The Clue de Verdaches
The old Verdaches swamps: a coal factory
A link in the history of the Alps….
Towards the end of the primary era, the continents are almost all welded together: Pangea is almost formed. The Variscan chain completes its uplift. At its feet, resulting from its very active erosion, sediments accumulate.
The clue de Verdaches, where the torrent “le Bès” flows, presents a landscape in two shades. Near the torrent, we observe dark and black rocks, and above, a huge light gray rocky bar that draws vertical walls.
The layers of the lower part of the clue contain a large proportion of organic matter transformed into charcoal, hence its black color. There are many plant fossils there. There are all kinds of tree ferns, giant horsetails related to those of our current wetlands or even large trees that no longer have equivalents today.
These fossils make it possible to date the rocks from -305 million years ago, ie from the Upper Carboniferous (primary era).
The different types of plants as well as the succession of layers and their organization make it possible to reconstruct the landscape and the climate of the time. A luxuriant forest occupied a marshy plain where sands and clays accumulated as well as plant debris. There was then a humid tropical climate.
In this type of confined, poorly oxygenated environment, organic debris that has fallen to the bottom of swamps is quickly buried by the regular arrival of sludge. Their burial over millions of years, under hundreds or even thousands of meters of sediment, allowed their slow transformation into coal.
The clear layers of the upper part of the clue are notched vertically by the Bès. These very hard rocks very rich in quartz are quartzites. They were formed in the Triassic (secondary era) from ancient sands and gravels resulting from the erosion of the residual reliefs of the Variscan range. These sediments were transported and deposited by rivers in a vast sub-desert plain.
… In connection with 22 other geosites: Argentera-Mercantour and Daluis
In the Clue de Verdaches, the Triassic quartzites lie directly on the Carboniferous: here the very end of the primary era is missing: the Permian. As a reminder, the normal succession of geological stages is Carboniferous, Permian then Triassic.
On the other hand, in the Daluis gorges, several hundred meters of Permian rocks were deposited under the Triassic. These deposits make it possible to tell what happened at that time, the erosion having entirely destroyed this piece of history in Verdaches.
Carboniferous soils are also present, in a localized manner, in the crystalline massif of Argentera-Mercantour. The history of this massif is old, with a large part of its rocks (including granites) which were formed in the primary era.
What we can decipher from the current landscape
Carboniferous rocks are rich in silica. Its presence makes the substrate acidic, unlike the rest of the rocks forming the Prealps of Digne. This is why the vegetation of the Verdaches clue is locally specific, acidophilic, with rare species.
If the current landscape of the Verdaches Clue owes a lot to the different composition of its rocks, it is also the result of tectonics and the folding of layers. Indeed, when crossing the clue by following the road, we first encounter the quartzites of the Triassic then the carboniferous layers of the Carboniferous and, again, the Triassic. It appears that the layers of the Triassic are folded and form a kind of vault. Quartzites are very hard, their folding has led to the intense fracturing that is observed today.
A little anecdote?
In the clue de Verdaches, an old stone quarry called "the Italians quarry" was in operation at the beginning of the XXrd century. Italian immigrants were hired there to extract and cut blocks of quartzites. This particularly hard rock was intended for the construction of structures, bridges and parapets, such as can be seen along the route du Bès.
Other mineral resources were the subject of occasional exploitation (mainly during the XVIIIrd and XIXrd centuries) such as anthracite, lead, copper and iron.
The Carboniferous is world famous for its large deposits of coal and its sedimentary basins several thousand meters thick. However, in the Verdaches clue, the situation on the surface is very different: the Carboniferous outcrops little and the coal is not very abundant.
What about in depth? In 1952, mining research led to a reconnaissance survey upstream of the clue. It was planned to descend to 600 meters. After having crossed 80 meters of quartzites, the hole intersects 370 meters of Carboniferous. Among many very thin layers of coal, only one exceeding 1 meter was found, but the bit got stuck at 453 meters. These meager findings were not sufficient to justify further research ...