The ridges of Lure

 Take to the skies and admire the beginnings of alpine folds

A link in the history of the Alps….

In the Late Cretaceous, the split in the South Atlantic completely separated South America from Africa. The African bloc, also separated from Antarctica, is driven back north. It is approaching the Europe-Asia block by crushing the oceanic space of the Tethys and its dependencies.

The end of the Alpine Ocean and the entire Tethys River is announced. The oceanic crust plunges by subduction under the Apulian plate which is a microplate in northern Africa.

The disappearance of ocean space does not occur everywhere at the same time, it is spread over several tens of millions of years (between -90 and -35 million years).

The Iberian plate (Spain, Sardinia, Corsica) slides eastwards during the Upper Cretaceous before becoming encrusted, pushed by Africa during the collision, in the southern margin of Europe (south of the future France). These tectonic events will give birth to a large chain of mountains: the Pyrenean-Provençal chain. Basse Provence is then intensely distorted.

The “Alpes de Haute-Provence”, located outside the chain, are much less affected. Nevertheless, they are folded, emerged and subject to strong erosion.

This Pyrenean-Provençal phase is crucial in the construction of subsequent landscapes, and the Lure mountain is in part inherited from these first reliefs.

Current landscape barrier between the Baronnies and Provence, in line with Mont Ventoux, the mountain of Lure is a large east-west fold of which all that remains, except near Sisteron, is the southern flank formed by a large slope.

The Lure ridge is bordered to the north by a large regional accident, which marks among other things the limit between the Provençal countries (including Lure) and the Baronnies. In geology, an accident is an abnormal contact surface such as a fault, a strike-slip or a thrust. From the Lure ridge, the entire landscape that opens to the north is typically organized in an east-west direction in very tight folds, unlike what can be seen to the south of the mountain.

The Provençal folds will be re-mobilized during the paroxysmal alpine folds: still in an east-west direction but this time pushed south. The Baronnies compartment passes under the mountain of Lure which straddles it.

… In connection with 22 other geosites

The northern limit of the Ventoux-Lure chain corresponds to a very old tectonic accident set in motion by the alpine uplift.  

About -130 million years ago, this accident limited a shallow sea platform to the south in which the Urgonian limestones of the Barremian were developed, visible, for example, in the gorges of Oppedette.

To the north, the deeper marine basin, called the Vocontian Basin, served as a receptacle for a quantity of sediments which formed thick series of alternating layers of limestone and marly layers. These remarkable fossiliferous series (ammonites) and their alternation between marl and limestone can be observed at Lake Castillon.

At the time when the Miocene Sea (-20 to -15 million years ago) occupied the marine gulf of Digne-Esclangon, the Ventoux-Lure massif formed one or two islands emerging from the waves of the Alpine sea (arm of the sea open to the western Mediterranean).

What we can decipher from the current landscape

The Barremian limestones of the Lure summit have gained altitude for 130 million years. During the Quaternary glaciations, they were strongly impacted by the harshness of the climate and the close vicinity of the great Durancian glacier.

Covered with snowfields, the limestones at the summit erupted under the action of frost (freezing) giving rise to large expanses of exploded slabs (ridge and slope) and significant scree on the steep northern slope. Although the conditions are no longer comparable, the ridge, long snowy and swept by the mistral, remains exposed to strong thermal differences which continue to shatter the rock.

A little anecdote?

Frost rocks are not good for construction. However, the natural cutting of limestone into small slabs facilitated their use to make dry stone constructions.

This is evidenced by a unique heritage that can be admired by walking through the Lure mountain: magnificent sheepfolds and their vaults, old cairns marking the paths ...

Jean Giono called these small slabs "singing stones" because of the soft screeching they produce when you walk on them ...

To mark the transition to the year 2000, an immense cairn was built where the stones of Lure mingle with those brought from all over the world by hikers-builders.