Les Eaux-Tortes, the Laverq valley
Thrust slicks, or when mountains meet
A link in the history of the Alps ...
With the collision, the two tectonic plates Europe and Apulia clash. From the end of the Eocene and the Oligocene (around -35 to -25 million years ago), the collision reached its climax in the eastern part of the Alps. This paroxysmal phase will progress in time towards the west, to reach later the future southern Alps and the subalpine chains.
The overlap of the Apulian microplate on that of Europe causes intense tectonic deformations. During the closure of the space between the two continents, the ocean floor and the continental margins are pushed north and west. The sedimentary piles deposited there therefore first move underwater (Upper Cretaceous / early Tertiary).
These large sedimentary and tectonic units can move over great distances.
When they are superimposed on other more stable layers at the edge of the pleated chain (that is to say on a foreland), they are called thrust sheets.
Partly dismantled in the frontal zone, these first submarine layers generate a quantity of blocks and all kinds of sedimentary mixtures. These are the materials that are superimposed on the Annot sandstone.
Thrust slicks arrive during the Oligocene (around -30 to -25 million years ago) in the surroundings of Barcelonnette and Allos. They fill the marine space which is then replaced by emerged lands. The following layers will advance, this time in the open air, to pile up on top of each other until the end of the Miocene (around -7 million years ago). They will make their last trip to the Pliocene on the back of the Digne aquifer (around -5 to -3 million years ago).
The Eaux-Tortes site offers a beautiful view of the front of these so-called “internal” water tables. This term designates here a set of tectonic and sedimentary zones which have moved very widely, by hundreds or thousands of kilometers. They are opposed to the external zones which have moved less compared to the foreland (the crystalline massifs like the Argentera-Mercantour, and their sedimentary covers which are the subalpine chains).
The left and right banks of the valley are very different. Left bank, from the top of the Estrop (2961 m) to the Eaux-Tortes torrent, the Grès d'Annot structure the landscape with their large sloping slabs. Opposite, on the right bank, stand imposing limestone reliefs with large vertical walls called Petite and Grande Séolanes. A few ravines carved out of the scree and glacial plateaus suggest alternating marly layers with harder layers. Several shreds of water tables are piled up on this slope. Some are composed of Flysch with Helminthoids which are rhythmic deposits (of turbidite type) containing serpentiform fossil traces attributed to kinds of worms. It was these flyschs that began their “journey” underwater in the Upper Cretaceous.
Links to the 22 other sites, Grès d'Annot, Lac d'Allos, Riou Bourdoux
The Eaux-Tortes depression was carved out by the Blanche du Laverq glacier. A glacial lock retains the water upstream. Due to the hardness of the sandstones, the Ice Age did not carve out a lake such as Lake Allos, and the small lake born from the melting of the glacier was quickly filled in to give a peaty environment.
At Lake Allos, the erosive power of the glacier has been facilitated by the presence of much more fragile terrain at the foot of the Tours, which is made of Annot sandstone. This lake, like all mountain lakes, is currently being filled.
We find the flysch at Helminthoïdes from the Pra Loup site with the remarkable view of the Riou Bourdoux and the entire chain on the right bank of the Ubaye. The Black Earths (-170 to -155 million years ago) of the valley support the upper part of the mountain where a rocky unit develops formed by a stack of small, lighter layers: this is the Flysch water table. to Helminthoïdes du Parpaillon.
Link with the current landscape
The valley owes its deep gash to the Blanche glacier, a tributary of the Ubaye glacier. It retains the marks of its Quaternary history with numerous veneers and glacial moraines, small lakes, peat bogs, glacial locks and sheep rocks.
The Eaux-Tortes peat bog is a sensitive natural area. It is home to many rare, protected, or endemic species. The site benefits from a discovery trail.
Anecdote?
The Laverq abbey is remarkable for its history. It was built in 1135 by the pioneer monks of the order of Chalais which follows the rule of St Benoit.
Composed of a fairly low frame with narrow openings and wooden shingled roofs, it stands out especially for its stone bell tower, with very simple and traditional lines. Nestled at the foot of the Grande Séolane, it is an integral part of the vast panorama of this valley floor.
It was subsequently attached to the Abbey of Boscodon in the Hautes-Alpes, built by the same order in the 7th century and today classified as a historical monument. The monks builders attached great importance to the symbolism of the elements. This abbey was established and oriented according to the rhythms of the sun, the surrounding rivers, the presence of a stone quarry (neither too close, to respect the silence on the site of construction of the abbey, nor too far not to increase supply constraints), the forest, people ... The importance of symbols is also found in the design of buildings: repetition of elements according to key figures (7 vaults, like the XNUMX days of the week), ratios of proportions in harmony with the golden ratio, etc.