Place de Selonnet, near the Town Hall, towards the Boulangère district. Follow the yellow PR markings along the road, which becomes a track, continue on the GR®6 for 200m. Turn left 3 times, cross the bridge over the Blanche river. Cross the D900 and take the road opposite towards Haute Liberne. Go up along the road, going around the houses to the left. Before the last farm, the path goes left across the fields. Be careful, the return of the route is along the D900 road, be careful.

The Grosse Pierre dates from the last great glaciation of the Quaternary, during which the Vallée de la Blanche was located at the southern European limit of the valleys clogged with ice. During the great glacial extensions of the Quaternary, the Seyne basin was invaded by the Ubaye glacier and to a lesser extent by that of the Durance. The glacier advanced through the Col Saint-Jean throughout the valley as far as the Col de Maure and Col du Fanget. It is likely that the ice was several hundred meters thick and that the surface of the glacier at its maximum was around 1700 m above sea level.

The Grosse-Pierre is a huge piece of sandstone whose mass can be estimated at around 550 tonnes. It was only in the second half of the 20th century, after much controversy, that the geodynamic significance of the erratic blocks was recognised. This is generally attributed to the Würm glaciation, which peaked around 000 years ago and was the last. It was preceded by the Riss glaciation (325.000 to 130.000 years ago), which was of greater extent.

Area : Blanche Serre-Ponçon