During the great glacial movements and extensions of the Quaternary period, the Seyne valley was invaded by the Ubaye glacier and to a lesser extent by the Durance glacier.

The glacier advanced through the Col Saint-Jean mountain-pass into the whole valley as far as the Col de Maure mountain-pass and the Col de Fanget mountain-pass

It is likely that the ice was several hundred metres thick and that the surface of the glacier at its maximum was around 1,700m in altitude.

La Grosse-Pierre (The Large Stone) is an erratic block, i.e. transported by the glacier and deposited here when the glacier melted.

It is a huge piece of sandstone whose mass can be estimated at around 550 tonnes.

It was only in the second half of the 19th century, after much controversy, that the geodynamic significance of the erratic blocks was recognised.

It is generally attributed to Würm Glaciation, which occurred around 20,000 years ago and was the last glaciation period. It was preceded by Riss Glaciation (325,000 to 130,000 years ago) which was more extensive.