The Velodrome

A collision between two cyclists?

A link in the history of the Alps ...

The second part of the tertiary era is called Neogene (-23 to -2,58 Ma), which means “new birth”. This “slice” of time is aptly named for the Southern Alps. Indeed, it is during this period, during a paroxysmal phase of deformations (folds, overlaps, uplift), that they will really take birth.

The collision has so far deeply affected the southern and eastern regions of the nascent Alps. In the Neogene, it will be the turn of its western and northern part: propagating in space and time, alpine deformations have often been compared to a wave. In this context, the Vélodrome is a spectacular fold, the geometry of which evokes a competitive cycle track. Within it, the deformations of the layers are slow and progressive: it began to take its shape around -7 million years and ended around -2 million years.

To wrinkle sedimentary layers… the sediment first has to settle. In the case of the Vélodrome, at the same time as the layers pile up, the tectonics appear and fold the layers, modifying the landscape. Already, around -20 million years ago, the landscape revolution was underway. The landscape before the Velodrome was very flat, slightly inclined towards the south, at an altitude close to 0 m. The rivers flowing down from the Alps carried clay, sand and pebbles towards the sea.

What sea? A new marine space, the peri-alpine sea came into being from -20 million years ago with the opening to the south of the western Mediterranean and with the formation of depressions all around the uplifting alpine chain.

This alpine sea drew a large marine gulf in the direction of Digne. Its northern and northeastern limits have been identified within the Velodrome thanks to its fossil beaches. The Vélodrome fold is made up of a series of sedimentary layers, initially deposited at the bottom of this marine gulf. A first set of deposits was deposited, very sandy at the base then clay-sandy, these are the marine molasses (see box). With the intensification of the deformations, erosion is very active and the sediments, transported by rivers and streams, are more and more abundant until gradually filling the sea. This is how the marly and conglomeratic deposits ( with pebbles) of the yellow continental molasse succeed (around -12 million years ago) to the marine molasse. When it settles around -12 to -3 million years ago, the layers of sea molasse are already partly straightened.

During and after the deposition of continental molasse, the entire stack of strata is deformed as a whole. The layers of sandstone of the sea molasse, slightly straightened, are then crushed and driven back towards the south. From strongly inclined, they pass vertically. Then, always pushed south, they even end up being overturned.

This large fold is then covered by the Digne thrust layer between -5 and -3 million years ago. Without the very late and deep erosion of the Bès valley, it would be impossible to see it. Over the last million years, the uprising of the massifs (Blayeul, Vélodrome) and the digging of the valleys have contributed to erode the sedimentary pile of the water table above the Vélodrome, revealing the organization of the strata and the reliefs.

Paleo-Bes fluvial deposits, 2,6 to -2 million years old, also deformed, complete the landscape and history of the Vélodrome.

... in connection with 22 other geosites: the Pénitents des Mées, Gorges du Verdon, Maurin marbles

The sediments of the Vélodrome tell the story of the Miocene marine gulf (from -20 to -12 million years ago) filled by the detrital inputs (pebbles, sands, clays) resulting from the erosion of the Alps. This Gulf of Digne-Valensole has given way to a delta landscape, the sea receding towards the south. Voluminous deposits of pebbles have accumulated forming the conglomerates of Valensole that can be found today in the landscapes of the Valensole plateau and at the exit of the Gorges du Verdon, on the site of the Pénitents des Mées or in the south-east. from Sisteron and around Digne.

On the Vélodrome site, pebbles are proportionately less abundant compared to clay and marly deposits. Indeed, the conglomerates of Valensole are replaced there by the yellow molasse.

What we can decipher from the current landscape

The first visual reference is that of the molasse marine with its imposing gray sandstone bars: to the south of the velodrome (downstream) they plunge towards the south, then upstream they plunge this time towards the north, then straighten up at the vertical (northern limit of the Velodrome).

While climbing on the small hillock of the panoramic site of Vieil Esclangon, it is possible to see fossils of "scallops" in the molassic sandstones, which confirms the marine origin of these sandstones. They also contain pebbles. Some of them are green, others are green and white. We recognize there rocks from the ancient ocean floor! When the marine sandstones were deposited, these “oceanic green rocks” were part of the alpine landforms subject to erosion: this was a witness to the collision. At the heart of the Vélodrome, the impressive vertical Facibelle blade catches the eye. It is a good landmark that marks the passage between gray marine molasse and yellow continental molasse.

A little anecdote?

This sumptuous landscape, a Mecca of Alpine geology, is also a land of sometimes unusual encounters. One can discover there golden messages engraved in the rock " Ambulo ergo sum », An Art Refuge, a nature sanctuary…

Mathematicians, philosophers, astronomers, historians, architects, naturalists, painters, geologists, contemporary artists or simple hikers, each with his gaze and vision of the world finds a source of reflection and contemplation in front of this spectacle of a moving Earth.

For another discovery of the place: https://musee-gassendi.org/fr/accueil/art-contemporain-nature-pays-dignois/