What remains of huge masses in movement
About 200,000 years ago, during the third glaciation known as the Riss glaciation, the Gesso Valley was covered by two large glacial branches: the first, called Entracque glacier, was formed by the glaciers of Barra, Rovina and Bousset; the second, that of Valdieri, was formed by the union of the glacier of Valletta and Valasco with that of Meris. The two main glacial tongues joined upstream of Valdieri to form the great Gesso glacier, about 300 metres thick and almost two kilometres wide, which extended to the straits of Andonno. During the last glaciation, the Würm glaciation, the two branches of the glacier were separated from each other.
The gigantic glaciers that descended from the peaks along the valleys in the Quaternary Era have left deep traces of their passage, which has shaped the current landscape of the Maritime Alps. This is evidenced by the inclined plates and the rounded bumps (roches moutonnées) seen in the high valleys, the result of the abrasion by the massive glaciers moving over the compact crystalline rocks. Vast areas of rock smoothed by the ancient glaciers can be found, for example, on the Baus plateau and along the mule track that goes up to the Lago soprano della Sellain the upper Vallone della Meris.
Other very evident traces left by the Quaternary glaciers are the basins and cirques, hollowed out where the rocky substratum was most easily erodible. At the highest altitudes, these basins are now largely occupied by lakes, while at intermediate altitudes they have almost all been filled by debris carried by watercourses and have thus been transformed into large plains, such as the Pra del Rasùr and the Praièt above San Giacomo di Entracque, Pian della Casa and Valasco above Terme di Valdieri, those of Prato and Chiòt in the Vallone della Meris.
The debris brought down by the glaciers is still visible today in the morainic deposits of Tetti Bandito and San Lorenzo di Valdieri, which can also be seen from the main road, and in those of Esterate (Serra dei castagni) and Polveriera.
The current glaciers of the Maritime Alps are very small compared to the ancient ones, however they are noteworthy because they are the southernmost in the Alps. Most of them are concentrated on the Italian side of the Clapier-Maledìa-Gelas group. The largest of them all is the Clapier Glacier, which is also the southernmost glacier, located just 45 kilometres as the crow flies from Monte Carlo. Over the last few decades, following the general retreat, the glaciers of the Maritime Alps have also been considerably reduced in size and are now close to disappearing altogether**: the Clapier Glacier is now confined to what was once its eastern branch, while the two main glaciers of the Gelas now struggle to emerge from the debris.