Allos Lake

Last memory of the ice age

A link in the history of the Alps ...

From the start of the Quaternary, around -2,58 million years ago, a succession of glacial phases are interspersed with shorter and warmer episodes, called interglacial phases.

The glaciations in the Alps have had a strong effect on the erosion of the chain, although it is still subject to the constraints of the collision and to increasingly important vertical movements. Indeed, the Alps are gaining altitude.

The last glaciation, which took place in the Alps between -90 years and around -000 years, has left its mark. Among them, the Allos lake bears witness to the action of glaciers and their melting after the last glacial maximum which marks the maximum extension of the ice caps.

With its 54 hectares of open water, Lac d'Allos is the largest natural mountain lake in Europe. Currently about forty meters deep, it occupies the bottom of a large basin dominated by towers formed of sandstone. The basin is limited by a rim forming an ancient glacial lock.

This great depression was carved out by a glacier. Indeed, around -20 years ago, during the coldest period of the last glaciation, glaciers occupied a large part of the Haut Verdon valley. The mountainous cirque of Tours and the Mountain of Laus was the zone of reception of the snow and the place of its transformation into ice. The glacier, thus formed in altitude, then descended in a north-westerly direction to join other glacial tongues as far as the main Verdon glacier, which was then 000 kilometers long.

At the end of the glaciation, the retreat of the glaciers was quite rapid in the southern Alps. Glaciers, which sometimes over-dug the bedrock in its most fragile areas, gave way, by melting, to bodies of water or lakes. These basins acted as receptacles for both meltwater and runoff water and for sediment, mud and pebbles descended from the reliefs.

Thanks to its dimensions and its depth, Lake d'Allos has recorded a lot of information in its sediments. When the glacier melted, the surrounding vegetation grew and pollens sank to the bottom of the lake. Various organisms have also accidentally fallen there while others have grown there. Then, the organic remains were buried at the bottom of the lake, in the mud. These annual accumulations of sediments with their procession of organic remains have constituted real archives of the history of the lake and its environment. 

Researchers have drilled holes in the sediments of the lake. Thus thanks to pollens, they dated the age of the first lake sediments and deduced that the glacier retreated around 13 years before the present; the “present” being 000 for the quaternarists.

The data provided by the lake make it possible to follow, over the last 13 millennia, climate change as well as the impact of man on vegetation and the environment for around 8000 years.

In connection with 22 other geosites: Grès d'Annot, Eaux-Tortes, Riou Bourdoux

The Tours du Lac are made of Annot sandstone. The sandstone mass is traversed by numerous faults. They facilitated the work of erosion and the cutting of the Towers.

As in Laverq and Ubaye (see the Eaux tortes and Riou Bourdoux files), the landscape has been sculpted by glaciers. It presents tectonic scales and layers of thrusts (here Flysch du Pelat) which cover rocky sets that have not been moved much like the Annot sandstone.

At Eaux-Tortes in Laverq, the hardness of the Annot sandstone only allowed the deepening of a series of shallow basins. Quickly filled, they evolved into peat bogs, without forming a lake.

What we can decipher from the current landscape

Around Lac d'Allos, the landscape is very contrasted. The towers' sandstone piles are suddenly interrupted by a fault that leads to their juxtaposition with another mountainous set with “gentler” slopes and with the ridge that ends up surrounding the lake. 

The effects of erosion underline the diversity of the rocks and the large sedimentary and tectonic complexes which contribute to the originality of the site.

We can be surprised at the absence of a weir! In fact, generally, the waters of glacial lakes escape downstream through a spillway. This is a notch dug in a glacier lock or in a moraine. Here, after a brief underground journey, the resurgence of water from the lake gives birth to the Serpentine stream.

Make no mistake, there is no serpentinite (metamorphic rock) around the lake. The stream winds “only” in the Laus plane where it promotes the development of a magnificent wetland.

A little anecdote?

At the age of 82, Alexandra David Néel, an adventurer in love with Tibet and a writer, spent a few winter nights in tents in the snow at Lake Allos at 2300 meters above sea level. She was accompanied by her adopted son the Lama Aphur Yongden.

The last Yeti of the Alps would have been glimpsed near the Encombrette shortly after Alexandra David Neel's stay.

In 1908, then President of the Council of Ministers, Georges Clémenceau rode a mule to Lake Allos. Accompanied by many personalities, he went to the lake on the occasion of a study for the construction of a huge water reservoir dedicated to supplying the Var department, then deprived of resources.