land and deepening in areas subject to subsidence along large faults.
From there, the sea will occupy continuously for nearly 130 million years what will become Haute-Provence and a large part of the southern Alps. The marine layers will follow one another and pile up over several thousand meters.
The ammonite slab, dated -198 million years ago, is part of this beginning of the marine adventure in the Alps. Before becoming the steeply inclined slab of rock that we know today, it was an ancient, horizontal seabed where mud and remains of marine animals were deposited, then transformed into limestone and fossils.
Ammonites are shelled marine mollusks that became incredibly diverse during the Secondary Era when they widely populated seas and oceans. Some species that have lived for a very short time on a geological scale are good “date” fossils.
On the slab, all ammonites belong to a single species with a few rare exceptions. This low representativeness is surprising given the great biodiversity of ammonites from the start of the Jurassic. This species (Coroniceras multicostatum) is known at various sites in Western Europe. However, the extraordinary spectacle offered by 1550 large ammonites concentrated over 320 m2 only exists in Digne-les-Bains. It is a world reference!
… In connection with 22 other geosites
Ammonites diversified abundantly during the secondary era. Their shapes and sizes are very variable.
The "star" of the slab is a large shell species (15-70 cm in diameter) with adjoining turns and ornamentation with well-marked ribs.
A getaway to the side of Lake Castillon will give you an illustration of the diversity of these animals in the Lower Cretaceous with “unrolled” ammonites.
What we can decipher from the current landscape
This ancient seabed, initially horizontal, was located 750 to 1000 meters lower than today. Digne-les-Bains is 600 meters above sea level, but the sea level was not that high 200 million years ago.
It is the formation of the Alps, with the collision of the African and Eurasian continents (see chapter 3 of the History of the Alps), which caused the crushing of all the layers formerly under the sea, folded them, pushed them , moved and stacked. The pushed back sea has given way to the mountains.
The inclination of the ammonite slab is one of the witnesses to the upheaval of the landscapes during the alpine adventure.
A little anecdote?
The ammonite slab was discovered during road widening works in the first half of the XNUMXth century.rd century. The Haute-Provence geological reserve has conducted several excavation sites to bring to light a larger surface (today 320m²), and thus, more than 1550 ammonites. Protection of the site, studies, promotion for the public, have made the slab famous throughout the world.
This is how a delegation from the city of Kamaïshi (Japan) went to Digne-les-Bains in the early 1990s to acquire the slab and take it home!
It was of course unthinkable to sell such heritage classified as a National Nature Reserve and to which the Dignois are very attached. The Reserve team then molded the slab (at the time 150 m² with 600 ammonites) which is now on display in a museum in Kamaïshi, a town twinned with Digne-les-Bains.