The Grand Vallon river roughly follows the geological fault-line of a major break in the earth's crust to which it gave its name, the Grand Vallon Geological Fault-Line. The Grand Vallon river is one of the most northly extremities of the geological fault-line which runs to Aix-en-Provence through the Durance valley and which has produced several earthquakes throughout history.

It has a long history: it was already active in the Lower Jurassic period, the effects of which can be seen around the area of Nibles, for example.

Here, the Grand-Vallon Fault-Line is revealed by the contrast of terrain-types it brings into contact: on the right, the red rocks formed in the continental environment during the Oligocene period (about 30 million years ago) and on the left, the thick series of dark clayey limestones of the Digne geolotical-nappe (thrust-sheet) formed under the sea in the Lias period (180 to 170 million years ago). Thus, the western compartment on the left is several hundred metres lower than the eastern one.

The sinuosity, or curvity of the geological fault-line allowed gypsum to infiltrate along it and to rise to the surface.