La Clue de Sisteron and the Rocher de la Baume
The Porte de la Provence open to alpine history
A link in the history of the Alps….
One or more collisions?
The clue de Sisteron is locally referred to as “Porte de la Provence”. It cuts a rocky ridge dated from the Upper Jurassic (from -150 to -145 million years ago). Its deformation began with the first folds of the alpine cycle during the so-called Pyrenean-Provençal phase, around -70 million years ago.
During most of the Lower Cretaceous (-145 to -120 million years ago), Iberia was located much further northwest than today. It belonged to the same micro-plate as Corsica and Sardinia, located in its eastern extension. Then, around -115 million years ago, the opening of part of the North Atlantic Ocean and the Bay of Biscay helped to move Iberia to the south-east.
The Upper Cretaceous, from -80 million years ago, marks the beginning of the Pyrenean-Provençal phase proper. The Corso-Sardinian block slides from west to east, along a huge fault. At this time, an oceanic space still exists between the southwest of France and Spain. However, the movement towards the north of the African bloc begins to crush Iberia against the European plate, at the level of the south of France. This compression gives rise to the first reliefs of the Pyrenean-Provençal chain: from the Pyrenees to Basse-Provence, via Sardinia and Corsica which had not yet rotated to occupy their current position (this rotational movement will not take place). that around -17 to -15 million years ago).
Basse-Provence is thus gradually emerged, eroded and then intensely deformed. Located a little outside the chain, the emergence of Haute-Provence comes a little later, around -70 million years ago. It is then deformed into folds oriented east-west, including the one that will later give the Bar of Sisteron.
Then around -35 million years ago, it is the beginning of the Alpine orogeny which results from the collision of Apulia (micro-plate in northern Africa) with the European margin, at the current location of the northern Italy. The paroxysmal alpine folds which take place between -15 to -5 million years ago will rejuvenate and accentuate the old east-west folds of the first phase.
Clearly posterior, the notch in the clue may have been inherited from the Messinian crisis (from -5,9 to -5,3 million years ago) but there is no evidence to prove it. Its current morphology, for its part, is linked to the action of glaciers during the Quaternary and, particularly, during the last 300 years.
… In connection with 22 other geosites
The Sisteron clue substantially materializes the downstream limit of the great Durance glacier during the penultimate Riss glaciation (from -300 to -000 years). During the last glaciation (from -100 to -000), the glacier stopped a few kilometers north of the clue.
Unlike the site of Lake Allos which corresponded to the source of a glacier and the receptacle of snow, the site of the Sisteron clue corresponded to a glacial front.
From the Lure ridge, you can admire all the Pyrenean-Provençal folds
, oriented east-west, and the Baronnies to which the Baume rock (clue de Sisteron) belongs.
What we can decipher from the current landscape
Due to the hardness of this limestone bar from the Upper Jurassic, erosion has left it in relief, while the softer and marly terrain that surrounds it has been deeply eroded.
The natural doors, like narrowings, are as many opportunities to promote exchanges or on the contrary to close them (clue or cluse: from the Latin claudere: close). The high-perched areas are strategic places of control particularly sought after for building fortresses. The visit of the citadel will allow you to discover part of this history.
Today, Sisteron is a city of tourist reception and passage. With the opening of the motorway tunnel, the Baume rock no longer plays its role of barrier.
A little anecdote?
If limestones are very hard rocks, they are nevertheless eaten away by acidic water such as infiltration water, loaded with carbonic acid and humic acid. Also, they can be hollowed out of karstic cavities as indicated by the evocative name of the rock of the Baume. In Provence and Haute Provence, a balm is a cave.
The rock is well and truly holed naturally. The curious, good walkers, will be able to see it by discovering the Trou de l'Argent hike in Sisteron. The Baume rock is also a particularly popular climbing site for all levels.