Conglomerates are sedimentary rocks, of detrital origin (small loose materials, particles from a larger mass of rock), whose fragments are a varying composite-mix bound together by a natural cement. When these fragments are in the form of pebbles, as is the case here, we speak of "poudingue" (like a mixed-fruit-cake-pudding). From the Upper Miocene period (8 million years ago) to the end of the Pliocene period (1.65 million years ago), the Valensole basin-area corresponds to a vast zone where the rivers (Paleo-Asse, Paleo-Durance) deposited a quantity of rocks that originated from further afield (green rocks coming from the central Alps) or from closer-by (local Jurassic period limestone). Over millions of years, hundreds of metres of pebbles have accumulated here, forming the Valensole plateau-plain, flatlands, and its surroundings. The considerable thickness of the deposited conglomerates under our feet and in front of us (the Hill of Saint Joseph) bears witness to this geological phenomenon and period.
Today, these lands have been deeply gouged by the torrents of mountain-rivers. Here the ravine is called the Graves Ravine.
The Valensole conglomerates (mixed composites of various rocks) were studied in the 19th century by the geologist Élie de Beaumont.
In the middle of the 20th century, the discovery of a deposit of fossils from small mammals in the Municipal District of Puimoisson made it possible to date the top of the rock formation to the end of the Pliocene period (about 1.65 million years ago).